quality control for overcoring stress measurement data · can be compared to strength envelope of...
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P O S I V A O Y
F I N - 2 7 1 6 0 O L K I L U O T O , F I N L A N D
P h o n e ( 0 2 ) 8 3 7 2 3 1 ( n a t . ) , ( + 3 5 8 - 2 - ) 8 3 7 2 3 1 ( i n t . )
F a x ( 0 2 ) 8 3 7 2 3 7 0 9 ( n a t . ) , ( + 3 5 8 - 2 - ) 8 3 7 2 3 7 0 9 ( i n t . )
Quality Controlfor Overcoring StressMeasurement Data
Apr i l 2006
POSIVA 2006 -03
Matt i Haka la
POSIVA 2006-03
Apr i l 2006
POSIVA OY
F I - 27160 OLK I LUOTO, F INLAND
Phone (02 ) 8372 31 (na t . ) , ( +358 -2 - ) 8372 31 ( i n t . )
Fax (02 ) 8372 3709 (na t . ) , ( +358 -2 - ) 8372 3709 ( i n t . )
Matt i Haka la
Gr idpo in t F in l and Oy
Quality Controlfor Overcoring StressMeasurement Data
ISBN 951-652 -126 -6ISSN 1239-3096
The conc lus ions and v i ewpo in ts p resen ted i n the r epo r t a r e
those o f au tho r ( s ) and do no t necessa r i l y co inc ide
wi th those o f Pos i va .
Tekijä(t) – Author(s)
Matti Hakala, Gridpoint Finland Oy
Toimeksiantaja(t) – Commissioned by Posiva Oy
Nimeke – Title
QUALITY CONTROL FOR OVERCORING STRESS MEASUREMENT DATA
Tiivistelmä – Abstract The in situ state of stress is one of the key rock mechanical factors considering the safety and stability of underground excavations for civil and mining engineering purposes in greater depth, but at the same time measuring and interpretation of stress comes more difficult. Normally the in situ stress interpretation is based on final strain readings and there have not been practical and objective tools to judge transient strain behaviour during overcoring. This study was set up by nuclear waste management companies Posiva Oy (Finland) and Svensk Kärnbränslehantering AB (Sweden) to improve the quality of interpretation of overcoring stress measurement results. The primary product of the project is quality control capability of overcoring stress measurement data. For this purpose a computer program was developed which can simulate the transient strains and stresses during the overcoring in any in situ stress and coring load conditions. The solution is based on superpositioning of elastic stresses and the basic idea can be applied also for different overcoring probes with minor modifications and recalculation of stress tensors. The measured strains can be compared to calculated ones to check if the measured transient behaviour is accordant with interpreted in situ state of stress. If not, the in situ state of stress can be calculated based any transient or final strain values. The transient stresses can be compared to strength envelope of intact rock and thereby estimate core damage potential. Technically the developed OCS-code fulfilled all the objectives and the numerical error was found to be less than 5%. The analysed case studies showed clearly the advance of having objective method study the reliability of stress measurement data. On the other hand, the interpretation of in situ state of stress from on early strains is difficult because the solution is very sensitive for measured strains and coring advance. The report also includes a comprehensive list of factors to be considered when performing in situ stress measurement and procedures to interpret measuring data in order to improve the quality of interpretation.
Avainsanat - Keywords
overcoring, in situ stress, measurement, quality control, simulate, transient strain, program ISBN ISBN 951-652-126-6
ISSN ISSN 1239-3096
Sivumäärä – Number of pages 106
Kieli – Language English
Posiva-raportti – Posiva Report Posiva Oy FI-27160 OLKILUOTO, FINLAND Puh. 02-8372 (31) – Int. Tel. +358 2 8372 (31)
Raportin tunnus – Report code
POSIVA 2006-03 Julkaisuaika – Date
April 2006
Tekijä(t) – Author(s) Matti Hakala, Gridpoint Finland Oy
Toimeksiantaja(t) – Commissioned by Posiva Oy
Nimeke – Title
IRTIKAIRAUSJÄNNITYSTILAMITTAUSTIEDON LAADUNVARMISTUS JA TULKINTA
Tiivistelmä – Abstract
In situ jännitystila on yksi keskeisimmistä louhittavuuteen ja turvallisuuteen vaikuttavista kallio-parametreistä kun suunnitellaan syvälle rakennettavia kalliotiloja tai louhoksia. Toisaalta jänni-tystilan mittaaminen ja tulkinta käy yhä haastavammaksi tutkimussyvyyden kasvaessa. Yleisesti jännitystilan tulkinta perustuu mittauksessa saataviin lopullisiin muodonmuutoksiin, eikä käytet-tävissä ole ollut käytännöllistä menetelmään kairauksen aikaisten transienttien muodonmuutosten tulkintaan. Tämä työ käynnistettiin käytetyn polttoaineen loppusijoituksesta vastaavien yhtiöiden Posiva Oy (Suomi) and Svensk Kärnbränslehantering AB (Ruotsi) toimeksiannosta ja sen tarkoituksena on irtikairausjännitystilamittausten tulkinnan laadun parantaminen. Tässä työssä kehitettiin valmius irtikairausjännitystilamittauksen laadun varmistamiseksi. Tätä varten tehtiin tietokoneohjelma, joka simuloi irtikairauksen aikana mitattavia transientteja muo-donmuutoksia vapaasti asetettavassa in situ jännitystilassa ja kairauksesta aiheutuvassa kuor-mituksessa. Ohjelma perustuu kimmoisten jännitystilojen superponointiin ja se on mukautet-tavissa pienin muutoksin ja jännitystensoreiden uudelleen laskennalla eri irtikairauskennoille. Ohjelmalla verrataan mitattuja muodonmuutoksia laskettuihin ja varmistaa että transientit muo-donmuutokset vastaavat tulkittua in situ jännitystilaa. Ellei näin ole voidaan in situ jännitystila tulkita uudelleen transienteista tai lopullisista muodonmuutoksista. Jännitystilavaurion toden-näköisyys saadaan arvioitua vertaamalla transientteja maksimi ja minimi jännitystiloja ehjän kiven murtopintaan. Tekniseltä toteutukseltaan kehitetty OCS-tietokoneohjelma täytti työn tavoitteet ja sen tulosten virheet ovat alle 5%. Läpikäydyt todelliset mittaustulokset osoittivat selvästi objektiivisen mene-telmän hyödyn mittaustulosten luotettavuuden arvioinnissa. Toisaalta kävi ilmi, että in situ jän-nitystilan tulkinta irtikairauksen alkuvaiheen muodonmuutoksista on erittäin ongelmallinen, sillä tulos on erittäin herkkä muodonmuutos ja etenkin kairauksen etenemän mittaustarkkuudelle. Raportissa on esitetty kattava lista irtikairausjännitystilamittaukseen vaikuttavista tekijöistä ja menetelmiä mittaustulosten tulkinnan laadun parantamiseksi.
Avainsanat - Keywords irtikairaus, in situ jännitystila, mittaus, laadunvarmistus, simuloida, transientti muodonmuutos, ohjelma ISBN ISBN 951-652-126-6
ISSN ISSN 1239-3096
Sivumäärä – Number of pages 106
Kieli – Language Englanti
Posiva-raportti – Posiva Report Posiva Oy FI-27160 OLKILUOTO, FINLAND Puh. 02-8372 (31) – Int. Tel. +358 2 8372 (31)
Raportin tunnus – Report code
POSIVA 2006-03 Julkaisuaika – Date
Huhtikuu 2006
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TABLE OF CONTENTS Abstract Tiivistelmä TABLE OF CONTENTS ..................................................................................... 1 LIST OF SYMBOLS ........................................................................................... 3 PREFACE .......................................................................................................... 5 1 INTRODUCTION.......................................................................................... 7 2 OVERCORING METHOD ............................................................................ 9 3 FACTORS TO BE CONSIDERED IN OVERCORING STRESS MEASUREMENT ............................................................................................. 17
3.1 Technical auditing (TA) and Quality assurance (QA) ........................... 17 3.2 Nature of in situ stress.......................................................................... 19 3.3 Assumptions......................................................................................... 20 3.4 Prior to overcoring ................................................................................ 23 3.5 During the overcoring ........................................................................... 24 3.6 Right after overcoring ........................................................................... 26 3.7 Biaxial testing ....................................................................................... 26 3.8 Core mapping....................................................................................... 26 3.9 Interpretation and reporting .................................................................. 27
4 OC-QUALITY CONTROL TOOL ................................................................ 33 5 SENSITIVITY STUDIES............................................................................. 49
5.1 Effect of loading conditions and material parameter values ................. 50 5.2 Effect of heterogeneity, anisotropy and pilot hole geometry ................. 53 5-5. Sensitivity of inverse solution ............................................................... 54
6 CASE STUDIES ......................................................................................... 57
6.1 Äspö KK0045G01 stress measurements.............................................. 58 6.2 Äspö KF0093A01 stress measurements .............................................. 75 6.3 AECL URL, Hole 209-022-0C2............................................................. 87
7 DISCUSSION AND RECOMMENDATIONS .............................................. 93 8 CONCLUSIONS ....................................................................................... 101 REFERENCES............................................................................................... 103 APPENDIX – 1. FLAC DATA FOR HOEK-CELL MODEL .............................. 105
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LIST OF SYMBOLS Roman Letters bearingσi bearing angle of principal stress i=1, 2 or 3 from north (degrees) dd dip direction angle from north (degrees) dip dip angle from horizontal (degrees) dipσi dip angle of principal stress i=1,2or 3 from horizontal (degrees) E Young’s modulus (Pa) mi intact rock parameter in Hoek & Brown failure criterion () si intact rock parameter in Hoek & Brown failure criterion () SH major horizontal in situ stress (Pa) Sh minor horizontal in situ stress (Pa) Sv vertical in situ stress (Pa) x,y,z Cartesian axes N,E,V Cartesian axes aligned with North, East and Vertical r,φ,z Polar axes A,B Strain gauge axes, two dimensional PA Axial pressure below drill bit (Pa) PS Shear stress at drill bit rock interface (Pa) WP Borehole fluid pressure (Pa) Greek Letters ε strain (m/m) σ1,2,3 major, intermediate and minor principal stress (Pa) σ stress tensor σs secondary stress tensor σij stress tensor component acting in the j-direction on a plane normal to the
i-axis (Pa), if i=j then notation σi instead of σij is used, σH major horizontal in situ stress (Pa) σh minor horizontal in situ stress (Pa) σv vertical in situ stress (Pa) σucs uniaxial compressive strength (Pa) σci crack initiation stress (Pa) σcd crack damage stress (Pa)
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σt tensile strength (Pa) ν Poisson’s ratio ( )
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PREFACE
The initial idea for this work was introduced by Derek Martin (Department of Civil Engineering, University of Alberta, Edmonton, Alberta, Canada) and Rolf Christianson (Svensk Kärnbränslehantering AB). The final implementation was further developed by Matti Hakala (Gridpoint Finland Oy), Rolf Christianson and Jonny Sjöberg (SwedPower AB) This study was carried out under contract to Posiva Oy which has co-operation agreement with SKB. Posiva’s contact person was Heikki Hinkkanen. The author, Matti Hakala, has done major part of the work with professional help of John A. Hudson (Rock Engineering Consultants) and Erik Johansson (Saanio & Riekkola Oy). The report, especially the Chapter 3.2, includes many authors’ previously non-reported experiences of in situ stress measurements and interpretation of test results. The primary product of this project was a computer code. The code is not commercially available but it can be asked from the owners Posiva Oy or Gridpoint Finland Oy.
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1 INTRODUCTION Background The in situ state of stress is one of the rock mechanical key factors affecting to the safety and stability of openings in greater depth like facilities for disposal of spent nuclear fuel. At the same time measuring of in situ state of stress becomes more difficult and vulnerable with increasing depth and/or stress. In deep boreholes, the three dimensional stress tensor can be measured at one point using overcoring method or interpreted for larger volume by reopening of existing fractures (HTPF-method). Both Posiva in Finland and SKB in Sweden have carried out overcoring in situ stress measurements where uncertainties concerning mainly the orientation but also the magnitude have showed up. In crystalline rock unsatisfactory results can be the identification of disturbed measurements but also the anisotropy or heterogeneity of rock. Disturbing factors can be any stress or strain induced damage of pilot hole wall or core, unacceptable behaviour of glue or glue contact or change in temperature. This study was set on by Posiva and SKB in order to develop the quality of overcoring stress measurement interpretation methods. Objectives The initial idea was to interprete in situ state of stress based on early strains, when the core damage potential is low. A suitable approach for this solution was introduced by Fouial et al 1998. During the pretesting phase it was found out that the early strain solution will be very sensitive and therefore the project was focused to verify that measured transient strains correspond the interpreted in situ state of stress and estimate core damage potential based on transient stresses at strain gauge location. The primary product of the project is a computer program which can simulate the transient strains and stresses during the overcoring in any in situ stress and coring load conditions. The measured strains can be compared to calculated ones to check if the measured transient behaviour coinsides with interpreted in situ state of stress. If not, the in situ state of stress can be calculated based any transient or final strain values. The transient stresses can be compared to strength envelope of intact rock and thereby estimate core damage potential. With developed code a sensitivity study for selected overcoring case is done and five in situ stress measurement cases are analysed. Further, a list of factors to be considered during the overcoring in situ stress measurement is presented. Assumptions To be a practical quality control tool the calculations of transient strains and stresses for any in situ state of stress have to be done in relative short time. For this reason the solution is based on superposing of precalculated secondary stresses caused by each relevant primary load component. This approach assumes that the rock is continuous, homegeneous and linear elastic. Further the rock is assumed to be isotropic although the solution can also be applied for known anisotropy. The precalculation of secondary
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stresses presumes that overcoring geometry and Poisson’s ratio is known. For this project the Borre Probe (SwedPower’s Leeman-Hiltscher probe) geometry was selected; pilot hole diameter is 36 mm and the inner and outer overcoring diameters are 62 mm and 76 mm. The precalculation was done with three different Poisson’s ration values 0.15, 0.25 and 0.35, and any other value is interpolated or extrapolated. Similar superposition solutions have been previously used in core damage studies by Li (1997) and Hakala (1999). Allthoug the precalculations are done for Borre Probe geometry the same computer code can be applied to any overcoring method or probe requiring basically that the secondary stress tensors are recalculated for this geometry. If the coring dimensions are the same as for Borre probe the current code can be used for user defined gauge positions and orientations. Only limitation is that inclined strain gauges have to be in 45° or –45° angle relative to the borehole axis
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2 OVERCORING METHOD Probes for 3D-measurement The overcoring methods applicable to define three dimensional in situ stress field with single measurement can be divided in two categories based on the strain gauge position. In first group the strain gauges are glued on approximately 50 cm long pilot hole wall and the hole pilot is overcored (Borre Probe, CSIRO-HI cell, CSIR-cell, modified CSIR-cells and ANZI-cells). Table 2.1 summarizes the dimensions and the most important properties of commonly used probes and in Figure 2-1 shows the position of strain gauges with full strain patterns around pilot holes in one reference case. In second group the strain gauges are glued on conical or spherical cutting in the flattened bottom of drill hole and only short overcoring, less than 100 mm, is needed. More detailed presentations of different probes are given in a book by Amadei and Stephansson (1997). Table 2-1. Properties of common 3D overcoring probes. The core diameter is smallest normally used value. Probe name diameters strain gauges gluing monitoring φ pilot φ core position / orientation Rosettes/Probe Logger (L)/Wires (W) (mm) (mm) (degree) depth, (m)
Borre probe 36 > 62 0/0, 0/90, 0/45 R L, 1000 120/0, 120/90, 120,45 240/0, 240/90, 240,45
CSIRO-HI 38 > 62 323/0, 300/90, 300/45 P W, 30 163/45, 163/135, 180/45 83/0, 60/90, 60/45 300/135, 210/90, 90/90
CSIR 38 > 62 0/0, 0/90 0/45, 0/135 R L, 150 120/0, 120/90 120/45, 120/135 W, 30 240/0, 240/90 240/45, 240/135
ANZI 56 > 82 0/0, 0/90, 0/45 P W, 50 29 > 52 60/45, 60/90, 60/135 120/0, 120/90, 120/45 180/45, 180/90, 180/135 240/0, 240/90, 240/45 300/45, 300/90, 300/135
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s1
Principal stresses relative to Äspö x-axis -120°:s1 = 26.1 MPa, dip 39.2°, dd 27°s2 = 17.3 MPa, dip 28.5°, dd 271°s3 = 8.9 MPa, dip 37.7°, dd 156°
Figure 2-1. Angular plot of strain gauge orientations and strain magnitudes (signals) for different 3D-stress measurement probes. Continuous lines are strain patterns around pilot hole for axial, circumferential, 45 and 135 degrees inclined strain gauges in Äspö KK0045G01 Level 2:2 measurement case.
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Basic assumptions and interpretation In pilot hole probe methods nine or twelve strain gauges with different orientations on pilot hole wall are monitored when the probe is released by overcoring from the secondary stress state around an ‘infinite’ pilot hole (Figure 4-1). Beacause the stress state around circular hole is known in closed form the in situ state of stress can then be calculated from the released strains assuming that the rock is continuous, homogeneous and behaves linear elastic around the circular pilot hole (Leeman 1970). The material have to be either isotropic, transversely isotropic or orthotropic. Further, it is assumed that the bottom of the pilot hole or full scale hole does not have affect on the secondary stress state at probe level before overcoring. The calculation of in situ stress requires at least six independent final strain readings and the elastic parameters of rock. For isotropic rock the elastic parameter values are defined by biaxial testing of overcored rock cylinder but for transversely isotropy or orthotropy at least three uniaxial or indirect tensile test specimens with different angles of anisotropy are needed (Amadei, 1996 and Chen et al. 1998). The success of measurement is defined by the continuity and stability of strains gauge readings during the overcoring and linearity during the biaxial testing. However the judgement of continuity and stability is quite subjective. The loading and unloading cycle in biaxial cell gives information on linear elasticity and the comparison of different axial, circumferential or inclined strain gauge readings gives information on the degree of anisotropy or heterogeneity. In bottom hole probe methods the basic assumptions are alike with the exception that the bottom hole secondary stress state can not be solved in closed form but instead factors based on numerical analyses are used to interpret the in situ state of stress. Procedure for overcoring in situ stress measurement A list of main phases of in situ stress measurement was build up in order to have possibilities to perform a successful overcoring in situ stress measurement in hard crystalline rock conditions (Figure 2-2). The list is based on the International Society for Rock Mechanics (ISRM) (1987) suggestions and complemented with experiences by the author. A detailed discussion of these phases is presented later in this report. Figure 2-3 shows the evolution of measured strains and cell temperature from cell installation (phase 8) to end of biaxial testing (phase 12): 1) Site characterisation
- collect information on jointing, fracturing, intact rock strength and estimate orientation and maximum magnitude of in situ stress.
2) Estimation of the potential for successful measurement.
- Probability to have measuring point fulfilling the continuity, homogeneity and linear elasticity
3) Drill hole position and orientation.
- What is the most potential orientation when taking account jointing and the assumed in situ state of stress.
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4) Drilling of full size hole to measurement depth.
5) Starting level for measurements.
- The starting point must be far enough from known disturbing factors (caverns, fracture zones etc).
6) Pilot drilling.
7) Acceptance of pilot hole.
- Does the cell installation point fulfil the requirements for continuity, homogeneity and linear elasticity.
8) Cell installation.
- Proper glue for present temperature and water conditions - cell orientation.
9) Hardening of glue and cell monitoring.
- To be sure that glue is hardened before measurement,online monitoring.
10) Overcoring and cell monitoring.
- Monitoring advance, temperature and strains during the overcoring and having stable in situ temperature and strain readings before breaking the core.
11) Stabilize the cell.
- Wait before breaking the core that cell temperature and monitored readings are stabilized to get real final released strain values.
12) Biaxial testing.
- Right after overcoring at in situ temperature to minimize errors.
13) Geological logging of core.
- Check continuity and homogeneity.
14) Biaxial test interpretation and definition of elastic parameters.
- Check the linear elastisity and isotropy or degree of anisotropy, in case of anisotropy different interpretation method or additional laboratory tests of sub cores are needed to define elastic parameters.
15) Check the transient strain response and definition of strain values used for stress calculation.
- Continuous development of transient strains and stable final readings; the code developed in this work is designed for this phase.
16) Stress calculation, sensitivity study and estimation of errors and validity.
- The code developed in this work can be used to calculate stress based on early strains for each measurement.
17) Guidelines for further use.
- Conclude all in situ stress measurements from the area with geology and give suggestions which values to use in further design work.
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1) site characterization
2) estimation of potential for suggesfull measurement
3) drill hole position and orientation
4) drilling of full size hole
5) starting level for measurement
6) pilot drilling
7) acceptance of pilot hole
8) cell installation
9) hardening of glue and cell monitoring
10) overcoring
11) stabilizing the cell, breaking core
12) biaxial testing
13) geological logging of core
14) biaxial test interpretation
15) transient strain check
16) stress calculation, sensitivity study
17) guidelines for further use
Site
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Figure 2-2. Flowchart for overcoring in situ stress measurements The quality control code developed in this work is applicable for phases 15 and 16 but this report gives also suggestions for phases 1-3,5,7 and 9-14.
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Figure 2-3. The evolution of measured strains and cell temperature from cell installation to end of biaxial testing. Total time for presented CSIRI-HI cell measurement was approximately 40 hours (A,B,C,D,E,F are identifiers for different rosettes and value is the orientation of strain gauge i.e. 0 is for axial strain gauge and 90 for tangential). Result variation It must be remembered that measured in situ stress is a point tensor and the defined value has uncertainties related to geological factors, accuracy of each measurement method and data analyses. Therefore it is important to understand and control all known error sources. Based on Amadei & Stephansson(1997) up to 20% scatter in magnitude and +-20 degree scatter in orientations must be accepted when having a serie of stress measurements in typical rock conditions. Word Stress Map, the global database for contemporary tectonic stress data from the Earth's crust, divides overcoring in situ stress measurement results to five categories (Muller et al., 2000) (Table 2-2): Neither of the two estimates does cover systematic human errors or mistakes.
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Table 2-2. Five categories to classify overcoring in situ stress measurement results (Muller et al., 2000) A-Quality - Average of consistent (std. dev. <=12°) measurements in two or more boreholes
extending more than two excavation radii from the excavation wall and far from any known local disturbances, depth >300m.
B-Quality - Multiple consistent (std. dev. <20°) measurements in one or more boreholes extending
more than two excavation radii from excavation well, depth >100m. C-Quality - Average of multiple measurements made near surface (depth >5-10m) at two or more
localities in close proximity with std. dev. <=25. - Multiple measurements at depth >100m with 20< std. dev. <25°. D-Quality - All near surface measurements with std. dev. >15°, depth <5m. - All single measurements at depth. - Multiple measurements at depth with std. dev. >25°. E-Quality - Multiple measurements at a single site or locality with no significant mean
(std. dev. >40°).
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3 FACTORS TO BE CONSIDERED IN OVERCORING STRESS MEASUREMENT In the following the technical auditing and quality assurance procedure for overcoring in situ stress measurement developed by SKB are presented (Christianson & Hudson, 2002). Further, the nature of in situ stress is shortly presented and a detailed ideal list of different factors affecting to in situ stress measurement and interpretation is given. 3.1 Technical auditing (TA) and Quality assurance (QA) SKB have thought that combined implementation of TA and QA will significantly improve site investigation procedures, both specifically for stress measurements and in general for all measurements in the forthcoming site investigations. There are two basic types of Technical Auditing: - Soft audit for establishing the features of the site investigation measurement or modeling method and whether the right approach has been adopted in principle. - Hard audit for detailed analysis of the site investigation measurement or modeling method to establish the relevant variables, mechanisms and parameters, whether the modeling is relevant, and whether the design is appropriate and robust. The soft audit is especially helpful in considering the best approach to a procedure but, because of the multitude of specific factors potentially influencing the stress measurements and the need to ensure that the numerical values are correct, it is necessary to use the hard audit for rock stress measurements. The auditing steps occur from the decision to measure the rock stress right through to the presentation of the final information (Table 3.1). In developing the auditing procedure, the audit subject areas are identified first, together with the fundamental questions associated with each subject area.
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Table 3-1. Components of Technical Auditing (TA) strategic checklist for SKB in situ rock stress measurements (Christianson & Hudson, 2002)
Technical Auditing Component Checklist
Subject Area A: Stress Measurement Objective and Background
1. STATEMENT OF THE MEASUREMENT OBJECTIVE • What is the purpose of the measurements? • What is the accuracy expected? • What confirmatory procedures are to be adopted? 2. STATEMENT OF THE STRESS MEASUREMENT BACKGROUND • Have the problems with in situ rock stress measurements been identified? • Has a list of the problems been made? • Have the best literature references been identified and studied? • Has the project been discussed with someone who has practical experience of measuring stresses,
and with the specific method to be used?
Subject Area B: Stress Measurement Method
3. SPECIFICATION OF THE STRESS MEASUREMENT METHOD • What stress measurement method is to be used?
• What are the physical processes involved? • What influence might site conditions have on the results from the method to be used?
• What problems have been identified in the past. 4. CONFIRMATION OF METHOD ADEQUACY • Given the statements on the Audit Sheets produced so far, is the stress measurement capable of
measuring the required rock stress? 5. AVAILABILTY OF A QA PROCEDURE • Is a QA procedure available for the stress measurement method? • If so, has the QA procedure been checked — for both theoretical and practical experience aspects
— to ensure that it is adequate, given the objective and the known problems with stress measurements?
• Is the existing QA procedure adequate? • If a suitable QA procedure is not available, can an adequate one be generated? 6. STRESS MEASUREMENT PROTOCOL • Is a protocol being developed for the use of the stress measurement method that incorporates the
TA and QA aspects?
Subject Area C: Stress Data Reduction, Interpretation, Validation and Presentation
7. DATA RECORDING, RELIABILITY AND REDUCTION • What procedures are in place to ensure that the data will be recorded accurately and safely? • Have all the hazards with stress measurements (see Audit Subject Areas A & B) been addressed? • What procedures are in place to ensure that the raw data obtained are reliable? • How will the data be reduced? • What procedures are in place to ensure that mistakes will not occur during data reduction? • Is there a protocol with a case example available for this subject area, 7? 8. DATA INTERPRETATION • How are the data to be interpreted and the trends identified? 9. DATA VALIDATION • Are results compatible with existing relevant data and trends at the site? • Are the site conditions within the assumptions for the method used? • Are the determined elastic properties of the rock realistic?
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Table 3-1 continued. Components of Technical Auditing (TA) strategic checklist for SKB in situ rock stress measurements (Christianson & Hudson, 2002) 10. PRESENTATION OF STRESS MEASUREMENT RESULTS • How are the stress measurements to be presented in a clear form?
Subject Area D: Technical Auditing Conclusions
11. STRESS MEASUREMENT ADEQUACY • Have the stress measurements been conducted adequately — given the objective (Audit Subject
Area 1) and the existing scientific, practical and site knowledge. • Is the documentation of the quality control during measurement, data reduction and data
interpretation reliable? 12. OVERALL TECHNICAL AUDITING STATEMENT • What are the overall TA conclusions given the individual conclusions in Items 1-11 above? • What recommendation are to be made concerning the work? The TA audit sheets should be completed before the measurements are to be conducted, initially by the Client, and then confirmed by the stress measurement Contractor. The work itself can then be guided by the QA, although the QA should be modified as necessary to include all the elements of the TA. The Quality Assurance procedures applied to the measurements will be implemented in line with the results of the TA analysis and the QA arrangements in place. It is valuable to develop a process to identify critical factors influencing the quality of the services to be carried out. For example, the systematic use of checklists is a measure to enable traceability. This procedure will enable a continuous line of logic to be documented from the original concept of the required measurements right through to the final storage and presentation of results. There is a good chance that the appropriate QA procedures would have detected and eliminated at an early stage the stress data reduction problem described in this report, but the combined TA/QA approach is much more secure. Of course, it is not possible to be 100% sure of eliminating all ill-founded concepts and incorrect measurements or data reduction, but the proposed methodology will significantly reduce the number of errors made in future measurements. 3.2 Nature of in situ stress The in situ state of stress is a three dimensional tensor quantity, which can be defined by six tensor components σx, σy, σz, σxy, σyz, σzx (x,y and z axis forms an orthogonal coordinate system) or three principal stresses and their orientations (σ1, bearingσ1, dipσ1, σ2, bearingσ2, dipσ2, σ3, bearingσ3, dipσ3) (Figure 3-1). The quite often used form of the maximum horizontal stress (σH) minimum horizontal stress (σh) and vertical stress (σV) is therefore a simplification ignoring the tensor components σyz and σzx (assuming that z is vertical axis). It must also be kept in mind that because of tensor nature the magnitudes and orientations of principal stresses are connected and therefore calculation of average value must be based on averages of six tensor components (Hudson & Harrison, 1997 p. 53).
20
a)
b) Figure 3-1. a) Illustration of the development of two shear forces on each face of an infidecimal cube and b) stress tensor components (Hudson & Harrison , 2000) 3.3 Assumptions In selecting the drill hole position, orientation and measuring points , the following must be considered:
- In situ stress measurement assumes that rock around the measurement point is Continuous, Homogeneous and Isotropic and behaves Linearly Elastic (CHILE), but solution for transversely isotropic rock exist also.
- the stress continuity requires that the probe is installed to the position where secondary stresses are the same as around infinite circular hole. This involves that the bottom of larger diameter hole nor the bottom of pilot hole does not affect to stress state at strain gauge location. The error is minor if the distance from both bottom's to strain gauge position is at least five times the radius of holes (Figure 3-2).
21
- if the overcoring is done with 76 mm diameter equipment and pilot hole with 36 mm drill pit then the continuity assumes at least 285 mm (5* 76 mm /2 + 5* 38 mm /2) but preferably 570 mm intact pilot (Figure 3-2). In perpendicular direction at least 115 mm (6 * 38 mm / 2) but more likely 210 mm intact rock radius is needed. The continuity perpendicular to bore hole can be estimated only by the orientation of joints or fractures mapped from core samples before the measuring point. Further, the stress calculation assumes constant boundary stress around this preferred intact rock block of 0.57 m * 0.42 m * 0.42 m, which can easily be violated (Figure 3-3).
5R - 10R
R r
Acceptable straingauge position
Minimum pilot hole length is 5r + 5R,but preferably 10r + 10R
6r - 11r
Acceptable straingauge position
Minimum intactrock radius is 6r, but
preferably 11r
5r - 10rProbe
Figure 3-2. Definition of minimum continuous homogeneous volume for overcoring.
Pilot hole
Minumum volume of continuous homogeneous rock for overcoring
Probe
Joints
Principal stress vectors
Figure 3-3. In situ stress calculation assumes constant boundary stress around the minimum continuous homogeneous volume.
22
- The homogeneous and isotropic nature of rock within this 0.1 m3 block can be judged based on geological inspection of pilot hole core and cores close to the measuring point.
- Kim &Franklin (1987) suggest that the strain gauge length should be at least 10 mm and Irwin et al. (1987) et al says that performance of CSIR-cell is reliable if strain gauge length is over the grain size. For laboratory testing ISRM (1996) suggested that length of strain gauge should be ten times the average grain size. It must be also remembered that longer strain gauge will average more strains around circular borehole.
- The geological mapping includes rock type, foliation, mineral size, alteration, weathering, joint positions, dip and dip direction of joints, and joint surface parameters JRC and Ja.
- The linear elastic behaviour of rock can be tested before the stress measurement but the possible damage of rock at measuring point is hard to predict because it involves information of the stress, which is going to be measured. Any direct or indirect information of microcracking on core surface is an evidence of non-linear and inelastic behaviour. Core disking or borehole breakout is a clear evidence of unacceptable conditions for stress measurement. Stress failures around measurement point can be minimised by aligning the borehole parallel to major principal stress. In case of anisotropic rock the effect of anisotropy can be minimized by aligning the borehole perpendicular to foliation, but if the plane of foliation is a plane of weakness then this improves pilot hole stability but increases risk for core disking. In Scandinavian stress conditions the core damage initiates when in situ stress perpendicular to borehole is over two times the tensile strength of rock. Based on Kirsch equations the pilot hole breakout is probable if in situ stress perpendicular to borehole axis is over 30% of the uniaxial compressive strength of intact rock (around circular hole the maximum tangential stress is 2…3 times the maximum in situ stress). Normally the linearly elastic behaviour is tested with biaxial cell after the stress measurement.
- If the rock exhibits time dependent behaviour or if the strength of rock is exceeded, part of the stresses around pilot hole will be relaxed prior to stress measurement. Time dependency of rock can be tested by laboratory tests and the stress failure can be studied by visual core damage study, microscopic study or p-wave measurement. Indications of these both can be seen in biaxial test results. Although it is possible to define these error courses the effect on measured stress state is hard to define.
- The in situ stress tends to align with geological structures i.e. fracture zones, joints, bedding / foliation.
23
3.4 Prior to overcoring Before the start of actual overcoring stress measurement following should be considered and checked:
- Checking the stability/calibration of strain reading equipment. This can be done with two sequential measurements of precision resistor.
- Checking the compass or other orientation measurement device of probe.
- Cleaning of pilot hole wall by flushing and/or mechanically.
- Temperature of rock, the amount and quality of leaking water and the needed pot life affects to selected glue. In high humidity environments exothermic epoxy resin surfaces do not set well (Irwin et al., 1987). It is noted that in low temperatures the hardening/curing time of the glue can be considerably longer than given by the manufacturer or glue softens again if temperature increases (Irwin et al., 1987).
- The thickness of glue layer affects to hardening time (Martino et al., 1997). Thicker glue layer leads to longer hardening time.
- The onset of hardening process of glue can be delayed considerably if the temperature is low. The onset of curing can be confirmed by doing the mixing of glue in advantageous temperature.
- The success of gluing and properties of glue affects to possible debonding, time dependent strain in glue and if the elastic parameters of rock-glue-gauge system are the same during the overcoring and biaxial testing.
- Gluing can be tested by a) doing a reference gluing on drilling site and let it harden in the same conditions as in borehole, and b) monitoring the strain response during the hardening time (not possible with all gauge types and limited to short boreholes). The glue hardening is normally exothermic reaction so, temperature increase and equal strains on all axial, tangential and similarly inclined gauges is assumed before the readings stabilise and glue is hardened (Figure 3-4).
- If possible, the strain stability should be checked before overcoring. It is not reasonable to start overcoring if the strain travelling within the time needed for overcoring is remarkable compared to strains to be measured (measured strains are normally 100 to 1000 microstrains and the overcoring takes 15 min to one hour). Acceptable strain drift (average trend) could thereby be about 10 microstrains per 15 min.
24
-400
-200
0
200
400
600
800
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0
Time ( h )
Mic
roSt
rain
( m
m/m
m )
0
2
4
6
8
10
12
Tem
pera
ture
( °C
)
A0
A90
A45
B45
B135
B90
C0
C90
C45
C135
E90
F90
cell temp.
Figure 3-4. An example of measured strains during the hardening of glue of CSIRO-HI cell and the cell temperature (A,B,C,D,E,F are identifiers for different rosettes and value after that is the orientation of strain gauge i.e. 0 is for axial strain gauge and 90 for tangential). Hardening time suggested by glue manufacturer was 16 h. 3.5 During the overcoring During the actual overcoring phase following shoud be considered:
- The coring advance rate is normally defined by drilling conditions but the strain sampling rate must be considered also. It is suggested that reading interval is less that 8% of hollow core diameter (8% * 62 mm is 5 mm). Further the drill bit position for each strain measurement should be defined in accuracy of +-2% of the hollow core diameter (+- 2% * 62 mm is +-1.2 mm).
- Based on study by Cai and Thomas (1993) temperature change can have several effects on overcoring stress measurement. Temperature change in lead wires can introduce tens of microns per one degree. This can be compensated by using three wire configuration with Wheatstone bridge. Also temperature has an effect on strain gauge resistance. Most complicated are the strains caused by thermal expansion of rock. The temperature change is caused by friction between drill bit and rock and by uncontrolled flushing water. Both temperature loads are changing with advancing coring and are complicated to eliminate.
- In tests done at Underground Research Laboratory (URL) it has been reported that 15 degrees change in temperature produces 22%-59% change in measured strains (Martino et al., 1997). It was suggested that the temperature change should be less
25
than 2 degrees. The drill bit friction increases the rock temperature therefore the flushing water should be controlled and about 2 degrees below the rock temperature. Laboratory tests done by Cai and Thomas (1993) showed that the measured strains by thermal expansion of rock core are –19 µε/°C to 75 µε/°C depending on probe used and rock type. The measurements were done using three wire quarter bridge which eliminates the thermal effect on lead wires.
- If the temperature change can not be controlled it is suggested to take stable initial readings in stable temperature without and with flushing on. Corresponding final stable readings are taken in the same stable temperature after overcoring with flushing on and off. Thus we get two strain differences for each strain gauge, which should be close to each other if the measurement and all assumptions are fulfilled.
- The drilling loads (trust force and torsion) and water level in borehole is suggested to be monitored if possible. They don’t affect to final readings, but they have affect on transient strains and they may cause or trigger core damage.
-200
0
200
400
600
0 10 20 30 40 50 60 70 80 90 100
OC-advance ( cm ), cell position = 29 cm
Mic
rost
rain
10
12
14
16
18
Tem
pera
ture
( de
gree
s )
A0 A90
A45 B45
B135 B90
C0 C90
C45 D135
E90 F90
Temp.
OC - stopped, core break at bottom level of pilot holeCell position
Figure 3-5. Effect of rock temperature change to measured strains during and after overcoring of CSIRO-HI cell (A,B,C,D,E,F are identifiers for different rosettes and value is the orientation of strain gauge i.e. 0 is for axial strain gauge and 90 for tangential).
OC-advance in cm before OC-stop and in minutes after that
26
3.6 Right after overcoring Right after overcoring following is suggested:
- To take final stable strain readings in stable temperature with flushing on and off. The temperature should be the same as before overcoring. This might involve that the barrels are left to borehole for an hour or two.
- Checking of core, cables and that the pilot hole is in the centre of hollow core, which is assumed in the calculation of in situ state of stress.
- Checking the stability/calibration of strain reading equipment, this can be done with two sequential measurements of precision resistor.
3.7 Biaxial testing Related to biaxial testing following should be considered:
- The biaxial testing is suggested to be done right after overcoring since this will minimize the effects of all time dependent behaviours in rock or glue.
- The temperature of core sample and biaxial cell should be close to in situ temperature to minimize thermomechanical effects.
- If the tested sample is clearly longer than the biaxial cell considerably tension is induced to the core right after cell. In case of 38 mm/62 mm diameter hollow core the tension can be up to 50% and in case of 38 mm / 92 mm core 35% of applied pressure. Values are based on axisymmetric FLAC simulation where core length is twice the cell length (Appendix 1).
- Loading and unloading is normally done incrementally and the highest load is suggested to be held about 15 min (two readings), so the linear elastisity and time dependency can be studied.
- The applied cell pressure don’t have to be close to in situ stress to get strain response from corresponding strain range because biaxial loading produces much higher strains under lower pressure level.
3.8 Core mapping Mapping of the core around the vicinity of in situ stress measurement location indicates the possible disturbing factors which is valuable information for interpretation of in situ state of stress:
- The geological mapping includes rock type, foliation, main minerals, mineral size, alteration, weathering and mapping of possible joint surfaces including position, dip, dip direction and joint surface parameters JRC and Ja.
- Visual and possibly measured indicates of core damage (micro fractures, fractures, core disking and possibly p-wave velocity in different orientations also).
27
- Photographs around the outer and inner surface or cross sections of hollow core.
- Core can be cut to inspect inner surface and to study gluing. 3.9 Interpretation and reporting Interpretation of in situ state of stress is based mainly on overcoring stress measurement data and biaxial loading data but beside this all supporting site data, borehole data and notes and measurements during the whole measurement procedure should be available. Following lists information that is should found from the report and principles for interpretation. General
- Reporting should include map with drill hole position and orientation, measuring points, magnetic north, possible local co-ordinate system, excavations near the measuring point and their dimensions, core logging data, all measured raw data, all exceptions from suggested methods, declaration of used methods, results, source of errors and applicability of results and all general information from investigation (i.e. investigation dates, contractor name, used equipments, etc.).
Biaxial testing
- Strain difference between end of coring and beginning of biaxial testing gives information on time dependency of glue-rock system. If the difference is big, it is not clear if the deformation parameters defined in biaxial test are the same as during the overcoring.
- Young’s modulus and Poisson’s ratio are defined from biaxial test results. In case of homogeneous and isotropic rock all a) axial, b) tangential and c) 45/135 degrees inclined strain gauges should have almost identical linear reversal behaviour (Figure 3-6). Further the strain should be constant during the hold at the maximum pressure. Non-linearity refers to non-linear behaviour of rock, core damage or improper gluing. Permanent strain refers to core damage or improper gluing. Time dependent behaviour can be related to rock or glue. Different response within gauge group is related to non-homogeneity, anisotropy, core damage or improper gluing. Homogeneity and/or isotropy of rock can be tested by calculating Young’s modulus and Poisson’s ratio of each rosette or b) calculating the response of inclined gauges from axial and tangential gauges in each rosette:
28
-1000
-800
-600
-400
-200
0
200
400
0 2 4 6 8 10 12 14 16 18
Cell pressure ( MPa )
Mic
rost
rain
A0
A90
A45
B45
B135
B90
C0
C90
C45
C135
E90
F90
Figure 3-6. Biaxial test strain response of CSIRO-HI cell gauges in case of homogeneous linearly elastic rock (A,B,C,D,E,F are identifiers for different rosettes and value is the orientation of strain gauge i.e. 0 is for axial strain gauge and 90 for tangential). For this rock-glue-probe system the defined Young’s modulus is 59 GPa, Poisson-ratio 0.32 and average homogeneity test difference 5%.
)(2
2min
2max
2max
tan RRRPE−∆
∆=
ε (3-1)
tanεεν
∆∆
−= axi
(3-2)
ε45,135 = (εa + εtan) / 2 (3-3) and further:
err = 100% (ε45,135_measured – ε45,135(εa,εtan)) / ε45,135_measured (3-4)
If the difference of deformation parameters of each rosette is less than 30% the rock can be considered homogeneous and isotropic (Eitzenberger 2002), same can be assumed for the error of inclined strain gauges.. If visual observations or biaxial results indicates anisotropy, the parameters can be defined at minimum with three
29
uniaxial or indirect tensile tests to apply anisotropic solution for in situ state of stress (Amadei, 1996). In case of heterogenous rock overcoring strains can be compensated based on biaxial response (Cai and Thomas 1993).
- The Young’s modulus and Poisson’s ratio are defined from unloading phase which simulates better the overcoring of rock (to zero pressure which is the situation in overcoring also). It is suggested to base the calculation on slopes of linear fits and the offset strain of linear fit at zero pressure can be used as measurement of deviation/successful.
Overcoring
- It is suggested that before any further actions the whole strain history from cell installation to the end of biaxial testing is plotted to get an general idea of; a) the stability of measurement, b) fulfillment of CHILE conditions, c) effect of temperature and d) error considerations. It is reasonable to reset all measured to zero value at the time of start of overcoring (Figure 2-3).
- In order to define the strains for stress calculation the stresses are plotted as a function of coring advance and they are resetted to zero at the point of zero advance (Figure 3-7). The strains readings should be constant in the beginning and in the end of overcoring to fulfill the minimum distance requirements (assumption of stress state corresponding an infinite hole at gauge position). In case of 38 mm diameter pilot and 76 mm diameter overcoring most of the strains should take place in coring advance range 100 mm before to 75 mm after the strain gauge position. The strains should develop smoothly with advancing coring with following phases; a) stable reading, b) local maximum or minimum before strain gauge position, c) local maximum or minimum after strain gauge position and d) final stable value. Further all axial strain gauges should give the same final value. Regardless of in situ state of stress all exceptions from previous mentioned behavior are indicates of; temperature change, core damage, debonding of gauges, geological anomaly, time dependent behavior of rock or glue or malfunctioning in strain reading unit.
- The calculation of in situ state of stress is based on the strain difference after and before overcoring. In before overcoring state; 1a) CHILE conditions should have been valid from the in situ state (before pilot hole drilling) and 1b) the strain gauges should have been in position corresponding infinite borehole. The strain values after overcoring should represent state in which; 2a) all stresses are totally released from that part of the core where strain gauges are 2b) the CHILE conditions have been valid during the overcoring and 2c) the temperature of the core is the same as before overcoring.
- Experinces from URL have showed that residual stresses of overcored rock cylinder are normally minor compared to measured ones (Martino, 1997).
- At the moment there are no practical methods to use transient strain values to define the success of measurement or use those in the interpretation of the in situ state of stress.
30
-200
-100
0
100
200
300
400
500
600
700
-200 -150 -100 -50 0 50 100 150Coring advance ( mm )
µStra
in (
)
G1 ( R1_axi )G2 ( R1_tan )G3 ( R1_incl. )G4 ( R2_axi )G5 ( R2_tan )G6 ( R2_incl. )G7 ( R3_axi )G8 ( R3_tan )G9 ( R3_incl. )G1m ( R1_axi )G3m ( R1_incl. )G2m ( R1_tan )G4m ( R2_axi )G6m ( R2_incl. )G5m ( R2_tan )G7m ( R3_axi )G9m ( R3_incl. )G8m ( R3_tan )
a. b. c. d.
Figure 3-7. The measured and calculated transient strains for Borre probe measurement. Following phases can be found in strain behavior: a) stable value, b) local maximum or minimum before strain gauge position, c) local maximum or minimum after strain gauge position and d) final stable value. The strain gauge position is equal to zero coring advance.
- No measured values should be ignored if no clear evidences of erroneous reading can be defined.
- Reliability for the measurement can be calculated based on the correlation or cumulative difference between the strains used for stress calculation and the strains back calculated from defined in situ state of stress.
- If several measurements are done in the same geological region the mean principal stresses, magnitudes and orientations, can be calculated based on mean values for stress tensor components σx, σy, σz, σxy, σyz, σzx
- An essential part of in situ stress measurement report is the; a) interpretation in respect of local and regional geology, b) estimation of success, c) error sources and the error level and d) suggested values for future use.
- Cai and Thomas (1993) compared deformation parameter determined by biaxial testing of different rock types and probes with uniaxial test values. The results showed that Young’s modulus values were very similar but Poisson’s ratio values were quite different. However, Poisson’s ratio doesn’t have much influence on stress calculation.
31
- Cai and Thomas (1993) compared in laboratory conditions the resulting in situ stress values determined with CSIR, CSIRO, UNSW and USBM cells. The results by CSIR and CRIRO cells showed that for homogeneous material the error is less than 10 %. For real rocks the error is –23% to +19%. The CSIRO-cell has somewhat better performance in heterogeneous materials but none of the cells worked well for thinly bedded conditions.
32
33
4 OC-QUALITY CONTROL TOOL Description The major product of this work is a computer code to simulate transient strains developed during overcoring. The code assumes continuous, homogeneous, isotropic and linear elastic behavior of rock. The loading condition can be defined freely as any combination of six independent in situ stress tensor components and three overcoring loads. Major limitation is the lack of thermal loads. At the moment the code is limited to Borre probe geometry, but it can be modified to any overcoring probe (Hallbjörn et al. 1990). In Borre probe the pilot hole is 36 mm diameter and the overcoring is done with 76 mm diameter main bore hole (Figure 4-1). Diameter of overcored core is 62 mm. The probe has three strain gauge rosettes each having axial, tangential and 45 degrees inclined strain gauge (Figure 4-2). The rosettes are 120 degrees apart from each other. The rosettes are installed at 16 cm from collar of the pilot hole. The orientation of rosettes is read after installation. Total overcoring length is at least 40 cm. During the overcoring each gauge is read after every one minute. The average overcoring speed is 3 cm/min. Applicability The developed program can be used:
a) For strain check i.e. compare the measured strains with the computed ones. b) To identify erroneous strain gauges and estimate the amount of error for individual
strain gauges or unexplained strains for all strain gauges. c) To estimate core damage potential by comparing elastic principal stresses on pilot
hole wall to strength of the rock and. d) Estimate the in situ state of stress based on the early strains readings by using a
inverse solution. Application approach
So far, for secondary stresses at the vicinity of bottom hole no analytical solutions exist. Currently, the in situ state of stress calculation is based on final stable strain readings and the analytical solution for infinite hole with circular or elliptical cross-section (Leeman, 1970). Therefore, the numerical modeling is the only possibility to study transient stresses and strains. To be as a practical quality control tool for stress measurement, the transient strain curve calculation has to be quick. Normally a 3D numerical calculation with adequate number of coring steps will take one or two days, but with precalculation results it is possible to preproduce transient strain curves for any loading condition in a minute.
34
Figure 4-1. Phases of Borre probe overcoring measurement (modified from Ljunggren and Klasson 1996)
35
120°
120°
120°
0
90°
R1
R3
R2
Strain gauge rosett e ( )seen from center of borehole
R1
G2 ( G5, G8 )
G1 ( G4, G7 )
G3 ( G6, G9 )
Borehole axis Figure 4-2. Borre probe strain gauge rosette (left) and strain gauge configuration (right) (modified from Ljunggren and Klasson 1996).
As input data for transient strain curve calculation following is needed:
a) Orientation of borehole i.e. bearing and dip (Figure 4-3), b) Position and orientation of strain gauges i.e. angular location on pilot hole
perimeter and dip related to borehole axis (Figures 4-4 and 4-5), c) The in situ state of stress, usually given in form of principal stress σ1, bearingσ1,
dipσ1, σ2, bearingσ2, dipσ2, σ3, bearingσ3, dipσ3 which can be transformed into tensor form σN, σE, σV, σNE, σEV, σVN , where N, E and V refers to compass points and vertical (Figure 4-6)
d) Young’s modulus and Poisson’s ratio from biaxial test of overcored core. e) Optionally drilling loads: axial stress (PA), shear stress (PS) at drill bit rock
interface and drilling fluid pressure (PW) on drill and pilot hole walls.
36
E
N
V
y
x
z
bearing, β
dip, γ
borehole
Figure 4-3. Definition of bore hole orientation and borehole coordinate system x,y,z.
y
x
z
z
rθ
eθ
er
ezstrain gauge
Figure 4-4. Definition of strain gauge angular position θ in borehole polar coordinate system r,θ,z.
37
z
eθ
er
ez
φ
dip of strain gauge = φ
eA
eB
θ
Figure 4-5. Definition of strain gauge dip φ and strain gauge coordinate system A.B in borehole polar coordinate system r,θ,z.
E
N
V
bearing, β
dip, γ
σ1
σ2 σ3
Figure 4-6. Definition of bearing and dip for principal stress components.
38
Similarly as the in situ stress measurement, the code is based on CHILE conditions (Continuous, Homogeneous, Isotropic and Linearly Elastic). In CHILE conditions the secondary stress state σs
x, σsy, σs
z, σsxy, σs
yz, σszx at borehole wall corresponding certain
overcoring advance x in known in situ state of stress (σN, σE, σV, σNE, σEV, σVN) and drilling loading condition (PA, PS, PW) can be re-produced by superimposing the secondary stresses from each primary load σs
x(i), σsy(i), σs
z(i), σsxy(i), σs
yz(i), σszx(i), i =
σN, σE, σV, σNE, σEV, σVN, PA, PS, PW).The precalculation of stress tensors are done in borehole coordinate system. The secondary stresses are non-linearly dependent on Poisson’s ratio, therefore secondary stresses from each primary load have to be calculated with adequate number of Poisson’s ratio values to get good estimates for secondary stresses of any Poisson’s ratio. For this code 0.15, 0.5 and 0.35 were selected providing minor error for any Poisson’s ratio between 0.05 and 0.45. Young’s modulus does not affect to secondary stresses instead it has linear effect on strains. From the secondary stresses at gauge position and Young’s modulus strain for any known strain gauge can be calculated. The secondary stresses of strain gauge area are calculated using 3D finite difference method code FLAC3D (Itasca Consulting Group, Inc., 1997). To simulate overcoring from the position of 190 mm before the strain gauges to 160 mm after strain gauges, 34 overcoring steps is used (Figures 4-7 and 4-8). The element length in borehole direction varies from 1 mm to 30 mm. The quarter model size is about 18 Mb, so the calculation time for this model is about one day depending on primary load. For full model, the size is about four times larger and calculation time will be 2.5 to 4 days. From the nine primary loads five can be calculated with quarter model (σxx, σyy, σzz, PA, PW), two with half model (σxz, σyz) and two with full model (σxy, PS). This implies that calculations for one Poisson’s ratio can be done in about two and a half weeks.
39
r = 18 mmPILOT
r = 360 mm = 20 * r
BOUNDARY
PILOT
132 mm= 3.5 * rOVER CORING
190 mm= 5.0 * rOVERC ORING
160 mm= 9.0 * rPILOT
132 mm= 3.5 * rOVERCOR ING
Strain gauge level
r = 38 mmOVERCO RING
Strain gauge area in core
Rock beside the coring volumeRock on drill bit pathRock to be overcored = Core
Figure 4-7. Dimension for FLAC3D tensor calculation model
40
190 mm
-160 mm
5 mm-5 mm
Strain gauge area
Figure 4-8. Simulated 34 overcoring steps with varying length, and strain gauge area.
With each of the nine primary loads three simulations with different Poisson’s ratio value were done. Each of these 27 simulations has 1+34 calculation phases (pilot hole phase and 34 overcoring phases). After reaching the equilibrium in each overcoring state, the secondary stress tensor of each element on strain gauge is written to a file. As a result of precalculations we have 9 * 3 * 35 secondary stress tensor files, each including 72*5 stress tensors. From these tensors it is possible to preproduce with good accuracy a secondary stress state corresponding any combination of following factors: - In situ state of stress. - Three drilling loads. - Poisson’s ratio between 0.15 and 0.35. - Coring advance from -190 mm to 160 mm compared to strain gauge position.
From these stress tensors a mean stress state at any at strain gauge area can be interpolated. This mean stress calculation is based at minimum on five but usually on ten stress tensors (Figures 4-9 and 4-10). The strain gauge strain is calculated based on the mean stress. The flowchart of developed code is shown in Figure 4-11.
41
5° = 1.57 mm
0.15 mm
2.0 mm
X
Z
Y
10.0 mm
Figure 4-9. Element size and resolution on strain gauge area.
X
Z
Y
X
Z
Y
X
Z
Y
axial
tangential
inclined
Figure 4-10. Basic idea for stress averaging for differently orientated strain gauges.
42
1) read stress tensor files for three Poisson's rato values
2) spread tensor values around pilot hole if symmetry was used in tensor calculation
3) Input case data
- bearing and dip of borehole
- in situ principal stresses in form of magnitude, bearing and dip
- drilling loads: Pa, Ps and Wp
- strain gauge rosette bearings
- Young's modulus and Poisson's ratio
4) convert in situ stress to global tensor form to get tensor multipliers
5) global tensor form to borehole tensor form to get tensor multipliers
6) check if inverse solution is asked to be done
7a) superimpose borehole cartesian stress tensors by taking account in situ stress multipliers and poisson's ratio
Repeat phases 7a to 11a nine times, in each time one primary load multiplier is set to one and the others are equal to zero
8a) calculate average strain gauge stresses by taking account true angular position Produce unit load-strain matrix for inverse solution
9a) transforms secondary stresses in cartesian borehole coordinate system to borehole polar coordinate system
10a) transforms stresses in borehole polar system into strain gauge stresses
11a) calculate strain gauge strains
12) calculate and find maximum principal stresses
13) find calculated strain vaues for measured ones
14) If asked, do inverse solution for in situ state of stress
15) produce plots
16) modify plots
17) If asked, reject strain gauges and redo inverse solution
18) change input values and recalculate
19) end
Figure 4-11. Flowchart of the developed OCS-code.
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User interface The developed code is a Microsoft Excel workbook. The programming is done with Visual Basic for Application macros. The workbook code includes a quick guide and more detailed manual. The session is started by reading in precalculated stress tensors and is continued by inputting case data and measured strain data (Figure 4-12). The case and measurement data can be written in manually, copied from other application or read from ASCII-file. The format of ASCII-file is given in manual. Once inputted, the transient strains corresponding the given in situ state of stress can be calculated and compared to measured ones. The comparison can be done in form of absolute transient values, transient differences or transient unexplained strains (Figures 4-13 and 4-14).
Figure 4-12. OCS code; input of case data with plot of measured strains.
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Figure 4-13. OCS code; comparison of measured and calculated strains.
Figure 4-14. OCS code; difference of measured and calculated strains.
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Stress path for maximum compression, deviatoric stress and tension at strain gauge area can be studied and compared to strength of intact rock (Figures 4-15 and 4-16). The strength plot includes user defined envelopes for peak strength, crack damage strength, crack initiation strength and σ3 = 5% σ1 (Figure 4-17). The Hoek-Brown peak strength envelope is defined by uniaxial compressive strength (σUCS) and tensile strength (σT) assuming s = 1 (Hoek & Brown 1997). The crack damage envelope is defined by uniaxial crack damage strength (σCD) and the same m-value as for peak strength. For crack initiation envelope deviatoric stress (σ1-σ3) is assumed to be equal to uniaxial crack damage strength (σCI). Based on presentation of Read et al. (1998) crack damage strength is assumed to be the true strength of the rock. The rock is assumed to damage, if stresses exceed the crack initiation surface and confinement is less than 5% of maximum stress. This damage can reduce the crack damage strength. The rate of strength reduction is not given but it can be defined from in situ observations.
Figure 4-15. OCS-code; development of stress maximums with advancing overcoring and location of maximums around pilot hole
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Figure 4-16. OCS-code, stress paths for the maximum compression and tension with rock strength envelopes.
scd
sci
Stress path during overcoring inlocation where maximum compression takes place
Stress path during overcoring inlocation where maximum tension takes place
Secondary stress on pilothole wall with biggersignal
Strength envelope,defined by , and s=1s sucs t
s t
sucs
Crack initiation envelope,- s s1 cis 3 =
Crack damage envelope,defined by , s=1 scd andm ( strength envelope )
Envelope for= 0.05 s3 s1
HIGH DAMAGEPOTENTIAL
DAMAGE POSSIBLE
DAMAGE EVIDENT
Figure 4-17. Interpretation of elastic stress paths during the overcoring at strain gauge location when superimposed over strength envelopes.
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Finally in situ state of stress can be calculated from early strains using an inverse method (Figure 4-18). In inverse solution unwanted strain gauges can be rejected from solution. Any input value can be changed and calculations repeated without reading again the stress tensors. The visibility of resulting and measured data can be controlled, calculated values can be copied to other applications and resulting figures can be copied or plotted.
Figure 4-18. OCS-code; inverse solution for in situ stress tensor. Accuracy of the method The accuracy of the code result was compared with the analytical solution for infinite circular hole in homogenous media. For the code perfect input data values are assuming perfect input data. Two models with different boundary conditions, fixed and stress, were also compared to evaluate the error caused by extent of the boundary and boundary condition. The radial boundaries of final calculation model had to be extended to value of 9 overcoring radius to limit the effect of model boundaries to transient stresses below 2%. The upper and lower boundaries are at 7 overcoring radius from the gauge position. Element size at strain gauge area (1 cm * 360°), which is divided to 72 * 5 elements, is 2 mm * 5° * 0.15 mm (height * width * thickness) (Figure 4-7). The maximum error of the code includes accuracy of stress tensor calculation, error caused by strain gauge location interpolation and error caused by Poisson’s ratio interpolation/extrapolation. The maximum error was studied with one real measurement
48
data and with one case having 10 MPa uniaxial in situ stress only. In both cases Poisson’s ratio was varied from 0.05 to 0.5. The maximum error in real case was –0.6 to 0.4 MPa, which is –3.7% to 3.1% of average in situ stress. With uniaxial in situ stress the maximum error was –0.5 MPa to 0.5 MPa being –5.1% to 5.1% of average in situ stress. The average total error was ± 1% of average in situ stress.
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5 SENSITIVITY STUDIES The following sensitivity study is not comprehensive but aimed to understand the effect of certain factors on transient strains and interpreted in situ state of stress. Three different types of sensitivity studies were done. First, relatively small changes in loading conditions and material parameter values were applied (A-cases). The effect on interpreted in situ state of stress was also calculated from final strains. The second group of cases are to study heterogeneity, anisotropy and pilot hole geometry (B-cases). Finally, the sensitivity of inverse solution for in situ state of stress was studied (C-cases). As a reference case KK0045G01 level 34.77 m in situ stress measurement from Äspö Hard Rock Laboratory was selected (Figure 5-1).
Figure 5-1. Geographical location of Äspö HRL (left) and general layout of the HRL facilities (right).
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5.1 Effect of loading conditions and material parameter values The idea was to see how sensitive transient and final strains are for changes in loading conditions and material parameter values. As a result differential strains at 40 mm, 15 mm before strain gauges and final differential strains were compared (Tables 5-1 to 5-3). The effect on interpreted in situ state of stress was also calculated from final strains (Table 5-4). The analyzed cases were: 0) reference case A1) strain at the center of strain gauge A2) bearing(σ1) +10 degrees A3) bearing (σ1) –10 degrees A4) dip(σ1) – 10 degrees A5) magnitude(σ1) –10% A6) Young’s modulus of rock –20% A7) Young’s modulus of rock +20% A8) Poisson’s ratio of rock –20% A9) Poisson’s ratio of rock +20% A10) drill bit axial load of 10 MPa A11) drill bit shear stress of 5 MPa A12) fluid pressure of 1 MPa (equal to 100 m water head) Table 5-1. Overcoring advance 40 mm before strain gauges. Strain gauge strain values for reference case and corresponding strain differences for sensitivity study cases.
Rosette 1 Rosette 2 Rosette 3
G1 ( R1_axi ) G2 ( R1_tan ) G3 ( R1_incl. ) G4 ( R2_axi ) G5 ( R2_tan ) G6 ( R2_incl. ) G7 ( R3_axi ) G8 ( R3_tan ) G9 ( R3_incl. )
µ εα µ ε90 µ ε45 µ εα µ εα µ εα µ εα µ εα µ εα
Ref. case, absolut values 107 -29 34 123 -103 -3 122 -83 38
Ref. case center - Ref. case -6 0 0 -7 -1 -2 -11 0 -2
Ymod 80% - Ref. case 17 -8 5 19 -15 -1 16 -13 4
Ymod 120% - Ref. case -12 5 -4 -13 10 1 -11 9 -3
Prat 80% - Ref. case -2 3 -1 -2 1 1 -2 1 0
Prat 120% - Ref. case 1 -3 1 1 0 -1 1 -1 0
Sigma 1, Hrot +10_deg - Ref. case 1 0 3 -2 5 0 2 -6 -3
Sigma 1, Hrot -10_deg - Ref. case 0 -3 -4 2 -4 2 -2 6 2
Sigma 1 Vrot-10_deg - Ref. case -11 1 -6 -9 -3 -5 -10 4 -2
Sigma 1 decreased 10% - Ref. case -5 0 -1 -5 6 1 -3 1 -3
Drill bit pressure 10 MPa - Ref. case -15 3 -6 -15 3 -6 -15 3 -6
Drill bit shear stress 5 MPa - Ref. case 0 0 0 0 0 0 0 0 0
Water pressure 1 MPa - Ref. case -4 3 -1 -4 3 -1 -4 3 -1
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Table 5-2. Overcoring advance 15 mm before strain gauges. Strain gauge strain values for reference case and corresponding strain differences for sensitivity study cases.
Rosette 1 Rosette 2 Rosette 3
G1 ( R1_axi ) G2 ( R1_tan ) G3 ( R1_incl. ) G4 ( R2_axi ) G5 ( R2_tan ) G6 ( R2_incl. ) G7 ( R3_axi ) G8 ( R3_tan ) G9 ( R3_incl. )
µ εα µ ε90 µ ε45 µ εα µ εα µ εα µ εα µ εα µ εα
Ref. case, absolut values 131 234 -48 132 617 653 135 509 283
Ref. case center - Ref. case 0 -1 -1 0 5 3 0 3 1
Ymod 80% - Ref. case 33 59 -12 33 154 163 34 127 71
Ymod 120% - Ref. case -22 -39 8 -22 -103 -109 -23 -85 -47
Prat 80% - Ref. case 26 8 26 26 18 11 26 15 22
Prat 120% - Ref. case -27 -6 -26 -27 -18 -12 -27 -15 -23
Sigma 1, Hrot +10_deg - Ref. case 0 -6 32 0 -65 -20 0 72 -13
Sigma 1, Hrot -10_deg - Ref. case 0 33 -11 0 46 1 0 -79 10
Sigma 1 Vrot-10_deg - Ref. case -42 34 -2 -42 92 3 -43 3 0
Sigma 1 decreased 10% - Ref. case -9 6 21 -10 -63 -87 -10 -2 21
Drill bit pressure 10 MPa - Ref. case 1 -1 0 1 -1 0 1 -1 0
Drill bit shear stress 5 MPa - Ref. case 0 0 0 0 0 0 0 0 0
Water pressure 1 MPa - Ref. case -7 -26 -17 -7 -26 -17 -7 -26 -17 Table 5-3. Totally overcored. Final strain gauge strain values for reference case and corresponding strain differences for sensitivity study cases.
Rosette 1 Rosette 2 Rosette 3
G1 ( R1_axi ) G2 ( R1_tan ) G3 ( R1_incl. ) G4 ( R2_axi ) G5 ( R2_tan ) G6 ( R2_incl. ) G7 ( R3_axi ) G8 ( R3_tan ) G9 ( R3_incl. )
µ εα µ ε90 µ ε45 µ εα µ εα µ εα µ εα µ εα µ εα
Ref. case, absolut values 236 -2 57 278 -104 131 367 -104 148
Ref. case center - Ref. case -1 1 1 3 -1 -3 -3 0 0
Ymod 80% - Ref. case 56 -3 12 67 -19 37 87 -22 35
Ymod 120% - Ref. case -38 2 -8 -44 13 -24 -58 15 -23
Prat 80% - Ref. case 7 2 6 4 1 3 -1 3 -1
Prat 120% - Ref. case -9 -2 -6 -5 -1 -4 1 -3 1
Sigma 1, Hrot +10_deg - Ref. case -9 3 12 7 6 6 2 -9 -18
Sigma 1, Hrot -10_deg - Ref. case 11 -7 -11 -7 -3 -5 -4 11 16
Sigma 1 Vrot-10_deg - Ref. case -8 -5 -10 -13 -10 -15 -19 2 -1
Sigma 1 decreased 10% - Ref. case -4 -4 4 -18 9 -16 -23 3 -7
Drill bit pressure 10 MPa - Ref. case -15 -5 -10 -15 -5 -10 -15 -5 -10
Drill bit shear stress 5 MPa - Ref. case 0 0 -1 0 0 1 0 0 0
Water pressure 1 MPa - Ref. case -16 3 -6 -16 3 -6 -16 3 -6 When the in situ state of stress is calculated from final strains, the applied changes are considered as errors i.e. if strains are calculated with 20% lower Young’s modulus, the stress interpretation is done with Young's modulus value of the reference case.
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Table 5-4. Calculated in situ stress values for sensitivity study cases.
σ1 σ2 σ3
MPa dip, ° dd, ° MPa dip, ° dd, ° MPa dip, ° dd, °
Ref. case 26.3 40 14 17.3 27 258 8.7 38 144
Ref. case center 26.2 40 14 17.2 27 258 8.7 38 145
Ymod 80% 32.9 40 14 21.6 27 258 10.9 38 144
Ymod 120% 21.9 40 14 14.4 27 258 7.3 38 144
Prat 80% 27.4 42 14 18.3 29 254 10.1 34 142
Prat 120% 25.2 38 14 16.3 26 262 7.2 42 147
Sigma 1, Hrot +10_deg 26.2 40 24 17.3 27 268 8.8 38 154
Sigma 1, Hrot -10_deg 26.3 40 4 17.2 28 248 8.7 38 135
Sigma 1 Vrot-10_deg 26.3 31 15 17.3 30 265 8.7 44 141
Sigma 1 decreased 10% 23.6 40 14 17.3 27 258 8.8 38 144
Drill bit pressure 10 MPa 26.3 40 14 17.3 27 258 8.7 38 144
Drill bit shear stress 5 MPa 26.3 40 14 17.3 27 258 8.7 38 144
Water pressure 1 MPa 25.3 40 14 16.3 27 258 7.7 38 144 The study were loading conditions and material parameter values were slightly varied showed that:
- Young’s modulus is important parameter having close to 100% linear effect on stress magnitudes.
- The effect of Poisson’s ratio is moderate to all strains and interpreted stress, the effect is 20% to 60% on stress magnitude and few degrees to orientation.
- 10° uncertainty in strain gauge orientation can produce at maximum 20% error to strain values.
- The rotation of whole gauge system produces equal error to principal stress orientations.
- 10% change in major principal stress requires 5% to 10% general change in strains. - 10 MPa drill bit pressure has general -5% to -20% effect on transient strains when
coring has not passed the strain gauge position. - Drill bit shear stress has minor effect on transient strains. - Drill bit loads do not have affect on final strains. - 1 MPa (=100 m water) drilling fluid pressure decreases all strains generally 5% to
10% and all interpreted in situ principal stresses by its magnitude. - In case of studied factors there is no general trend that the effect is higher and lower
to transient stresses than to final stresses, except in case of drilling loads which does not affect to final strain.
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5.2 Effect of heterogeneity, anisotropy and pilot hole geometry Few sensitivity study calculations were done with different material model assumptions simulating heterogeneity of rock i.e. crystals of different minerals. In one case very small deformation anisotropy was used. Finally the effect of overcoring diameter on transient strains was studied: B1) Heterogeneous rock, +-35% deviation of elastic parameters B2) 2 cm spherical crystal at rosette 1 location, difference of Young’s modulus -20% B3) 2 cm spherical crystal at rosette 1 location, difference of Young’s modulus -80% B4) Transversely anisotropic rock, difference of Young’s modulus 10% B5) 96 mm overcoring instead of 76 mm The models for heterogeneous rock, anisotropy and crystal were calculated to in situ stress equilibrium under boundary stress conditions before overcoring. The resulting in situ state of stress is calculated using reference case material parameter values (Table 5-5). The results show that only a local crystal with very low modulus have clear effect on interpreted stress. The results for 96 mm overcoring should be the same as for reference case and error is caused by model boundaries. On the other hand, the transient strains are clearly different for bigger overcoring diameter (Table 5-6). Basically the transient overcoring range is longer and smoother. The maximum and minimum principal stresses at stain gauge location were 53.2 MPa and 7.1 MPa for 96 mm overcoring and 57 MPa and –18 MPa for 76 mm overcoring. This means that thin walled overcoring is more sensitive for core damage than thicker one. Table 5-5. Calculated in situ stresses for sensitivity study B cases.
σ1 σ2 σ3
MPa dip, ° dd, ° MPa dip, ° dd, ° MPa dip, ° dd, °
Ref. case 25.9 38 16 17.3 30 259 8.9 37 143
Heterogeneous rock, +-35% 25.2 36 17 16.5 34 257 9.0 35 138
2 cm crystal - local deviation -20% 25.2 35 22 16.3 36 262 9.3 35 141
2 cm crystal - local deviation -80% 24.3 28 32 13.7 51 261 7.6 25 136
Anisotropy, 10% 25.9 39 15 17.4 30 258 9.3 37 142
96 mm overcoring 25.0 36 15 17.1 28 262 8.2 41 144 Table 5-6. Transient strains for three coring advances for 76 mm (reference case) and 96 mm overcoring (caseB5).
Rosette 1 Rosette 2 Rosette 3
G1 ( R1_axi ) G2 ( R1_tan ) G3 ( R1_incl. ) G4 ( R2_axi ) G5 ( R2_tan ) G6 ( R2_incl. ) G7 ( R3_axi ) G8 ( R3_tan ) G9 ( R3_incl. )
µ εα µ ε90 µ ε45 µ εα µ εα µ εα µ εα µ εα µ εα
76 mm overcoring, 40 mm 69 -30 21 77 -62 -4 66 -51 17
96 mm overcoring, 40 mm 143 -38 25 154 -84 57 189 -82 60
76 mm overcoring, 15 mm 226 -14 49 266 -76 146 348 -87 141
96 mm overcoring, 15 mm 228 35 23 234 -5 238 350 -24 148
76 mm overcoring, final 131 235 -50 132 621 654 135 505 284
96 mm overcoring, final 105 241 -58 106 617 619 112 509 284
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5-5. Sensitivity of inverse solution The developed code has capability to solve the in situ state of stress based on any measured transient (early strains) or final strains. If applied for exact strain values and corresponding coring advances, calculated by numerical simulation of overcoring, the solution is exact. In real case there is always some error relating to measuring of coring advance, drilling loads and measuring accuracy. Following sensitivity study is done for coring advance only. It is based on calculated strain values of the previously used Äspö reference case. Three coring advances of 60 mm, 15 mm and 5 mm before strain gauge location were studied. The errors for coring advances are shown in Figures 5-2 to 5-4. Resulting stress tensors shows that if there is some uncertainty in measurement of coring advance the solution is sensitive and being most sensitive close to strain gauge position. If inverse solution is going to be used, the coring advance have to be measured with higher than +- 1 mm accuracy and even with this accuracy the inverse solution can not be used for coring advances close to strain gauge position.
-20
-10
0
10
20
30
40
50
sxx syy szz sxy syz szx
In situ stress tensor component
Stre
ss (
MP
a )
-15 mm
0
+10 mm
Figure 5-2. Effected of coring advance to inverse solution for in situ stress tensor if true coring advance is -60 mm and assumed advances are -75 mm or -50 mm.
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-20
-10
0
10
20
30
40
50
sxx syy szz sxy syz szx
In situ stress tensor component
Stre
ss (
MP
a )
-5 mm
0 mm
+5 mm
Figure 5-3. Effected of coring advance to inverse solution for in situ stress tensor if true coring advance is -15 mm and assumed advances are -20 mm or -10 mm.
-20
-10
0
10
20
30
40
50
sxx syy szz sxy syz szx
In situ stress tensor component
Stre
ss (
MP
a )
-2.5 mm
0 mm
+2 mm
Figure 5-4. Effected of coring advance to inverse solution for in situ stress tensor if true coring advance is -5 mm and assumed advances are -7.5 mm or -3.0 mm.
56
57
6 CASE STUDIES The procedure in all case studies was to analyse first the biaxial data, then compare measured transient strains with calculated ones assuming closed form solution for in situ state of stress. Finally, inverse solution for in situ state of stress is proceeded and transient strains are compared to measured ones again. The following five in situ stress measurements were analyzed using the developed OCS-code: - Äspö KK0045G01 Level 2:2 (at 34.77 m) - Äspö KK0045G01 Level 2:3 (at 35.48 m) - Äspö KF0093A01 MP1 (at 32.14 m) - Äspö KF0093A01 MP4 (at 35.38 m) - AECL URL, Hole 209-022-0C2 (at 11.91 m) The first and second Äspö Borre Probe cases were analysed in detail, but the other two were studied in more general way. The measurement at AECL was done with modified twelve gauge CSIR. The selected cases are from Äspö Hard Rock Laboratory (HRL) where the Swedish Nuclear and Fuel Waste Management Co (SKB) has tested the reliability of different in situ stress methods under controlled conditions. “The HRL is located below an island close to the coast of south-east Sweden (Figure 5-1). The host rock is a diorite with a Young’s modulus of 75 – 80 GPa and a uniaxial compressive strength in the range of 160 – 210 MPa. The stress measurement campaigns have been carried out in various phases. The first investigation phase up to 1990 was carried out from the surface, and included hydraulic fracturing and overcoring using the Borre probe. The next phase up to 1995 covered stress measurements during excavation of the tunnels, using the CSIRO gauge. During the operational phase after 1995, stress measurements been carried out from the tunnels using overcoring with the CSIRO and Borre gauges, as well as hydraulic fracturing. By comparing the results from the various stress measuring campaigns between levels 320 to 500 m it was concluded that data from different measurement projects, and from different locations at the site, could display significant differences in orientations, magnitudes and repeatability. Differences in stress magnitudes with depth of 50% and variations in trend of the major principal stress of ± 30-40° within the test locations were considered to indicate a problem with the reliability of the stress measurement data.”(Christiansson & Hudson 2002). From the borehole KK0045G01 two sequential measurements were studied; Level 2:2 (34.77 m) and Level 2:3 (35.48 m). The procedure in case study was to look first the measured strains, analyse the biaxial data, do the closed form solution, compare measured transient strains with calculated ones and finally do inverse solution for in situ state of stress and compare transient strains to measured ones again. Due to misunderstanding, all orientation values are rotated 120 degree counter clockwise (CCW). In the two borehole KF0093A01 measurements the temperature logging did not
58
work. The core diameter was also slightly slimmer than usual and the core had to be taped at the biaxial cell openings to prevent leaking oil. The fifth case is from Underground Research Laboratory, Canada where Atomic Energy of Canada Ltd has studied in situ state of stress and other rock mechanical issues during the years 1982-1997. The selected case Hole 209-022-0C2 (at 11.91 m) is about 240 m below ground surface and situated in homogeneous medium to coarse grained grey granite. The measurement was done with modified twelve gauge CSIR-probe and the three 135 inclined strain gauges were not studied. 6.1 Äspö KK0045G01 stress measurements Measurement data Borehole, probe and strain gauge orientations of the measurements at depths 34.77 m and 35.48 m are presented in Table 6-1. Figures 6-1 and 6-2 shows the measured strain gauge readings during the overcoring measurement and the values selected for in situ stress calculation. Noteworthy is the 4.5 degree temperature increase in both cases, the time dependent behavior of most extended strain gauges (34.77 m: G5, G6, G8 and 35.48 m: G2, G5) and definition of calculation values basically from position where drill bit is 5 - 6 cm over the gauge position, but some values have been changed during the stress calculation (34.77 m: G2 and 35.48: G7 and G8). The temperature seems to have minor effect on readings, because it is not shown on all tangential or inclined strain gauges. The most strained gauges suffer from debonding because no time dependent behavior is shown on less strained strain gauges. The definition of strain values for stress calculation are reasonable because in ideal case (simulation) the strain readings are relative stable when coring has passed gauge position by 5 cm. Also, the effect of debonding is probably lowest at this position. The only question is the value for axial strain; in both measurements the lowest measured value was ignored and axial strain was decided to be average of two highest values. Also the stress calculation value for gauge 8 at 35.48 m is not clear. It is not acceptable that measured values are ignored if no evidences for erroneous reading can be shown. In this case there are no such clear evidences, only the gauge1 at 34.77 m has different transient path.
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Table 6-1. Borre probe gauge configuration for two Äspö borehole KK0045G01 stress measurements. Äspö HRLDemo tunnelNon-Corrected Orientations (Gauges & Stresses)
Borehole: KK0045G01 Level 2:2 Level 2:3Vertical hole
Depth [m] 34.77 35.48Bearing [°] 300 359
(magnetic needle reading from zero-mark on compass)Dip [°] 90 90
Normal Configuration for Borre ProbeRosette & strain gauge orientation
Orientation relative to Magnetic NorthBearing [°] Bearing [°]
Rosette # 1 30 89 (relative to Magnetic North)2 270 329 (relative to Magnetic North)3 150 209 (relative to Magnetic North)
Rosette # Strain gauge # Dip Dir [°] Dip [°] Dip Dir [°] Dip [°]1 1 30 90 89 90 (relative to Magnetic North)1 2 300 0 359 0 (relative to Magnetic North)1 3 300 45 359 45 (relative to Magnetic North)2 4 270 90 329 90 (relative to Magnetic North)2 5 180 0 239 0 (relative to Magnetic North)2 6 180 45 239 45 (relative to Magnetic North)3 7 150 90 209 90 (relative to Magnetic North)3 8 60 0 119 0 (relative to Magnetic North)3 9 60 45 119 45 (relative to Magnetic North)
Normal Configuration for Borre ProbeOrientation relative to Äspö x-axisÄspö x-axis is 13.222 gon (11.9 degrees) west of Magnetic North
Bearing [°] Bearing [°]Rosette # 1 41.9 100.9 (relative to Äspö x-axis)
2 281.9 340.9 (relative to Äspö x-axis)3 161.9 220.9 (relative to Äspö x-axis)
Rosette # Strain gauge # Dip Dir [°] Dip [°] Dip Dir [°] Dip [°]1 1 41.9 90 100.9 90 (relative to Äspö x-axis)1 2 311.9 0 10.9 0 (relative to Äspö x-axis)1 3 311.9 45 10.9 45 (relative to Äspö x-axis)2 4 281.9 90 340.9 90 (relative to Äspö x-axis)2 5 191.9 0 250.9 0 (relative to Äspö x-axis)2 6 191.9 45 250.9 45 (relative to Äspö x-axis)3 7 161.9 90 220.9 90 (relative to Äspö x-axis)3 8 71.9 0 130.9 0 (relative to Äspö x-axis)3 9 71.9 45 130.9 45 (relative to Äspö x-axis)
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SG-position OC-stop
-200
-100
0
100
200
300
400
500
600
700
-20 0 20 40 60 80 100
Advance / time ( cm/s )
Mic
ro s
trai
n
6
8
10
12
14
16
18
20
22
24
Tem
p ( d
egre
es )
G1, R1-axi
G2, R1-tan
G3, R1-45
G4, R2-axi
G5, R2-tan
G6, R2-45
G7, R3-axi
G8, R3-tan
G9, R3-t45
G1sc, R1-axi
G2sc, R1-tan
G3sc, R1-45
G4sc, R2-axi
G5sc, R2-tan
G6sc, R2-45
G7sc, R3-axi
G8sc, R3-tan
G9sc, R3-t45
SG temp
Figure 6-1. Äspö KK0045G01 34.77 m, measured strain gauge readings during the overcoring measurement and the values selected for in situ stress calculation (Gn is for measured and Gnsc is for stress calculation value).
SG-position OC-stop
-200
-100
0
100
200
300
400
500
600
700
-20 0 20 40 60 80 100
Advance / time ( cm/s )
Mic
ro s
trai
n
6
8
10
12
14
16
18
20
22
24Te
mp
( deg
rees
)G1, R1-axi
G2, R1-tan
G3, R1-45
G4, R2-axi
G5, R2-tan
G6, R2-45
G7, R3-axi
G8, R3-tan
G9, R3-t45
G1sc, R1-axi
G2sc, R1-tan
G3sc, R1-45
G4sc, R2-axi
G5sc, R2-tan
G6sc, R2-45
G7sc, R3-axi
G8sc, R3-tan
G9sc, R3-t45
SG temp
Figure 6-2. Äspö KK0045G01 35.48 m, measured strain gauge readings during the overcoring measurement and the values selected for in situ stress calculation (Gn is for measured and Gnsc is for stress calculation value).
61
Table 6-2 and Figure 6-3 shows the strain gauge readings in the end of overcoring, in the beginning of biaxial testing and the difference (time interval and the temperature in biaxial test is unknown). The differences are high compared to measured values. Possible reasons are strain gauge drift, hardening of glue, time dependent behaviour of rock or temperature change. If it is gauge drift or temperature it does not affect to stress calculation, but in other two cases we can not be sure that the strain gauge response in biaxial test was the same as it was during the overcoring. After overcoring has stopped time dependent strain takes place in most strained gauges, and it is most likely caused by debonding of gauges (Figures 6-1 to 6-3). Anyhow the uncertainties can be minimised by doing the biaxial testing immediately after overcoring in a same temperature. Table 6-2. Äspö KK0045G01. Strain gauge readings in the end of overcoring, in the beginning of biaxial testing and the difference.
G1, R1_axi G2, R1_tan G3, R1_45 G4, R2_axi G5, R2_tan G6, R2_45 G7, R3_axi G8, R3_tan G9, R3_4534.77 mEnd of OC 303 245 422 250 1404 760 639 902 2284Beg. of Biaxial 233 3 41 318 1273 777 269 362 1706Delta -70 -242 -381 68 -131 17 -370 -540 -578
35.44 mEnd of OC 397 664 1242 178 124 675 274 631 1189Beg. of Biaxial 386 656 945 -85 465 569 543 704 774Delta -11 -8 -297 -263 341 -106 269 73 -415
-600
-400
-200
0
200
400
0 500 1000 1500 2000 2500
Strain after over coring ( µstrain )
Stra
in d
iffer
ence
( µs
train
)
Axial Gauges
Inclained Gauges
Tangential Gauges
Figure 6-3. Äspö KK0045G01 34.77 m and 35.48 m. Change in strain gauge readings from the end of overcoring until biaxial testing as a function of strain after overcoring.
62
Biaxial testing In both biaxial measurements slight hysteresis is seen but no permanent deformation exist (Figures 6-4 and 6-5). In axial gauges the deviation is minor and in inclined gauges it is moderate, but the gauge 5 deviates from other tangential gauges in both cases.
-600
-500
-400
-300
-200
-100
0
100
200
0 2 4 6 8 10
Cell pressure ( MPa )
Mic
rost
rain
G1, R1_axi
G2, R1_tan
G3, R1_45
G4, R2_axi
G5, R2_tan
G6, R2_45
G7, R3_axi
G8, R3_tan
G9, R3_45
Figure 6-4. Äspö KK0045G01 34.77 m, biaxial test response.
-600
-500
-400
-300
-200
-100
0
100
200
0 2 4 6 8 10
Cell pressure ( MPa )
Mic
rost
rain
G1, R1_axi
G2, R1_tan
G3, R1_45
G4, R2_axi
G5, R2_tan
G6, R2_45
G7, R3_axi
G8, R3_tan
G9, R3_45
Figure 6-5. Äspö KK0045G01 35.48 m, biaxial test response.
63
When continuous, homogenous, isotropic and linearly elastic material is assumed the definition of elastic parameters are based on axial and tangential strain response and the inner and outer diameter ratio:
)(2
2min
2max
2max
tan RRRPE−∆
∆=
ε , (6-1)
tanεεν
∆∆
−= axi
, (6-2) where Rmax is the outer radius and Rmin is the inner radius of hollow core. Normally calculation is based on unloading part because the rock cylinder was unloaded in overcoring stress measurement also. In the calculation different methods can be used. SwedPower calculates first E for each Pi, εtan(Pi) pair and ν for each εaxi(Pi),εtan(Pi) pair. After that average E and ν for unloading range P=3-8 MPa is calculated. Note, that this calculation method does not assume linear elastic behavior. Another method is to do linear fit for all unloading Pi, εtan(Pi) and Pi, εaxi(Pi) values and then calculate average values based on linear fits. Table 6-3 summarises the deformation parameters defined with different methods and different pressure ranges. Table 6-3. Äspö KK0045G01. Resulting elastic parameters and the anisotropy error factor defined from unloading of biaxial cell using two different methods. Depth 34.77 35.48
E ν A_err E ν A_errPoint values ( SwedPower )
3…8 MPa, all gauges 66 0.26 56 0.17
Linear Fit0…10 MPa, Rosette 1 63 0.26 0 % 64 0.23 7 %0…10 MPa, Rosette 2 90 0.39 17 % 54 0.19 -29 %0…10 MPa, Rosette 3 63 0.26 -27 % 66 0.23 -10 %
0…10 MPa, all gauges 70 0.29 61 0.210…10 MPa, G5 ignored 63 0.26 65 0.23
3…8 MPa, Rosette 1 64 0.26 -2 % 65 0.23 6 %3…8 MPa, Rosette 2 92 0.40 17 % 55 0.19 -27 %3…8 MPa, Rosette 3 64 0.27 -28 % 67 0.24 -11 %
3…8 MPa, all gauges 71 0.30 62 0.223…8 MPa, G5 ignored 64 0.27 66 0.23
For continuous, homogenous, isotropic and linearly elastic material the strain of 45 degrees inclined strain gauge can be calculated as an average of axial and tangential strain gauge values. Thereby following error factor caused by anisotropy or heterogeneity can be calculated (Figure 6-6):
64
45
tan45 )(*5.0(ε
εεε gentialaxiAoUS+−
= (6-3)
-10%-20% +10% +20%
Äspö x-axis - 120°
AoUS
S
W
30° / 0%
89° / 7%
157° / -27%
209° / -10%
270° / 17% 329° / -29%
α i = angular position of strain rosette relative to Äspö x-axis - 120°
r i = AoUS
Figure 6-6. Amount of unexplained strain (AoUS) for different strain gauge rosettes for both Äspö KK0045G01 34.77 m and 35.48 m biaxial test. The angle from North corresponds with angular position of rosette and radial distance is the amount of unexplained strain. The study of biaxial test result showed that a) There is a significant difference in deformation parameter values of different
rosettes. b) In both measurements the highest difference is caused by tangential gauge number
5 in rosette 2. c) There is indication of anisotropy but response of gauge 5 is not clear. d) The calculation method can have 5% to 10% effect on Young’s modulus value, and
there by to interpreted in situ stress magnitudes also. e) Generally SwedPowers method gives lower modulus and Poisson’s ratio and it does
not assume linear elastic behavior. f) The pressure range used for calculation has minor effect.
65
It should be noted that change in Young’s modulus have about linear 1:1 effect on the calculated in situ state of stress. Closed form solution Based on analytical closed form solution for in situ state of stress it is possible to back calculate axial, tangential and 45 degrees inclined strain on pilot hole wall i.e. strains for used strain gauges without fixing the angular position. Further, to compare measured and back calculated strain the measured values are superimposed on pilot hole strain plot (Figures 6-7 and 6-8). The difference between measured and back calculated strains and the location of strain rosette readings compared to slope of calculated strain pattern can be used to estimate the sensitivity of individual strain reading to the closed form solution. Note the 120 degree rotation in orientations, which is due to misunderstanding but does not effect the case study. The result of closed form solution indicates that:
a) The absolute and relative differences and the amount of unexplained strains are quite small in both cases and the correlation between measured and back calculated strains is almost perfect (Table 6-4).
b) Degree of anisotropy or heterogeneity of the rock is probably not strong because calculated and measured strains are close to each other.
c) The defined Poisson’s ratio value is reasonable or the effect of Poisson’s ratio is minor.
d) The accuracy of tangential strain for rosettes 2 and 3 at level 34.77 m and rosettes 1 and 2 at level 35.48 m can have remarkable effect on the resulting in situ state of stress, because tangential and inclined strain gauge gradient is high at these strain gauge positions.
e) The orientation of stress field is well known, although the magnitudes can vary in a range of defined Young’s modulus. In both measurements the Young’s modulus is about 64 GPa and Poisson’s ratio 0.25 if the gauge 5 reading is ignored. On the other hand no evidences off erroneous response exist.
f) Based on the two measurements and the elastic parameters defined by SwedPower the magnitude of σ1 and σ2 is known with ±20% and σ3 with ±30% accuracy. The orientation of principal stresses is known with ±7° accuracy (Table 6-5).
The study of biaxial test and final strain values of the two Äspö KK0045G01 cases showed that these two measurements can be considered relatively successful and full filling the assumptions of continuous, homogenous, isotropic and linearly elastic rock.
66
-800
-600
-400
-200
0
200
400
600
800
-800 -600 -400 -200 0 200 400 600 800
Analytical - 0, axi
Analytical - 90, tan
Analytical - 45, incl.
Stress calc value - 0, axi
Stress calc value - 90, tan
Stress calc value - 45, incl.
SG reading at +10cm - 0, axi
SG reading at +10cm - 90, tan
SG reading at +10cm - 45, incl.
Äspö x-axis - 120°
Principal stresses relative to Äspö x-axis -120°:σ1 = 26.1 MPa, dip 39°, bearing 27°σ2 = 17.3 MPa, dip 28°, bearing 271°σ3 = 8.9 MPa, dip 38°, bearing 156°
Äspö KK0045G01 34.77
angular positionfor Rosette 1
angular positionfor Rosette 2
angular positionfor Rosette 3
s1
Figure 6-7. Äspö KK0045G01 34.77 m. Strain values used to calculate in situ state of stress, measured strain values when drill bit is 10 cm ahead the gauge position and back calculated axial, tangential and 45 degrees inclined strains around pilot hole.
-800
-600
-400
-200
0
200
400
600
800
-800 -600 -400 -200 0 200 400 600 800
Analytical - 0, axi
Analytical - 90, tan
Analytical - 45, incl.
Stress calc value - 0, axi
Stress calc value - 90, tan
Stress calc value - 45, incl.
SG reading at +10cm - 0, axi
SG reading at +10cm - 90, tan
SG reading at +10cm - 45, incl.
Äspö x-axis - 120°
Principal stresses relative to Äspö x-axis - 120°:σ1 = 18.1 MPa, dip 29°, bearing 24°σ2 = 12.3 MPa, dip 35°, bearing 271°σ3 = 5.0 MPa, dip 41°, bearing 143°
Äspö KK0045G01 35.48
angular positionfor Rosette 1
angular positionfor Rosette 2
angular positionfor Rosette 3
s1 - 120°
Figure 6-8. Äspö KK0045G01 35.48 m. Strain values used to calculate in situ state of stress, measured strain values when drill bit is 10 cm ahead the gauge position and back calculated axial, tangential and 45 degrees inclined strains around pilot hole.
67
Table 6-4. Äspö KK0045G01. Difference between the strain values used in stress calculation and strain values calculated from interpreted in situ state of stress.
G1, R1_axi G2, R1_tan G3, R1_45 G4, R2_axi G5, R2_tan G6, R2_45 G7, R3_axi G8, R3_tan G9, R3_4534.77 mStress calc. values 144 226 -5 132 538 620 119 601 265Calculated 132 227 -5 132 537 619 132 600 265Delta 12 -1 0 0 1 1 -13 1 0Delta % 9 % 0 % 0 % 0 % 0 % 0 % -10 % 0 % 0 %
Correlation 1.00Unexplained / Total 1 %
35.48 mStress calc. values 91 566 139 132 518 338 112 151 308Calculated 112 566 139 112 520 339 112 152 309Delta -21 0 0 20 -2 -1 0 -1 -1Delta % -19 % 0 % 0 % 18 % 0 % 0 % 0 % -1 % 0 %
Correlation 1.00Unexplained / Total 0 % Table 6-5. Äspö KK0045G01. In situ principal stresses and the direction angles of both measurements, average in situ principal stress based on average tensor components and differences compared to average. Interpreted in situ state of stress ( MPa )
measuring point σ1 dip (°) bearing (°) σ2 dip (°) bearing (°) σ3 dip (°) bearing (°)
depth 34.77 m 26.1 39 27 17.0 28 271 8.9 38 156depth 35.48 m 18.1 29 24 12.0 35 271 5.0 41 143
Average in situ state of stress ( MPa )
σ1 dip (°) bearing (°) σ2 dip (°) bearing (°) σ3 dip (°) bearing (°)
22.0 35 25 15.0 31 270 7.1 39 150
Absolute difference ( MPa )
measuring point σ1 dip (°) bearing (°) σ2 dip (°) bearing (°) σ3 dip (°) bearing (°)
depth 34.77 m -ave. 4.1 4 2 2.5 -3 1 1.8 -1 6depth 35.48 m - ave. -3.9 -6 -1 -2.5 4 1 2.1 2 -7
Relative difference ( % )
measuring point σ1 σ2 σ3
depth 34.77 m -ave. 19 17 26depth 35.48 m - ave. -18 -17 -29
68
Transient strain analysis Comparing the measured and calculated transient strains following conclusions can be made (Figures 6-9abc and 6-10abc):
a) Majority of strain gauges have trend close to simulated response. b) Relative long interval (min 2 cm) in measurements makes the comparison
uncertain, for example Gauge 4 in Figure 5-9. c) It seems that measured coring advance is about 2 cm ahead the calculated one. This
is possible, because the advance is defined from starting and ending time of overcoring and total overcoring length. Further, normally the overcoring speed is 3 to 5 cm/min.
d) Because of uncertainty in coring advance it would be extremely hard to do stress interpretation based on early strains.
e) The final simulated strain values are very close to measured strain values around +50 mm coring advance, which were used for interpretation of in situ state of stress.
G1
G2
G3
-200
-100
0
100
200
300
400
500
600
700
-200 -150 -100 -50 0 50 100 150 200Coring advance ( mm )
Stra
in (
)
G1 ( R1_axi )
G2 ( R1_tan )
G3 ( R1_incl. )
G4 ( R2_axi )
G5 ( R2_tan )
G6 ( R2_incl. )
G7 ( R3_axi )
G8 ( R3_tan )
G9 ( R3_incl. )
m G1 ( R1_axi )
m G2 ( R1_tan )
m G3 ( R1_incl. )
m G4 ( R2_axi )
m G5 ( R2_tan )
m G6 ( R2_incl. )
m G7 ( R3_axi )
m G8 ( R3_tan )
m G9 ( R3_incl. )
Figure 6-9a. Äspö KK0045G01 34.77 m. Calculated and measured strain gauge responses in Rosette 1, zero advance is equal to gauge position.
69
G4
G5
G6
-200
-100
0
100
200
300
400
500
600
700
-200 -150 -100 -50 0 50 100 150 200Coring advance ( mm )
Stra
in (
)G1 ( R1_axi )
G2 ( R1_tan )
G3 ( R1_incl. )
G4 ( R2_axi )
G5 ( R2_tan )
G6 ( R2_incl. )
G7 ( R3_axi )
G8 ( R3_tan )
G9 ( R3_incl. )
m G1 ( R1_axi )
m G2 ( R1_tan )
m G3 ( R1_incl. )
m G4 ( R2_axi )
m G5 ( R2_tan )
m G6 ( R2_incl. )
m G7 ( R3_axi )
m G8 ( R3_tan )
m G9 ( R3_incl. )
Figure 6-9b. Äspö KK0045G01 34.77 m. Calculated and measured strain gauge responses in Rosette 2, zero advance is equal to gauge position.
G7
G8
G9
-200
-100
0
100
200
300
400
500
600
700
-200 -150 -100 -50 0 50 100 150 200Coring advance ( mm )
Stra
in (
)
G1 ( R1_axi )
G2 ( R1_tan )
G3 ( R1_incl. )
G4 ( R2_axi )
G5 ( R2_tan )
G6 ( R2_incl. )
G7 ( R3_axi )
G8 ( R3_tan )
G9 ( R3_incl. )
m G1 ( R1_axi )
m G2 ( R1_tan )
m G3 ( R1_incl. )
m G4 ( R2_axi )
m G5 ( R2_tan )
m G6 ( R2_incl. )
m G7 ( R3_axi )
m G8 ( R3_tan )
m G9 ( R3_incl. )
Figure 6-9c. Äspö KK0045G01 34.77 m. Calculated and measured strain gauge responses in Rosette 3, zero advance is equal to gauge position.
70
G1
G2
G3
-200
-100
0
100
200
300
400
500
600
700
-200 -150 -100 -50 0 50 100 150 200Coring advance ( mm )
Stra
in (
)G1 ( R1_axi )
G2 ( R1_tan )
G3 ( R1_incl. )
G4 ( R2_axi )
G5 ( R2_tan )
G6 ( R2_incl. )
G7 ( R3_axi )
G8 ( R3_tan )
G9 ( R3_incl. )
m G1 ( R1_axi )
m G2 ( R1_tan )
m G3 ( R1_incl. )
m G4 ( R2_axi )
m G5 ( R2_tan )
m G6 ( R2_incl. )
m G7 ( R3_axi )
m G8 ( R3_tan )
m G9 ( R3_incl. )
Figure 6-10a. Äspö KK0045G01 35.48 m. Calculated and measured strain gauge responses in Rosette 1.
G4
G5
G6
-200
-100
0
100
200
300
400
500
600
700
-200 -150 -100 -50 0 50 100 150 200Coring advance ( mm )
Stra
in (
)
G1 ( R1_axi )
G2 ( R1_tan )
G3 ( R1_incl. )
G4 ( R2_axi )
G5 ( R2_tan )
G6 ( R2_incl. )
G7 ( R3_axi )
G8 ( R3_tan )
G9 ( R3_incl. )
m G1 ( R1_axi )
m G2 ( R1_tan )
m G3 ( R1_incl. )
m G4 ( R2_axi )
m G5 ( R2_tan )
m G6 ( R2_incl. )
m G7 ( R3_axi )
m G8 ( R3_tan )
m G9 ( R3_incl. )
Figure 6-10b. Äspö KK0045G01 35.48 m. Calculated and measured strain gauge responses in Rosette 2.
71
G7
G8
G9
-200
-100
0
100
200
300
400
500
600
700
-200 -150 -100 -50 0 50 100 150 200Coring advance ( mm )
Stra
in (
)G1 ( R1_axi )
G2 ( R1_tan )
G3 ( R1_incl. )
G4 ( R2_axi )
G5 ( R2_tan )
G6 ( R2_incl. )
G7 ( R3_axi )
G8 ( R3_tan )
G9 ( R3_incl. )
m G1 ( R1_axi )
m G2 ( R1_tan )
m G3 ( R1_incl. )
m G4 ( R2_axi )
m G5 ( R2_tan )
m G6 ( R2_incl. )
m G7 ( R3_axi )
m G8 ( R3_tan )
m G9 ( R3_incl. )
Figure 6-10c. Äspö KK0045G01 35.48 m. Calculated and measured strain gauge responses in Rosette 3. Transient stress analysis In both cases it is a high potential to have core damage (Figures 6-11 and 6-12). At 34.77 m depth the tensile stress exceeds the tensile strength and at 35.48 m depth tensile stress is over crack damage envelope. Inverse solution For both cases inverse solution with two different elastic parameter values was done (Table 6-6). It was reasonable to do the solution only for strains 50 mm after strain gauge position because the coring advance was not directly measured and is thus not accurate enough (Figure 5-13 and 5-14). Also the point interval is not adequate. The inverse solution results principal stresses within –1.9 MPa to 1.5 MPa and their orientations within –4° to +6° compared to closed from solution (Table 6-6, Figures 6-13 and 6-14). The deviation on resulting values can be considered small.
72
0
50
100
150
200
250
-20.0 -15.0 -10.0 -5.0 0.0 5.0 10.0
Minor Principal Stress ( MPa )
Ma
jor
Prin
cipa
l Stre
ss (
MP
a )
σPEAK
σCR ACK DAM AGE
σ3 = 0.05 σ1
σCR ACK INIT IATION
Stress path for point: σ1,MA X
Stress path for point: σ3,MIN
σucs
σt
σcd
σci
195 MPa16.0 MPa123 MPa75 MPa
====
Intact rock parameters:
Position of monitring points:
σ1,MAX at 135°σ3 ,MIN at 217°
Figure 6-11. Äspö KK0045G01 34.77 m. Elastic stress path for maximum compression and tension superimposed over intact rock strength envelopes.
0
50
100
150
200
250
-20.0 -15.0 -10.0 -5.0 0.0 5.0 10.0
Minor Principal St ress ( MPa )
Maj
or P
rinci
pal S
tres
s ( M
Pa )
σPEAK
σCR ACK DA MAGE
σ3 = 0.05 σ1
σCR ACK INIT IATION
Stress path for point: σ1,MAX
Stress path for point: σ3,MIN
σucsσt
σcd
σci
195 MPa16.0 MPa123 MPa
75 MPa
====
Intact rock parameters:
σ 1,MAX at 127°σ3,MIN at 207°
Position of monitring points:
Figure 6-12. Äspö KK0045G01 35.48 m. Elastic stress path for maximum compression and tension superimposed over intact rock strength envelopes.
73
Summary The study the two Äspö KK0045G01 cases showed that these two measurement can be considered relatively successful and full filling the assumptions of continuous, homogenous, isotropic and linearly elastic rock. Major uncertainty concerns the time dependent behavior of most strained strain gauges, changes in strain readings between core removal and biaxial testing and the deviation of gauge 5 in biaxial test. Based on the two measurements and the elastic parameters defined by SwedPower the magnitude of σ1 and σ2 is known with ±20% and σ3 with ±30% accuracy. The orientation of principal stresses is known with ±7° accuracy. Table 6-6. Principal stresses and their orientations based on closed form solution and inverse solution with two sets of deformation parameters. Äspö KK0045G01 34.77 m Äspö KK0045G01 35.48 m
Closed form solution
E 66 GPa E 56 GPaν 0.26 ν 0.17
component value (MPa) trend ( ° ) plunge ( ° ) component value (MPa) trend ( ° ) plunge ( ° )σ1 26.1 27 39 σ1 18.1 24 29σ2 17.3 271 29 σ2 12.3 271 35σ3 8.9 156 38 σ3 5.0 143 41
Inverse solution 50 mm after strain gauges
E 66 GPa E 56 GPaν 0.26 ν 0.17
component value (MPa) trend ( ° ) plunge ( ° ) component value (MPa) trend ( ° ) plunge ( ° )σ1 24.2 31 36 σ1 17.7 27 25σ2 16.0 276 31 σ2 11.8 275 39σ3 8.1 157 40 σ3 3.5 141 41
E 70 GPa E 61 GPaν 0.29 ν 0.21
component value (MPa) trend ( ° ) plunge ( ° ) component value (MPa) trend ( ° ) plunge ( ° )σ1 27.5 30 37 σ1 19.6 27 26σ2 18.6 273 31 σ2 13.5 272 41σ3 10.0 156 37 σ3 4.7 139 38
74
-80
-60
-40
-20
0
20
40
60
-200 -150 -100 -50 0 50 100 150 200
Coring advance ( mm )
Stre
ss (
MP
a )
σN
σE
σV
σNE
σEV
σVN
Inverse solution for coring advance of 160 mm:component value (MPa) bearing ( ° ) dip ( ° )
σ1 24.7 32 36 σ2 16.7 277 30 σ3 8.1 158 39
Figure 6-13. Äspö KK0045G01 34.77 m. Inverse solution for stress tensor components.
-30
-20
-10
0
10
20
30
40
-200 -150 -100 -50 0 50 100 150 200
Coring advance ( mm )
Stre
ss (
MP
a )
σN
σE
σV
σNE
σEV
σVN
component value (MPa)
σ1 17.1 25 27 σ2 11.6 272 38 σ3 3.5 141 40
Inverse solution for coring advance of 160 mm:
bearing ( ° ) dip ( ° )
Figure 6-14. Äspö KK0045G01 35.48 m. Inverse solution for stress tensor components.
75
6.2 Äspö KF0093A01 stress measurements From the borehole KF003A01 two measurements were studied; Point 1 (32.14 m) and Point 2 (35.38 m). Borehole, probe and strain gauge orientations for measurements are presented in Table 6-7. Table 6-7. Borehole and strain gauge orientations for Äspö KF003A01 stress measurement. Äspö HRLBorehole: KF0093A01
Measuring point 1 Measuring point 4
Depth [m] 32.14 35.38Hole Bearing [°] 310 310
Dip [°] -2 -2(upward) (upward)
mpass Bearing [°] 300 235(magnetic needle reading from zero-mark on compass)
Rosette & strain gauge orientationOrientation relative to Magnetic North
Bearing [°] Bearing [°]Rosette # 1 30 325 (relative to Magnetic North)
2 270 205 (relative to Magnetic North)3 150 85 (relative to Magnetic North)
Rosette # Strain gauge # Dip Dir [°] Dip [°] Dip Dir [°] Dip [°]1 1 30 90 325 90 (relative to Magnetic North)1 2 300 0 235 0 (relative to Magnetic North)1 3 300 45 235 45 (relative to Magnetic North)2 4 270 90 205 90 (relative to Magnetic North)2 5 180 0 115 0 (relative to Magnetic North)2 6 180 45 115 45 (relative to Magnetic North)3 7 150 90 85 90 (relative to Magnetic North)3 8 60 0 355 0 (relative to Magnetic North)3 9 60 45 355 45 (relative to Magnetic North)
Orientation relative to Äspö x-axisÄspö x-axis is 13.222 gon (11.9 degrees) west of Magnetic North
Bearing [°] Bearing [°]Rosette # 1 41.9 336.9 (relative to Äspö x-axis)
2 281.9 216.9 (relative to Äspö x-axis)3 161.9 96.9 (relative to Äspö x-axis)
Rosette # Strain gauge # Dip Dir [°] Dip [°] Dip Dir [°] Dip [°]1 1 41.9 90 336.9 90 (relative to Äspö x-axis)1 2 311.9 0 246.9 0 (relative to Äspö x-axis)1 3 311.9 45 246.9 45 (relative to Äspö x-axis)2 4 281.9 90 216.9 90 (relative to Äspö x-axis)2 5 191.9 0 126.9 0 (relative to Äspö x-axis)2 6 191.9 45 126.9 45 (relative to Äspö x-axis)3 7 161.9 90 96.9 90 (relative to Äspö x-axis)3 8 71.9 0 6.9 0 (relative to Äspö x-axis)3 9 71.9 45 6.9 45 (relative to Äspö x-axis)
Biaxial response of 32.14 level measurement is relatively well linear elastic. The linearity of level 35.38 is slightly worse (Figures 6-15 and 6-16). Some hysteresis and minor permanent strains exist. On the other hand both measurements showed clear heterogeneity or anisotropy (Table 6-7). The Young’s modulus shows indicates of
76
anisotropy varying systematically between –13% and 26% from the average (Figure 6-17). The anisotropy is close to reported limiting value 1.3, over which anisotropic solution is recommended. Noteworthy is also the difference in Poisson’s ratio value for depth 32.14 resulting from different calculation method (Table 6-7). The correlation coefficient for all strain gauges is between 0.82 – 0.96 which is not good. The relative difference of measured and calculated values for inclined strain gauges was between -7% and +16%, which is quite small. Table 6-7. Reported and recalculated values for elastic parameters for Äspö KF003A01 stress measurements. Biaxial results
Measuring point 1 Measuring point 4
Depth [m] 32.14 35.38
Reported biaxial results for unloading from 8 MPa to 3 MPa
All E [GPa] 51 53ν 0.19 0.22
Linear fit for total unloading from 8 MPa to 0 MPa
R1 E [GPa] 41 46ν 0.12 0.19
a_err -1 % 5 %∆E/E_all -13 % -13 %
R2 E [GPa] 61 50ν 0.16 0.19
a_err -7 % 7 %∆E/E_all 28 % -6 %
R3 E [GPa] 45 67ν 0.11 0.30
a_err 10 % 16 %∆E/E_all -6 % 26 %
All E [GPa] 48 53ν 0.13 0.22
a_err 2 % 9 %
77
-600
-500
-400
-300
-200
-100
0
100
200
0 2 4 6 8 10
Cell pressure ( MPa )
Mic
rost
rain
G1, R1_axi
G2, R1_tan
G3, R1_45
G4, R2_axi
G5, R2_tan
G6, R2_45
G7, R3_axi
G8, R3_tan
G9, R3_45
Figure 6-15. Äspö KF0093A01 32.14 m, biaxial test response.
-600
-500
-400
-300
-200
-100
0
100
200
0 2 4 6 8 10
Cell pressure ( MPa )
Mic
rost
rain
G1, R1_axi
G2, R1_tan
G3, R1_45
G4, R2_axi
G5, R2_tan
G6, R2_45
G7, R3_axi
G8, R3_tan
G9, R3_45
Figure 6-16. Äspö KF0093A01 35.38 m, biaxial test response.
78
100%
100%0%
Äspö North
90°
Figure 6-17. Amount of unexplained strain (AoUS) for different strain gauge rosettes for both Äspö KF0093A01 32.14 m and 35.38 m biaxial test. The angle from North corresponds with angular position of rosette and radial distance is the amount of unexplained strain Based on calculated strain behavior the location of strain gauges was defined to be 3 cm earlier for measurement at depth 32.14 m and 5 cm earlier for measurement at depth 35.38 m. This is because the coring advance is not measured directly but defined from end level and average overcoring speed. With given closed form solution for in situ state of stress for depth 32.14 m measurement the transient strain behavior of rosettes 1 and 2 shows bad correlation with calculated strains, even for early strains (Figure 6-18). Noteworthy is the continuous and unexpected drift in all strain gauges, which was over 200 microns during the last 20 minutes (not showed in Figure 6-18). Temperature change is one relevant reason for this because it is seen in all strain gauges. If an inverse solution is done based on measured strains at 50 mm after strain gauges better correlation with measured and calculated strains is achieved (Figure 6-19 and Table 6-8). Interesting is that tensile stresses can cause damage with high potential and the side for maximum tension is the same as for lowest Young’s modulus values (Figures 6-17, 6-20 and 6-21).
79
Case: Äspö KF0093A01 MP1 ( 32.14 m )
Principal stress ( NEV c.s., comp=positive ): Bore hole orientation ( NEV c.s.) Drilling loads ( comp.=positive ) Strain gauges ( B.H. c.s. ) Elastic propertiescomponent value (MPa) trend ( ° ) plunge ( ° ) rosette bearing ( ° )
σ1 32.5 307 38 trend 310 ( ° ) Drilling fluid pressure 0.0 MPa 1 41.9 E 51 ( GPa )σ2 13.8 96 48 plunge -2.0 ( ° ) Drill bit axial pressure 0.0 MPa 2 281.9 ν 0.19 ( )σ3 8.7 204 16 tan(φ ) at drill bit - rock 0.0 ( ) 3 161.9 ν_methd. interpolated
Visibility of gauge strain
gauge calc meas.G1, r1_axi
G2, r1_tan
G3, r1_incl
G4, r2_axi
G5, r2_tan
G6, r2_incl
G7, r3_axi
G8, r3_tan
G9, r3_incl
all
Calculate in situ stress
FALSE
G1
G2
G3
-200
-100
0
100
200
300
400
500
600
700
800
-200 -150 -100 -50 0 50 100 150 200Coring advance ( mm )
µStra
in (
)
G1 ( R1_axi )
G2 ( R1_tan )
G3 ( R1_incl. )
G4 ( R2_axi )
G5 ( R2_tan )
G6 ( R2_incl. )
G7 ( R3_axi )
G8 ( R3_tan )
G9 ( R3_incl. )
m G1 ( R1_axi )
m G2 ( R1_tan )
m G3 ( R1_incl. )
m G4 ( R2_axi )
m G5 ( R2_tan )
m G6 ( R2_incl. )
m G7 ( R3_axi )
m G8 ( R3_tan )
m G9 ( R3_incl. )
Calculate Inverse solution
Figure 6-18a. Äspö KF0093A01 32.14 m. Calculated and measured strain gauge responses in Rosette 1, zero advance is equal to gauge position.
Case: Äspö KF0093A01 MP1 ( 32.14 m )
Principal stress ( NEV c.s., comp=positive ): Bore hole orientation ( NEV c.s.) Drilling loads ( comp.=positive ) Strain gauges ( B.H. c.s. ) Elastic propertiescomponent value (MPa) trend ( ° ) plunge ( ° ) rosette bearing ( ° )
σ1 32.5 307 38 trend 310 ( ° ) Drilling fluid pressure 0.0 MPa 1 41.9 E 51 ( GPa )σ2 13.8 96 48 plunge -2.0 ( ° ) Drill bit axial pressure 0.0 MPa 2 281.9 ν 0.19 ( )σ3 8.7 204 16 tan(φ ) at drill bit - rock 0.0 ( ) 3 161.9 ν_methd. interpolated
Visibility of gauge strain
gauge calc meas.G1, r1_axi
G2, r1_tanG3, r1_incl
G4, r2_axi
G5, r2_tanG6, r2_incl
G7, r3_axi
G8, r3_tan
G9, r3_inclall
Calculate in situ stress
FALSE
G7
G8
G9
-100
0
100
200
300
400
500
600
700
-200 -150 -100 -50 0 50 100 150 200Coring advance ( mm )
µStra
in (
)
G1 ( R1_axi )
G2 ( R1_tan )
G3 ( R1_incl. )
G4 ( R2_axi )
G5 ( R2_tan )
G6 ( R2_incl. )
G7 ( R3_axi )
G8 ( R3_tan )
G9 ( R3_incl. )
m G1 ( R1_axi )
m G2 ( R1_tan )
m G3 ( R1_incl. )
m G4 ( R2_axi )
m G5 ( R2_tan )
m G6 ( R2_incl. )
m G7 ( R3_axi )
m G8 ( R3_tan )
m G9 ( R3_incl. )
Calculate Inverse solution
Figure 6-18b. Äspö KF0093A01 32.14 m. Calculated and measured strain gauge responses in Rosette 3, zero advance is equal to gauge position.
80
Case: Äspö KF0093A01 MP1 ( 32.14 m )
Principal stress ( NEV c.s., comp=positive ): Bore hole orientation ( NEV c.s.) Drilling loads ( comp.=positive ) Strain gauges ( B.H. c.s. ) Elastic propertiescomponent value (MPa) trend ( ° ) plunge ( ° ) rosette bearing ( ° )
σ1 28.8 298 36 trend 310 ( ° ) Drilling fluid pressure 0.0 MPa 1 41.9 E 49 ( GPa )σ2 10.9 100 52 plunge -2.0 ( ° ) Drill bit axial pressure 0.0 MPa 2 281.9 ν 0.13 ( )σ3 7.1 201 9 tan(φ ) at drill bit - rock 0.0 ( ) 3 161.9 ν_methd. extrapolated
Visibility of gauge strain
gauge calc meas.G1, r1_axi
G2, r1_tanG3, r1_incl
G4, r2_axi
G5, r2_tanG6, r2_incl
G7, r3_axi
G8, r3_tan
G9, r3_inclall
Calculate in situ stress
FALSE
G1
G2
G3
-200
-100
0
100
200
300
400
500
600
700
-200 -150 -100 -50 0 50 100 150 200Coring advance ( mm )
µStra
in (
)
G1 ( R1_axi )
G2 ( R1_tan )
G3 ( R1_incl. )
G4 ( R2_axi )
G5 ( R2_tan )
G6 ( R2_incl. )
G7 ( R3_axi )
G8 ( R3_tan )
G9 ( R3_incl. )
m G1 ( R1_axi )
m G2 ( R1_tan )
m G3 ( R1_incl. )
m G4 ( R2_axi )
m G5 ( R2_tan )
m G6 ( R2_incl. )
m G7 ( R3_axi )
m G8 ( R3_tan )
m G9 ( R3_incl. )
Calculate Inverse solution
Figure 6-19a. Äspö KF0093A01 32.14 m. Calculated and measured strain gauge responses in Rosette 1 new solution for in situ state of stress, zero advance is equal to gauge position.
Case: Äspö KF0093A01 MP1 ( 32.14 m )
Principal stress ( NEV c.s., comp=positive ): Bore hole orientation ( NEV c.s.) Drilling loads ( comp.=positive ) Strain gauges ( B.H. c.s. ) Elastic propertiescomponent value (MPa) trend ( ° ) plunge ( ° ) rosette bearing ( ° )
σ1 28.8 298 36 trend 310 ( ° ) Drilling fluid pressure 0.0 MPa 1 41.9 E 49 ( GPa )σ2 10.9 100 52 plunge -2.0 ( ° ) Drill bit axial pressure 0.0 MPa 2 281.9 ν 0.13 ( )σ3 7.1 201 9 tan(φ ) at drill bit - rock 0.0 ( ) 3 161.9 ν_methd. extrapolated
Visibility of gauge strain
gauge calc meas.G1, r1_axi
G2, r1_tanG3, r1_incl
G4, r2_axi
G5, r2_tanG6, r2_incl
G7, r3_axi
G8, r3_tan
G9, r3_inclall
Calculate in situ stress
FALSE
G4
G5
G6
-400
-200
0
200
400
600
800
1000
-200 -150 -100 -50 0 50 100 150 200Coring advance ( mm )
µStra
in (
)
G1 ( R1_axi )
G2 ( R1_tan )
G3 ( R1_incl. )
G4 ( R2_axi )
G5 ( R2_tan )
G6 ( R2_incl. )
G7 ( R3_axi )
G8 ( R3_tan )
G9 ( R3_incl. )
m G1 ( R1_axi )
m G2 ( R1_tan )
m G3 ( R1_incl. )
m G4 ( R2_axi )
m G5 ( R2_tan )
m G6 ( R2_incl. )
m G7 ( R3_axi )
m G8 ( R3_tan )
m G9 ( R3_incl. )
Calculate Inverse solution
Figure 6-19b. Äspö KF0093A01 32.14 m. Calculated and measured strain gauge responses in Rosette 2 new solution for in situ state of stress, zero advance is equal to gauge position.
81
Case: Äspö KF0093A01 MP1 ( 32.14 m )
Principal stress ( NEV c.s., comp=positive ): Bore hole orientation ( NEV c.s.) Drilling loads ( comp.=positive ) Strain gauges ( B.H. c.s. ) Elastic propertiescomponent value (MPa) trend ( ° ) plunge ( ° ) rosette bearing ( ° )
σ1 28.8 298 36 trend 310 ( ° ) Drilling fluid pressure 0.0 MPa 1 41.9 E 49 ( GPa )σ2 10.9 100 52 plunge -2.0 ( ° ) Drill bit axial pressure 0.0 MPa 2 281.9 ν 0.13 ( )σ3 7.1 201 9 tan(φ ) at drill bit - rock 0.0 ( ) 3 161.9 ν_methd. extrapolated
Visibility of gauge strain
gauge calc meas.G1, r1_axi
G2, r1_tanG3, r1_incl
G4, r2_axi
G5, r2_tanG6, r2_incl
G7, r3_axi
G8, r3_tan
G9, r3_inclall
Calculate in situ stress
FALSE
G7
G8
G9
-100
0
100
200
300
400
500
600
700
-200 -150 -100 -50 0 50 100 150 200Coring advance ( mm )
µStra
in (
)
G1 ( R1_axi )
G2 ( R1_tan )
G3 ( R1_incl. )
G4 ( R2_axi )
G5 ( R2_tan )
G6 ( R2_incl. )
G7 ( R3_axi )
G8 ( R3_tan )
G9 ( R3_incl. )
m G1 ( R1_axi )
m G2 ( R1_tan )
m G3 ( R1_incl. )
m G4 ( R2_axi )
m G5 ( R2_tan )
m G6 ( R2_incl. )
m G7 ( R3_axi )
m G8 ( R3_tan )
m G9 ( R3_incl. )
Calculate Inverse solution
Figure 6-19c. Äspö KF0093A01 32.14 m. Calculated and measured strain gauge responses in Rosette 3 new solution for in situ state of stress, zero advance is equal to gauge position. Table 6-8. Original closed form solution and inverse solution at 50 mm after strain gauges for Äspö KF0093A01 32.14 m in situ stress measurement. Original closed form solution
component value (MPa) trend ( ° ) plunge ( ° )σ1 32.5 307 38 σ2 13.8 96 48 σ3 8.7 204 16
Inverse solution at 50 mm after strain gauges
component value (MPa) trend ( ° ) plunge ( ° )σ1 28.8 298 36 σ2 10.9 100 52 σ3 7.1 201 9
82
Case: Äspö KF0093A01 MP1 ( 32.14 m )
Principal stress ( NEV c.s., comp=positive ): Bore hole orientation ( NEV c.s.) Drilling loads ( comp.=positive ) Strain gauges ( B.H. c.s. ) Elastic propertiescomponent value (MPa) trend ( ° ) plunge ( ° ) rosette bearing ( ° )
σ1 32.5 306.6 37.9 trend 310.0 ( ° ) Drilling fluid pressure 0.0 MPa 1 41.9 E 51 ( GPa )σ2 13.8 96.0 47.9 plunge -2.0 ( ° ) Drill bit axial pressure 0.0 MPa 2 281.9 ν 0.19 ( )σ3 8.7 204.0 15.6 tan(φ ) at drill bit - rock 0.0 ( ) 3 161.9 ν_methd. interpolated
Strength properties
σucs 195 ( MPa )σt 16.0 ( MPa )σcd 123 ( MPa )σci 75 ( MPa )
Visibility of stress maximums
Principal stress pathσ1,max TRUEσ3,min TRUE( σ1 - σ3 ) max FALSE
Bearing for stress maximumson pilot hole wall ( ° )σ1,max 282.5σ3,min 12.5( σ1 - σ3 ) max 282.5
0
50
100
150
200
250
-20.0 -15.0 -10.0 -5.0 0.0 5.0 10.0Minor Principal Stress ( MPa )
Maj
or P
rinci
pal S
tress
( M
Pa
)
sucs
scd
sci
s3 = 0.05 s1
s1, MAX
s3, MAX
( s1 - s3 ) max
Figure 6-20. Stress paths for maximum tension and compression with rock strength envelopes. Äspö KF0093A01 32.14 m .
Case: Äspö KF0093A01 MP1 ( 32.14 m )
Principal stress ( NEV c.s., comp=positive ): Bore hole orientation ( NEV c.s.) Drilling loads ( comp.=positive ) Strain gauges ( B.H. c.s. ) Elastic propertiescomponent value (MPa) trend ( ° ) plunge ( ° ) rosette bearing ( ° )
σ1 32.5 306.6 37.9 trend 310.0 ( ° ) Drilling fluid pressure 0.0 MPa 1 41.9 E 51 ( GPa )σ2 13.8 96.0 47.9 plunge -2.0 ( ° ) Drill bit axial pressure 0.0 MPa 2 281.9 ν 0.19 ( )σ3 8.7 204.0 15.6 tan(φ ) at drill bit - rock 0.0 ( ) 3 161.9 ν_methd. interpolated
Visibility of stress maximums
Principal stressσ3 , σ1,max TRUEσ1 , σ1,max TRUEσ3 , σ3,min TRUEσ1 , σ3,min TRUEσ3 , ( σ1 - σ3 ) max TRUEσ1 , ( σ1 - σ3 ) max TRUE
Bearing for stress maximumson pilot hole wall ( ° )σ1,max 282.5σ3,min 12.5( σ1 - σ3 ) max 282.5
-20
-10
0
10
20
30
40
50
60
70
-200 -150 -100 -50 0 50 100 150 200Coring advance ( mm )
Stre
ss (
MP
a )
s3 at s1,max
s1 at s1,max
s3 at s3,min
s1 at s3,min
s3 at ( s1 - s3 ) max
s1 at ( s1 - s3 ) max
Figure 6-21. Development of maximum tension and compression with advancing overcoring. Äspö KF0093A01 32.14 m . For 35.38 m measurement the measured transient strains before strain gauge location coincides mainly with calculated ones, but after cell position larger differences exist (Figure 6-22).
83
Case: Äspö KF0093A01 MP2 ( 35.38 m )
Principal stress ( NEV c.s., comp=positive ): Bore hole orientation ( NEV c.s.) Drilling loads ( comp.=positive ) Strain gauges ( B.H. c.s. ) Elastic propertiescomponent value (MPa) trend ( ° ) plunge ( ° ) rosette bearing ( ° )
σ1 23.2 308 10 trend 310 ( ° ) Drilling fluid pressure 0.0 MPa 1 336.9 E 53 ( GPa )σ2 14.2 44 30 plunge -2.0 ( ° ) Drill bit axial pressure 0.0 MPa 2 216.9 ν 0.22 ( )σ3 6.9 202 58 tan(φ ) at drill bit - rock 0.0 ( ) 3 96.9 ν_methd. interpolated
Visibility of gauge strain
gauge calc meas.G1, r1_axi
G2, r1_tan
G3, r1_incl
G4, r2_axi
G5, r2_tan
G6, r2_incl
G7, r3_axi
G8, r3_tan
G9, r3_incl
all
Calculate in situ stress
FALSE
G1
G2
G3
-100
0
100
200
300
400
500
-200 -150 -100 -50 0 50 100 150 200Coring advance ( mm )
µStra
in (
)
G1 ( R1_axi )
G2 ( R1_tan )
G3 ( R1_incl. )
G4 ( R2_axi )
G5 ( R2_tan )
G6 ( R2_incl. )
G7 ( R3_axi )
G8 ( R3_tan )
G9 ( R3_incl. )
m G1 ( R1_axi )
m G2 ( R1_tan )
m G3 ( R1_incl. )
m G4 ( R2_axi )
m G5 ( R2_tan )
m G6 ( R2_incl. )
m G7 ( R3_axi )
m G8 ( R3_tan )
m G9 ( R3_incl. )
Calculate Inverse solution
Figure 6-22a. Äspö KF0093A01 35.38 m. Calculated and measured strain gauge responses in Rosette 1, zero advance is equal to gauge position.
Case: Äspö KF0093A01 MP2 ( 35.38 m )
Principal stress ( NEV c.s., comp=positive ): Bore hole orientation ( NEV c.s.) Drilling loads ( comp.=positive ) Strain gauges ( B.H. c.s. ) Elastic propertiescomponent value (MPa) trend ( ° ) plunge ( ° ) rosette bearing ( ° )
σ1 23.2 308 10 trend 310 ( ° ) Drilling fluid pressure 0.0 MPa 1 336.9 E 53 ( GPa )σ2 14.2 44 30 plunge -2.0 ( ° ) Drill bit axial pressure 0.0 MPa 2 216.9 ν 0.22 ( )σ3 6.9 202 58 tan(φ ) at drill bit - rock 0.0 ( ) 3 96.9 ν_methd. interpolated
Visibility of gauge strain
gauge calc meas.G1, r1_axi
G2, r1_tan
G3, r1_incl
G4, r2_axi
G5, r2_tan
G6, r2_incl
G7, r3_axi
G8, r3_tan
G9, r3_incl
all
Calculate in situ stress
FALSE
G4
G5
G6
-200
-100
0
100
200
300
400
500
600
-200 -150 -100 -50 0 50 100 150 200Coring advance ( mm )
µStra
in (
)
G1 ( R1_axi )
G2 ( R1_tan )
G3 ( R1_incl. )
G4 ( R2_axi )
G5 ( R2_tan )
G6 ( R2_incl. )
G7 ( R3_axi )
G8 ( R3_tan )
G9 ( R3_incl. )
m G1 ( R1_axi )
m G2 ( R1_tan )
m G3 ( R1_incl. )
m G4 ( R2_axi )
m G5 ( R2_tan )
m G6 ( R2_incl. )
m G7 ( R3_axi )
m G8 ( R3_tan )
m G9 ( R3_incl. )
Calculate Inverse solution
Figure 6-22b. Äspö KF0093A01 35.38 m. Calculated and measured strain gauge responses in Rosette 2, zero advance is equal to gauge position.
84
Case: Äspö KF0093A01 MP2 ( 35.38 m )
Principal stress ( NEV c.s., comp=positive ): Bore hole orientation ( NEV c.s.) Drilling loads ( comp.=positive ) Strain gauges ( B.H. c.s. ) Elastic propertiescomponent value (MPa) trend ( ° ) plunge ( ° ) rosette bearing ( ° )
σ1 23.2 308 10 trend 310 ( ° ) Drilling fluid pressure 0.0 MPa 1 336.9 E 53 ( GPa )σ2 14.2 44 30 plunge -2.0 ( ° ) Drill bit axial pressure 0.0 MPa 2 216.9 ν 0.22 ( )σ3 6.9 202 58 tan(φ ) at drill bit - rock 0.0 ( ) 3 96.9 ν_methd. interpolated
Visibility of gauge strain
gauge calc meas.G1, r1_axi
G2, r1_tan
G3, r1_incl
G4, r2_axi
G5, r2_tan
G6, r2_incl
G7, r3_axi
G8, r3_tan
G9, r3_incl
all
Calculate in situ stress
FALSE
G7
G8
G9
-100
0
100
200
300
400
500
-200 -150 -100 -50 0 50 100 150 200Coring advance ( mm )
µStra
in (
)
G1 ( R1_axi )
G2 ( R1_tan )
G3 ( R1_incl. )
G4 ( R2_axi )
G5 ( R2_tan )
G6 ( R2_incl. )
G7 ( R3_axi )
G8 ( R3_tan )
G9 ( R3_incl. )
m G1 ( R1_axi )
m G2 ( R1_tan )
m G3 ( R1_incl. )
m G4 ( R2_axi )
m G5 ( R2_tan )
m G6 ( R2_incl. )
m G7 ( R3_axi )
m G8 ( R3_tan )
m G9 ( R3_incl. )
Calculate Inverse solution
Figure 6-22c. Äspö KF0093A01 35.38 m. Calculated and measured strain gauge responses in Rosette 3, zero advance is equal to gauge position. The inverse solution based on strains 50 mm after strain gauges gives clearly better correspondence for transient strains after strain gauge position (Figure 6-23). The magnitude of resulting in situ state of stress is almost the same with original closed form solution, but intermediate and minor principal stresses are rotated about 12° around major principal stress (Table 6-9). In 35.38 measurement no damage is assumed (Figure 6-24). Table 6-9. Original closed form solution and inverse solution at 50 mm after strain gauges for Äspö KF0093A01 35.38 m in situ stress measurement. Original closed form solution
component value (MPa) trend ( ° ) plunge ( ° )σ1 23.2 308 10 σ2 14.2 44 30 σ3 6.9 202 58
Inverse solution at 50 mm after strain gauges
component value (MPa) trend ( ° ) plunge ( ° )σ1 23.5 307 9 σ2 14.3 45 42 σ3 6.9 207 47
85
Case: Äspö KF0093A01 MP2 ( 35.38 m )
Principal stress ( NEV c.s., comp=positive ): Bore hole orientation ( NEV c.s.) Drilling loads ( comp.=positive ) Strain gauges ( B.H. c.s. ) Elastic propertiescomponent value (MPa) trend ( ° ) plunge ( ° ) rosette bearing ( ° )
σ1 23.5 307 9 trend 310 ( ° ) Drilling fluid pressure 0.0 MPa 1 336.9 E 53 ( GPa )σ2 14.3 45 42 plunge -2.0 ( ° ) Drill bit axial pressure 0.0 MPa 2 216.9 ν 0.22 ( )σ3 6.9 207 47 tan(φ ) at drill bit - rock 0.0 ( ) 3 96.9 ν_methd. interpolated
Visibility of gauge strain
gauge calc meas.G1, r1_axi
G2, r1_tan
G3, r1_incl
G4, r2_axi
G5, r2_tan
G6, r2_incl
G7, r3_axi
G8, r3_tan
G9, r3_inclall
Calculate in situ stress
FALSE
G1
G2G3
-100
0
100
200
300
400
500
-200 -150 -100 -50 0 50 100 150 200Coring advance ( mm )
µStra
in (
)
G1 ( R1_axi )
G2 ( R1_tan )
G3 ( R1_incl. )
G4 ( R2_axi )
G5 ( R2_tan )
G6 ( R2_incl. )
G7 ( R3_axi )
G8 ( R3_tan )
G9 ( R3_incl. )
m G1 ( R1_axi )
m G2 ( R1_tan )
m G3 ( R1_incl. )
m G4 ( R2_axi )
m G5 ( R2_tan )
m G6 ( R2_incl. )
m G7 ( R3_axi )
m G8 ( R3_tan )
m G9 ( R3_incl. )
Calculate Inverse solution
Figure 6-23a. Äspö KF0093A01 35.38 m. Calculated and measured strain gauge responses in Rosette 1, zero advance is equal to gauge position.
Case: Äspö KF0093A01 MP2 ( 35.38 m )
Principal stress ( NEV c.s., comp=positive ): Bore hole orientation ( NEV c.s.) Drilling loads ( comp.=positive ) Strain gauges ( B.H. c.s. ) Elastic propertiescomponent value (MPa) trend ( ° ) plunge ( ° ) rosette bearing ( ° )
σ1 23.5 307 9 trend 310 ( ° ) Drilling fluid pressure 0.0 MPa 1 336.9 E 53 ( GPa )σ2 14.3 45 42 plunge -2.0 ( ° ) Drill bit axial pressure 0.0 MPa 2 216.9 ν 0.22 ( )σ3 6.9 207 47 tan(φ ) at drill bit - rock 0.0 ( ) 3 96.9 ν_methd. interpolated
Visibility of gauge strain
gauge calc meas.G1, r1_axi
G2, r1_tan
G3, r1_incl
G4, r2_axi
G5, r2_tan
G6, r2_incl
G7, r3_axi
G8, r3_tan
G9, r3_incl
all
Calculate in situ stress
FALSE
G4
G5
G6
-200
-100
0
100
200
300
400
500
600
-200 -150 -100 -50 0 50 100 150 200Coring advance ( mm )
µStra
in (
)
G1 ( R1_axi )
G2 ( R1_tan )
G3 ( R1_incl. )
G4 ( R2_axi )
G5 ( R2_tan )
G6 ( R2_incl. )
G7 ( R3_axi )
G8 ( R3_tan )
G9 ( R3_incl. )
m G1 ( R1_axi )
m G2 ( R1_tan )
m G3 ( R1_incl. )
m G4 ( R2_axi )
m G5 ( R2_tan )
m G6 ( R2_incl. )
m G7 ( R3_axi )
m G8 ( R3_tan )
m G9 ( R3_incl. )
Calculate Inverse solution
Figure 6-23b. Äspö KF0093A01 35.38 m. Calculated and measured strain gauge responses in Rosette 2, zero advance is equal to gauge position.
86
Case: Äspö KF0093A01 MP2 ( 35.38 m )
Principal stress ( NEV c.s., comp=positive ): Bore hole orientation ( NEV c.s.) Drilling loads ( comp.=positive ) Strain gauges ( B.H. c.s. ) Elastic propertiescomponent value (MPa) trend ( ° ) plunge ( ° ) rosette bearing ( ° )
σ1 23.5 307 9 trend 310 ( ° ) Drilling fluid pressure 0.0 MPa 1 336.9 E 53 ( GPa )σ2 14.3 45 42 plunge -2.0 ( ° ) Drill bit axial pressure 0.0 MPa 2 216.9 ν 0.22 ( )σ3 6.9 207 47 tan(φ ) at drill bit - rock 0.0 ( ) 3 96.9 ν_methd. interpolated
Visibility of gauge strain
gauge calc meas.G1, r1_axi
G2, r1_tan
G3, r1_incl
G4, r2_axi
G5, r2_tan
G6, r2_incl
G7, r3_axi
G8, r3_tan
G9, r3_inclall
Calculate in situ stress
FALSE
G7
G8
G9
-100
0
100
200
300
400
500
-200 -150 -100 -50 0 50 100 150 200Coring advance ( mm )
µStra
in (
)
G1 ( R1_axi )
G2 ( R1_tan )
G3 ( R1_incl. )
G4 ( R2_axi )
G5 ( R2_tan )
G6 ( R2_incl. )
G7 ( R3_axi )
G8 ( R3_tan )
G9 ( R3_incl. )
m G1 ( R1_axi )
m G2 ( R1_tan )
m G3 ( R1_incl. )
m G4 ( R2_axi )
m G5 ( R2_tan )
m G6 ( R2_incl. )
m G7 ( R3_axi )
m G8 ( R3_tan )
m G9 ( R3_incl. )
Calculate Inverse solution
Figure 6-23c. Äspö KF0093A01 35.38 m. Calculated and measured strain gauge responses in Rosette 3, zero advance is equal to gauge position.
Case: Äspö KF0093A01 MP2 ( 35.38 m )
Principal stress ( NEV c.s., comp=positive ): Bore hole orientation ( NEV c.s.) Drilling loads ( comp.=positive ) Strain gauges ( B.H. c.s. ) Elastic propertiescomponent value (MPa) trend ( ° ) plunge ( ° ) rosette bearing ( ° )
σ1 23.5 306.6 9.2 trend 310.0 ( ° ) Drilling fluid pressure 0.0 MPa 1 336.9 E 53 ( GPa )σ2 14.3 44.9 41.8 plunge -2.0 ( ° ) Drill bit axial pressure 0.0 MPa 2 216.9 ν 0.22 ( )σ3 6.9 206.6 46.7 tan(φ ) at drill bit - rock 0.0 ( ) 3 96.9 ν_methd. interpolated
Strength properties
σucs 195 ( MPa )σt 16.0 ( MPa )σcd 123 ( MPa )σci 75 ( MPa )
Visibility of stress maximums
Principal stress pathσ1,max TRUEσ3,min TRUE( σ1 - σ3 ) max FALSE
Bearing for stress maximumson pilot hole wall ( ° )σ1,max 47.5σ3,min 132.5( σ1 - σ3 ) max 47.5
0
50
100
150
200
250
-20.0 -15.0 -10.0 -5.0 0.0 5.0 10.0Minor Principal Stress ( MPa )
Maj
or P
rinci
pal S
tress
( M
Pa
)
sucs
scd
sci
s3 = 0.05 s1
s1, MAX
s3, MAX
( s1 - s3 ) max
Figure 6-24. Stress paths for maximum tension and compression with rock strength envelopes. Äspö KF0093A01 35.38 m. The analysis of the two Äspö KF0093A01 measurements indicates that rock or overcored specimen has deformation anisotropy, of 30%. The 32.14 measurement is clearly unreliable and there is a potential for core damage which explains the higher interpreted in situ state of stress. The 35.38 m measurement is more reliable, closed form and inverse solutions for in situ state of stress are close to each other. Based on
87
Young’s modulus value the maximum deviation for in situ stress magnitudes is between –12% and +26%. 6.3 AECL URL, Hole 209-022-0C2 The case is from Underground Research Laboratory, Atomic Energy of Canada Ltd. Test level is about 240 m below ground surface. Overcoring in situ stress measurement was done with modified CSIR-cell which has three strain gauge rosettes 120 degrees apart from each other and each of them has four strain gauges (axial, tangential, 45 and 135 degrees inclined). The three 135 degrees inclined gauges were ignored in this study. The biaxial response of case is reversible but clearly non-linear (Figure 6-15), which indicates damage in core. Also, the response of 120° rosette indicates unacceptable behavior resulting Young’s modulus of 103 MPa and Poisson’s ratio of 0.32. The average values of other two rosettes, E=50 GPa and n=0.20, are used in further analyses.
Figure 6-15. Biaxial test results of AECL URL, Hole 209-022-0C2, depth 11.91 m overcoring measurement. The comparison of strains with calculated in situ state of stress shows relatively good agreement for rosettes at 0° and 240°. Bad response for rosette at 120° was assumed based on biaxial result (Figures 6 – 16abc). Noteworthy is that strain gauge position is obviously 40 mm later than defined based on transient strain comparison. Also, lower
88
maximum and minimum values are caused by 10 mm thicker overcoring cylinder (86 mm vs. 76 mm), which also prevents early strain inverse solution.
Case: AECL URL, Hole 209-022-0C2, depth 11.91 m
Principal stress ( NEV c.s., comp=positive ): Bore hole orientation ( NEV c.s.) Drilling loads ( comp.=positive ) Strain gauges ( B.H. c.s. ) Elastic propertiescomponent value (MPa) trend ( ° ) plunge ( ° ) rosette bearing ( ° )
σ1 27.1 222 27 trend 187 ( ° ) Drilling fluid pressure 0.2 MPa 1 0.0 E 50 ( GPa )σ2 15.1 335 37 plunge -35.7 ( ° ) Drill bit axial pressure 1.0 MPa 2 120.0 ν 0.20 ( )σ3 10.8 106 41 tan(φ ) at drill bit - rock 0.5 ( ) 3 240.0 ν_methd. interpolated
Visibility of gauge strain
gauge calc meas.G1, r1_axi
G2, r1_tan
G3, r1_incl
G4, r2_axi
G5, r2_tan
G6, r2_incl
G7, r3_axi
G8, r3_tan
G9, r3_incl
all
Calculate in situ stress
TRUE
G1
G2
G3
-200
-100
0
100
200
300
400
500
600
-200 -150 -100 -50 0 50 100 150 200Coring advance ( mm )
µStra
in (
)
G1 ( R1_axi )
G2 ( R1_tan )
G3 ( R1_incl. )
G4 ( R2_axi )
G5 ( R2_tan )
G6 ( R2_incl. )
G7 ( R3_axi )
G8 ( R3_tan )
G9 ( R3_incl. )
m G1 ( R1_axi )
m G2 ( R1_tan )
m G3 ( R1_incl. )
m G4 ( R2_axi )
m G5 ( R2_tan )
m G6 ( R2_incl. )
m G7 ( R3_axi )
m G8 ( R3_tan )
m G9 ( R3_incl. )
Calculate Inverse solution
Figure 6-16a. Measured and calculated transient strains for rosetta at 0°. AECL URL, Hole 209-022-0C2, depth 11.91 m.
Case: AECL URL, Hole 209-022-0C2, depth 11.91 m
Principal stress ( NEV c.s., comp=positive ): Bore hole orientation ( NEV c.s.) Drilling loads ( comp.=positive ) Strain gauges ( B.H. c.s. ) Elastic propertiescomponent value (MPa) trend ( ° ) plunge ( ° ) rosette bearing ( ° )
σ1 27.1 222 27 trend 187 ( ° ) Drilling fluid pressure 0.2 MPa 1 0.0 E 50 ( GPa )σ2 15.1 335 37 plunge -35.7 ( ° ) Drill bit axial pressure 1.0 MPa 2 120.0 ν 0.20 ( )σ3 10.8 106 41 tan(φ ) at drill bit - rock 0.5 ( ) 3 240.0 ν_methd. interpolated
Visibility of gauge strain
gauge calc meas.G1, r1_axi
G2, r1_tan
G3, r1_incl
G4, r2_axi
G5, r2_tan
G6, r2_incl
G7, r3_axi
G8, r3_tan
G9, r3_incl
all
Calculate in situ stress
TRUE
G4
G5
G6
-200
-100
0
100
200
300
400
500
-200 -150 -100 -50 0 50 100 150 200Coring advance ( mm )
µStra
in (
)
G1 ( R1_axi )
G2 ( R1_tan )
G3 ( R1_incl. )
G4 ( R2_axi )
G5 ( R2_tan )
G6 ( R2_incl. )
G7 ( R3_axi )
G8 ( R3_tan )
G9 ( R3_incl. )
m G1 ( R1_axi )
m G2 ( R1_tan )
m G3 ( R1_incl. )
m G4 ( R2_axi )
m G5 ( R2_tan )
m G6 ( R2_incl. )
m G7 ( R3_axi )
m G8 ( R3_tan )
m G9 ( R3_incl. )
Calculate Inverse solution
Figure 6-16b. Measured and calculated transient strains for rosetta at 120°. AECL URL, Hole 209-022-0C2, depth 11.91 m.
89
Case: AECL URL, Hole 209-022-0C2, depth 11.91 m
Principal stress ( NEV c.s., comp=positive ): Bore hole orientation ( NEV c.s.) Drilling loads ( comp.=positive ) Strain gauges ( B.H. c.s. ) Elastic propertiescomponent value (MPa) trend ( ° ) plunge ( ° ) rosette bearing ( ° )
σ1 27.1 222 27 trend 187 ( ° ) Drilling fluid pressure 0.2 MPa 1 0.0 E 50 ( GPa )σ2 15.1 335 37 plunge -35.7 ( ° ) Drill bit axial pressure 1.0 MPa 2 120.0 ν 0.20 ( )σ3 10.8 106 41 tan(φ ) at drill bit - rock 0.5 ( ) 3 240.0 ν_methd. interpolated
Visibility of gauge strain
gauge calc meas.G1, r1_axi
G2, r1_tan
G3, r1_incl
G4, r2_axi
G5, r2_tan
G6, r2_incl
G7, r3_axi
G8, r3_tan
G9, r3_inclall
Calculate in situ stress
TRUE
G7
G8
G9
-400
-200
0
200
400
600
800
1000
1200
1400
-200 -150 -100 -50 0 50 100 150 200Coring advance ( mm )
µStra
in (
)
G1 ( R1_axi )
G2 ( R1_tan )
G3 ( R1_incl. )
G4 ( R2_axi )
G5 ( R2_tan )
G6 ( R2_incl. )
G7 ( R3_axi )
G8 ( R3_tan )
G9 ( R3_incl. )
m G1 ( R1_axi )
m G2 ( R1_tan )
m G3 ( R1_incl. )
m G4 ( R2_axi )
m G5 ( R2_tan )
m G6 ( R2_incl. )
m G7 ( R3_axi )
m G8 ( R3_tan )
m G9 ( R3_incl. )
Calculate Inverse solution
Figure 6-16c. Measured and calculated transient strains for rosetta at 240°. AECL URL, Hole 209-022-0C2, depth 11.91 m. The possibility for core damage is relatively high while transient tensile stresses over the crack damage stress exist (Figure 6-17). The maximum tension takes place between rosettes 240° and 0°.
Case: AECL URL, Hole 209-022-0C2, depth 11.91 m
Principal stress ( NEV c.s., comp=positive ): Bore hole orientation ( NEV c.s.) Drilling loads ( comp.=positive ) Strain gauges ( B.H. c.s. ) Elastic propertiescomponent value (MPa) trend ( ° ) plunge ( ° ) rosette bearing ( ° )
σ1 27.1 222.0 27.0 trend 187.2 ( ° ) Drilling fluid pressure 0.2 MPa 1 0.0 E 50 ( GPa )σ2 15.1 335.0 37.0 plunge -35.7 ( ° ) Drill bit axial pressure 1.0 MPa 2 120.0 ν 0.20 ( )σ3 10.8 106.0 41.0 tan(φ ) at drill bit - rock 0.5 ( ) 3 240.0 ν_methd. interpolated
Strength properties
σucs 200 ( MPa )σt 20.0 ( MPa )σcd 160 ( MPa )σci 80 ( MPa )
Visibility of stress maximums
Principal stress pathσ1,max TRUEσ3,min TRUE( σ1 - σ3 ) max TRUE
Bearing for stress maximumson pilot hole wall ( ° )σ1,max 62.5σ3,min 327.5( σ1 - σ3 ) max 62.5
0
50
100
150
200
250
-25.0 -20.0 -15.0 -10.0 -5.0 0.0 5.0 10.0Minor Principal Stress ( MPa )
Maj
or P
rinci
pal S
tress
( M
Pa
)
sucs
scd
sci
s3 = 0.05 s1
s1, MAX
s3, MAX
( s1 - s3 ) max
Figure 6-17. Paths for maximum tension and deviatoric stress at strain gauge location. AECL URL, Hole 209-022-0C2, depth 11.91 m.
90
Different closed form solutions and inverse solutions for final strains were made (Table 6-9). In all these solutions Young’s modulus was 50 GPa and Poisson’s ratio was 0.20. The closed form solution for nine gauges is closest to the original solution for twelve gauges. Table 6-9. Different closed form solutions and inverse solutions for AECL URL, Hole 209-022-0C2, depth 11.91 m. Closed form solution for nine gauges
stress component magnitude (MPa) trend (º) plunge (º) σ1 27.1 222 27 σ2 15.1 335 37 σ3 10.8 106 41
Original closed form solution for twelve gauges
stress component magnitude (MPa) trend (º) plunge (º) σ1 28.2 224 26 σ2 15.0 335 37 σ3 9.8 108 42
The inverse solution for nine final strains
stress component magnitude (MPa) trend (º) plunge (º) σ1 26.0 228 32 σ2 15.3 4 49 σ3 10.3 123 23
The inverse solution for six final strains, rosette at 120° ignored
stress component magnitude (MPa) trend (º) plunge (º) σ1 24.5 237 32 σ2 12.8 21 52 σ3 5.0 136 18
With the six gauge inverse solution the correlation between rosettes at 0° and 120° are highest, but quite bad for early strains of rosette at 120° (Figure 6-18ab). Therefore it can be assumed that the first three interpretations are reasonable. In these interpretations the magnitudes of principal stresses are close to each other but moderate scatter is resulted for σ2 and σ3 orientations.
91
Case: AECL URL, Hole 209-022-0C2, depth 11.91 m
Principal stress ( NEV c.s., comp=positive ): Bore hole orientation ( NEV c.s.) Drilling loads ( comp.=positive ) Strain gauges ( B.H. c.s. ) Elastic propertiescomponent value (MPa) trend ( ° ) plunge ( ° ) rosette bearing ( ° )
σ1 24.5 237 32 trend 187 ( ° ) Drilling fluid pressure 0.2 MPa 1 0.0 E 50 ( GPa )σ2 12.8 21 52 plunge -35.7 ( ° ) Drill bit axial pressure 1.0 MPa 2 120.0 ν 0.20 ( )σ3 5.0 136 18 tan(φ ) at drill bit - rock 0.5 ( ) 3 240.0 ν_methd. interpolated
Visibility of gauge strain
gauge calc meas.G1, r1_axi
G2, r1_tan
G3, r1_incl
G4, r2_axi
G5, r2_tan
G6, r2_incl
G7, r3_axi
G8, r3_tan
G9, r3_inclall
Calculate in situ stress
FALSE
G1
G2G3
G7
G8
G9
-400
-200
0
200
400
600
800
1000
1200
1400
-200 -150 -100 -50 0 50 100 150 200Coring advance ( mm )
µStra
in (
)
G1 ( R1_axi )
G2 ( R1_tan )
G3 ( R1_incl. )
G4 ( R2_axi )
G5 ( R2_tan )
G6 ( R2_incl. )
G7 ( R3_axi )
G8 ( R3_tan )
G9 ( R3_incl. )
m G1 ( R1_axi )
m G2 ( R1_tan )
m G3 ( R1_incl. )
m G4 ( R2_axi )
m G5 ( R2_tan )
m G6 ( R2_incl. )
m G7 ( R3_axi )
m G8 ( R3_tan )
m G9 ( R3_incl. )
Calculate Inverse solution
Figure 6-18a. Measured and calculated transient strains for rosette at 0° and 240°. AECL URL, Hole 209-022-0C2, depth 11.91 m.
Case: AECL URL, Hole 209-022-0C2, depth 11.91 m
Principal stress ( NEV c.s., comp=positive ): Bore hole orientation ( NEV c.s.) Drilling loads ( comp.=positive ) Strain gauges ( B.H. c.s. ) Elastic propertiescomponent value (MPa) trend ( ° ) plunge ( ° ) rosette bearing ( ° )
σ1 24.5 237 32 trend 187 ( ° ) Drilling fluid pressure 0.2 MPa 1 0.0 E 50 ( GPa )σ2 12.8 21 52 plunge -35.7 ( ° ) Drill bit axial pressure 1.0 MPa 2 120.0 ν 0.20 ( )σ3 5.0 136 18 tan(φ ) at drill bit - rock 0.5 ( ) 3 240.0 ν_methd. interpolated
Visibility of gauge strain
gauge calc meas.G1, r1_axi
G2, r1_tan
G3, r1_incl
G4, r2_axi
G5, r2_tan
G6, r2_incl
G7, r3_axi
G8, r3_tan
G9, r3_incl
all
Calculate in situ stress
FALSE
G4
G5G6
-200
-100
0
100
200
300
400
500
-200 -150 -100 -50 0 50 100 150 200Coring advance ( mm )
µStra
in (
)
G1 ( R1_axi )
G2 ( R1_tan )
G3 ( R1_incl. )
G4 ( R2_axi )
G5 ( R2_tan )
G6 ( R2_incl. )
G7 ( R3_axi )
G8 ( R3_tan )
G9 ( R3_incl. )
m G1 ( R1_axi )
m G2 ( R1_tan )
m G3 ( R1_incl. )
m G4 ( R2_axi )
m G5 ( R2_tan )
m G6 ( R2_incl. )
m G7 ( R3_axi )
m G8 ( R3_tan )
m G9 ( R3_incl. )
Calculate Inverse solution
Figure 6-18b. Measured and calculated transient strains for rosette at 120°. AECL URL, Hole 209-022-0C2, depth 11.91 m.
92
93
7 DISCUSSION AND RECOMMENDATIONS The current version of developed code can’t take account thermal effects and anisotropy. With minor development and calculation of new tensor files it is possible to simulate thermal strains with reasonable accuracy by interpolating effects of flush water temperature, drill bit contact temperature, overcoring speed and thermal properties of rock. Capability for limited transverse isotropy, known degree of anisotropy and free orientation, can also be developed. This requires that the degree of anisotropy is defined by using different Young’s modulus and Poisson’s ratio values in a direction perpendicular and along the plane of anisotropy. Preliminary, only one Young’s modulus ration and one Poisson’s ratio value will be used to keep number of precalculations in reasonable level (i.e. E⊥/E|| = constant and ν⊥/ν|| = constant, where || is for parallel to anisotropy plane and ⊥ is for perpendicular to anisotropy plane) Test phase of the code gave ideas of following helpful features to be further developed: - Ability to calculate coring advance for measured date based on algorithm to
minimize difference when compared to calculated strains. - Numbers to describe the quality of inverse solution like amount of unexplained
strains or correlation coefficient or R^2. - Inverse solution for mean σ1, σ2, σ3 and their orientations from selected range of
coring advance. - Strain pattern around pilot hole for given in situ state of stress and selected inverse
solution, showing strain gauge positions and measured strains also. - Scatter for selected inverse solution. As a result of this work a quality control strategy for overcoring in situ stress measurement data interpretation was formed. The developed code can be used to analyse objectively the measured transient strains, judge if CHILE condition are fulfilled, estimate the damage potential and in some cases to find better solution for in situ state of stress. This strategy is shown phase by phase in Figures 7-1 to 7-5.
94
Are the assumptions for continuity, homogeneity and isotropy fulfilled for the
probe and strain gauge location ?
Yes/NoNo
what is the estimation of disturbance, and, is it reasonable to continue analysis ?
Yesclassify strain gauges to ,
and based on information on improper gluing, fracturing, other damage
or bigger crystals with different deformation behaviour
good moderaterejected
Yes/No
Noreject measurement
Figure 7-1. Flowchart of core logging data analysis.
95
Is biaxial test result consistent with CHILE condition assumption ?
Yes/No
Isotropic
Nowhat is the estimation for disturbance and is it reasonable to go continue analysis ?
Yes/NoNo
reject measurement
Yesclassify strain gauges to
and based on information of non-linear elastic behaviour, time dependency
or permanent deformation. If clear evidences and possible, leave out strain
gauges
good, moderaterejected
Test isotropic and transversely isotropic assumptions for biaxial tests results
I / T
Continue with transversely isotropic assumpitions.
Developed OCQ-code can not be used.
Yescalculation of elastic parameter values
and their deviation from unloading with linear elastic assumption.
classify elastic parameters to and reliability
good, moderate low
Transversely Isotropic
Figure 7-2. Flowchart of biaxial test data analysis.
96
Were the strain readings stable before and after overcoring ?
Apply closed form solution for in situ state of stress. Classify results to
and reliabilitygood,
moderate low
Yes/NoNo
the usability of measured values is depending on the drift magnitude
compared to magnitude of released strains. Is it reasonable to continue
analysis ?
Yes/NoNo
reject measurement
Yeswas the cell temperature steady during the
overcoring ?
Yes/NoNo
the usability of transient strain data is depending on the magnitude of thermally
induced strains compared to released strains
Yes/NoNo
reject measurement
Yesclassify strain gauges to good, moderate
and rejected based on information of time dependency or unexpected behaviour
Figure 7-3. Flowchart of overcoring data analysis.
97
Are the trends and magnitudes of measured transient strains close to ones
calculated by the OCS-code ?
Yes/NoNo
calculate new in situ state of stress with inverse method using all accepted strain gauges, note that early strain solution is
possible only when coring advance accuracy is better than +- 2.5% of pilot
hole diameter ( +- 1 mm for 38 mm )
Is the calculated in situ stress field same from early strains and final strain ?
Yes/NoCheck failure potential ?
No failure potential.Measured transient strains are close to
calculated ones.
Check for both in situ stresses:- failure potential
- if the trends and magnitudes of measured transient strains close to ones
calculated by the OCS-code ?
Result ?
Noreject measurement
Considerably failure potential.Measured early strains are close to
calculated ones.Continue with early strain solution.
Failure potential or not.Measured transient strains don’t fit with
calculated ones.
Classify resulting in situ state of stress as or reliability.good, moderate low
Figure 7-4. Flowchart of transient strain data analysis.
98
Give estimate for average in situ state of stress and scatter for it.
Calculate in situ state of stress based on a) good strain gauges,
b) with good and moderate strain gauges and c) with all non-rejected strain gauges
If reasonable based on geology and location of measuring points, analyse all stress measurements representing same conditions together in the same way as
presented above. Take geology into account in interpretation.
Compare scatter with average stresses, take account considerations from previous
phases and reasonable common assumptions and judge the reliability of
measurement
If more than one measurement ?
No/Yes
Give suggestions for further use ie.for numerical simulations etc.
Figure 7-5. Flowchart of in situ stress interpretation. During this work discussion about optimum strain gauge configuration for overcoring probe was raised up. From author’s point of view it is first of all a hardware matter but a compromise between overcoring and biaxial interpretation also. In overcoring of homogeneous isotropic rock the strain patterns for 45º and 135º aligned strain gauges are non-symmetric but mirror images of each other. The tangential strain pattern is half-symmetric and the axial strain pattern is always a circle. In case of anisotropic rock the strain patterns can be more complicated. If the strain gauges with different orientation are placed more equally around pilot hole a better overall strain pattern is achieved. But from quality assurance point of view it is better to have at least four (axial, tangential, 45º and 135º inclined) strain gauges at the same point, thus giving possibility to calculate one strain from the three other ones. It is highly recommend to have center points of each strain gauge at the same level to make transient strain analyses more reliable, specially in case of any core damage. In biaxial testing of homogeneous isotropic rock all axial, tangential and 45º/135º aligned strain gauges have same response. But interpretation of transverse isotropy involves at least three rosettes each having axial, tangential and inclined strain gauges (Nunes 2002). If the number of strain
99
gauges is limited to 9 it is suggested to have three rosettes 120 degrees apart from each other. In case of 12 strain gauges a) fourth strain gauge can be added to each rosette, b) one three gauge rosette can be added or c) two tangential and one inclined strain gauges can be added. In all these configurations it is recommended to keep the three rosettes 120º apart from each other.
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8 CONCLUSIONS Technically the developed code fulfilled all the objectives. It can be used to inteprete in situ state of stress of CHILE rock based on early strains, compare measured transient strains to calculated ones and estimate the core damage potential based on elastic stresses at strain gauge area. The code is quick enough to be a practical tool and it can be used as one of the main components in an overall quality control procedure for overseeing overcoring measurements and their reduction. The basic idea can be applied also to other overcoring probes with minor modifications and recalculation of stress tensors. Based on case studies, the interpretation of in situ state of stress from on early strains is difficult because the solution is very sensitive for measured strains and coring advance. The normal heterogeneity of so called homogeneous rock, change in rock temperature and accuracy to define coring advance can ruin the interpretation. On the other hand the comparison of measured strains to calculated ones, finding unreliable strains responses, estimation of core damage potential and inverse solution for final strains worked well. The estimation of core damage potential requires of course laboratory test results of intact rock strength. The work showed clearly that good quality in situ overcoring stress measurement involves a great understanding of all affecting factors and controlled quality of preparation, measuring and interpretation work. Key matters are:
- Knowledge of the local geology i.e. if rock is continuous, homogeneous, isotropic and linear elastic both in stain gauge and pilot hole scale.
- Selection, stability and control of glue. - Temperature control during and after overcoring, depending on the probe used, even
one degree change in core temperature can have clear effect on resulting in situ state of stress.
- Measurement of coring advance, depending on the device the interval should be close to 5 mm and accuracy +- 1 mm.
- Interpretation of biaxial test results. - Strain monitoring from probe installation to after biaxial test state to identify possible
sources of unexpected response.
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REFERENCES Amadei, B. 1996. Importance of Anisotropy When Estimating and Measuring In situ Stress in Rock. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. Vol 33, No. 3, pp. 293-325. Amadei, B. and Stephansson, O. 1997. Rock stress and its measurement. Chapman & Hall, London. Cai, M. and Thomas, L. J. 1973. Performance of overcoring stress measurement device in various rock types and conditions. Trans. Instn Min. Meteall. 102, May-August 1993. Chen, C., Pan, E. and Amadei, B. 1998. Determination of Deformability and Tensile Strength of Anisotropic Rock Using Brazian Test. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. Vol 35, No. 1, pp. 43-61. Christiansson, R., Hudson, J., A., 2002. Quality control of in situ rock stress measurements: Lesson from the Äspö Hard Rock Laboratory, Sweden. Eitzenberger A., 2002. Determination of the Degree of Anisotropy on Cores From Äspö HRL. Svensk Kärnbränslehantering AB. Fouial, K., Alheib, M., Baroudi, H. and Trentsaux, C. 1998. Improvement in the interpretation of stress measurements by use of the overcoring method: development of a new approach. Engineering Geology 49, pp. 239-252. Hakala, M. 1998. Numerical study on core damage and interpretation of in situ state of stress. Helsinki, Finland: Posiva Oy. Posiva-99-25. Hallbjörn, L., Ingevald, K., Martna, J., Strindell., 1990. New automatic probe for measuring triaxial stress in deep boreholes. Tunneling Underground Spece Technology 1990 5(1/2):141-5. Hoek, E. & Brown, E.T., 1997. Practical estimates of rock mass strength, Int.Journal Rock Mechanics & Mining Science & Geomechanics Abstracts. 34(8), 1165-1186. Hudson, J. A. and Harrison, J. P. 1997. Engineering Rock Mechanics, An introduction to the principles. Elsivier Science Ldt Irwin, R. A., Garritty, P. and Farmer I. W., 1987. The effect of boundary yield on the results of in situ stress measurement using overcoring techniques. Int. J. Rock Mech. Min Sci. & Geomech. Abstr. Vol 24, No 1, pp 89-93. ISRM., 1996. Rock characterization testing and monitoring, ISRM Suggested methods. Brown E. T., editor. Pergamon Press, Oxford.
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FLAC3D (Itasca Consulting Group, Inc. ,1997). Fast Lagrengian Analyses of Continua in 3-Dimensions, version 2.0, Users Manual. Minneapolis, Minnesota, USA). Kim, K. and Franklin, J.A. (coordinators) 1987. Suggested methods for rock stress determination. Int. J. Rock Mech. Min Sci. & Geomech. Abstr. Vol 24, No 1, pp 55-73. Li Y., 1997. Drilling-induced core damage and its Relationship to crystal in situ state of stress and rock properties. Ph.D. thesis, University of Alberta. Edmonton, Alberta. Leeman, E. R., 1970. The CSIR ”Doorstopper” and triaxial rock stress measuring instrument. Rock Mechanics 3, pp 25-50. Ljunggren, C., Klasson, H., 1996. Rock stress measurement at Zedex test area, Äspö HRL. Äspö Hard Rock Laboratory, Technical note TN-96-08z, Stockholm. Mueller, B., Reinecker, J., Heidbach, O. and Fuchs, K. 2000. The 2000 release of the World Stress Map (available online at www.world-stress-map.org) Nunes, A.L.L.S. 2002. A new method for determination of transverse isotropic orientation and the associated elastic parameters for intact rock. Int. J. Rock Mech. Min. Sci., 39, pp. 257-273. Read, R.S., Chandler, N.A & Dzipk, E.J., 1998. In situ Strength Criteria for Tunnel Design in Highly-stressed Rock Masses. Int. J. Rock Mechanics & Mining Science, Vol. 35, No 3 pp. 261-278.
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APPENDIX – 1. FLAC DATA FOR HOEK-CELL MODEL new ; --- Hoek Cell test ; --- cylinder length is twise the Hoek cell length ; --- axi-symmetry Config axi ; --- model grid Grid 6,61 ; --- linear elastic material model Mo elast ; --- fix grid to model coordinate system ; --- 38 mm / 62 mm cylinder Gen 0.019,-0.2 0.019,-0.1 0.031,-0.1 0.031,-0.2 rat 1.2,1 j=1,16 Gen 0.019,-0.1 0.019,-0.015 0.031,-0.015 0.031,-0.1 rat 1.2,1 j=16,30 Gen 0.019,-0.015 0.019,0.015 0.031,0.015 0.031,-0.015 rat 1.2,1 j=30,33 Gen 0.019, 0.015 0.019,0.1 0.031,0.1 0.031,0.015 rat 1.2,1 j=33,47 Gen 0.019,0.1 0.019,0.2 0.031,0.2 0.031,0.1 rat 1.2,1 j=47,62 ; --- 38 mm / 92 mm cylinder ; Gen 0.019,-0.2 0.019,-0.1 0.046,-0.1 0.046,-0.2 rat 1.2,1 j=1,16 ; Gen 0.019,-0.1 0.019,-0.015 0.046,-0.015 0.046,-0.1 rat 1.2,1 j=16,30 ; Gen 0.019,-0.015 0.019,0.015 0.046,0.015 0.046,-0.015 rat 1.2,1 j=30,33 ; Gen 0.019, 0.015 0.019,0.1 0.046,0.1 0.046,0.015 rat 1.2,1 j=33,47 ; Gen 0.019,0.1 0.019,0.2 0.046,0.2 0.046,0.1 rat 1.2,1 j=47,62 ; --- elastic material parameter values Prop d 2700 b 40e9 sh 24e9 ; --- apply cell pressure of 10 MPa to middle half apply sxx=-10e6 i=7 j=16,47 ; --- delete model parts outside the cell i.e. cylinder length = cell legth ; model null j=1,15 ; model null j=47,62 ; --- calculate to force equilibrium step 5000 ; --- resulting fig pl hold bound mag 2000 yel bo apply max 5e3 red pl hold bo str ret
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a.) b) Figure 1. Model grid for 38 mm / 62 mm rock cylinder in Hoek cell (a) and original boundary, applied force vectors and 2000 times magnified deformed boundary (b).
1 (1)
LIST OF REPORTS 6.4.2006 POSIVA-REPORTS 2006 POSIVA 2006-01 Effects of Salinity and High pH on Crushed Rock and Bentonite -experimental Work and Modelling Ulla Vuorinen, Ari Luukkonen, Heini Ervanne, Jarmo Lehikoinen April 2006 ISBN 951-652-142-8 POSIVA 2006-02 Petrology of Olkiluoto
Aulis Kärki, Kivitieto Oy Seppo Paulamäki, Geological Survey of Finland April 2006 ISBN 951-652-143-6
POSIVA 2006-03 Quality Controll for Overcoring Stress Measurement Data Matti Hakala, Gridpoint Finland Oy April 2006 ISBN 951-652-126-6
