rock_mass

56
ROCK MASS CLASSIFICATION Presented by: Neil Benson

Upload: gaddargaddar

Post on 13-Apr-2015

16 views

Category:

Documents


3 download

TRANSCRIPT

Page 1: Rock_Mass

ROCK MASS CLASSIFICATION

Presented by:

Neil Benson

Page 2: Rock_Mass

Rock Mass ClassificationRock Mass Classification

• Why?• How does this help us in tunnel design?

Page 3: Rock_Mass

Rock Mass ClassificationRock Mass ClassificationWHY?WHY?

Page 4: Rock_Mass

Ground interactionGround interaction

Page 5: Rock_Mass

Summary of rock mass characteristics, testingSummary of rock mass characteristics, testingmethods and theoretical considerationsmethods and theoretical considerations

Page 6: Rock_Mass

Types of failure which occur in rock massesTypes of failure which occur in rock massesunder low and high in-situ stress levelsunder low and high in-situ stress levels

Page 7: Rock_Mass
Page 8: Rock_Mass
Page 9: Rock_Mass

Relationship between support pressureRelationship between support pressureand tunnel deformation for different ratios of rockand tunnel deformation for different ratios of rock

mass strength to in-situ stressmass strength to in-situ stress

Page 10: Rock_Mass

Estimates of support capacity for tunnelsEstimates of support capacity for tunnelsof different sizesof different sizes

Page 11: Rock_Mass

Engineering Rock Mass Classification SchemesEngineering Rock Mass Classification Schemes

• Developed for estimation of tunnel support

• Used at project feasibility and preliminary design stages

• Simple check lists or detailed schemes

• Used to develop a picture of the rock mass and its

variability

• Used to provide initial empirical estimates of tunnel

support requirements

• Are practical engineering tools which force the user to

examine the properties of the rock mass

• Do Not replace detailed design methods

• Project specific

Page 12: Rock_Mass

Terzaghi’s Rock Mass Classification (1946)Terzaghi’s Rock Mass Classification (1946)

• Rock Mass Descriptions

– Intact

– Stratified

– Moderately jointed

– Blocky and Seamy

– Crushed

– Squeezing

– Swelling

Page 13: Rock_Mass

Rock Quality Designation Index (RQD)Rock Quality Designation Index (RQD)(Deere et al. 1967)(Deere et al. 1967)

• Aim : to provide a quantitative estimate of rock mass

quality from drill logs

• Equal to the percentage of intact core pieces longer than

100mm in the total length of core

Page 14: Rock_Mass

RQDRQD

• Directionally dependant parameter

• Intended to indicate rock mass quality in-situ

• Adapted for surface exposures as ‘Jv’ number of

discontinuities per unit volume

• Used as a component in the RMR and Q systems

Page 15: Rock_Mass

Procedure for Measurement and Calculation of RQDProcedure for Measurement and Calculation of RQD

Page 16: Rock_Mass

Weathering of Basalt with depthWeathering of Basalt with depth

Page 17: Rock_Mass
Page 18: Rock_Mass

Multi parameter Rock Mass Classification Multi parameter Rock Mass Classification SchemesSchemes

• Rock Mass Structure Rating (RSR)

• Rock Mass Rating (RMR)

• Rock Tunnelling Quality Index (Q)

Page 19: Rock_Mass

Rock Mass Structure Rating (RSR) (1972)Rock Mass Structure Rating (RSR) (1972)

• Introduced the concept of rating components to arrive at

a numerical value

• Demonstrates the logic in a quasi-quantitative rock mass

classification

• Has limitations as based on small tunnels supported by

steel sets only

• RSR = A + B + C

Page 20: Rock_Mass

Rock Structure RatingRock Structure RatingParameter A: General area geologyParameter A: General area geology

Considers (a) rock type origin(b) rock ‘hardness’(c) geotechnical structure

Page 21: Rock_Mass

Considers (a) joint spacing(b) joint orientation (strike and dip)(c) direction of tunnel drive

Rock Structure RatingRock Structure RatingParameter B: Geometry : Effect of discontinuity patternParameter B: Geometry : Effect of discontinuity pattern

Page 22: Rock_Mass

Considers (a) overall rock mass quality (on the basis of A + B)(b) joint condition(c) water inflow

Rock Structure RatingRock Structure RatingParameter C: Groundwater, joint conditionParameter C: Groundwater, joint condition

Page 23: Rock_Mass

RSR support estimates for a 7.3m diameterRSR support estimates for a 7.3m diametercircular tunnelcircular tunnel

(After Wickham et al. 1972)

ExamplesRSR = 622” shotcrete1” rockbolts @ 5ft centres

RSR = 305” shotcrete1” rockbolts @2.5ft centresOR 8WF31 steelsets @ 3ft centres

Page 24: Rock_Mass

Geomechanics Classification orGeomechanics Classification orRock Mass Rating System (RMR) (Bieniawski 1976)Rock Mass Rating System (RMR) (Bieniawski 1976)

Based upon

• uniaxial compressive strength of rock material

• rock quality designation (RQD)

• spacing of discontinuities

• condition of discontinuities

• groundwater conditions

• orientation of discontinuities

Page 25: Rock_Mass

Rock Mass Rating SystemRock Mass Rating System

• Rock mass divided into structural regions

• Each region is classified separately

• Boundaries can be rock type or structural, eg: fault

• Can be sub divided based on significant changes, eg:

discontinuity spacing

Page 26: Rock_Mass

Rock Mass Rating SystemRock Mass Rating System

Page 27: Rock_Mass

Rock Mass Rating SystemRock Mass Rating System

BUT: 1976 to 1989 Bienawski

• System refined by greater data

• Ratings for parameters changed

• Adapted by other workers for different situations

• PROJECT SPECIFIC SYSTEMS

Page 28: Rock_Mass

Development of Rock Mass Rating SystemDevelopment of Rock Mass Rating System

Page 29: Rock_Mass

Rock Mass Rating SystemRock Mass Rating System

(After Beiniawski 1989)

Page 30: Rock_Mass

Guidelines for excavation and support of 10mGuidelines for excavation and support of 10mspan rock tunnels in accordance with the RMR systemspan rock tunnels in accordance with the RMR system

(After Beiniawski 1989)

Page 31: Rock_Mass

Rock Tunnelling Quality Index QRock Tunnelling Quality Index Q

• Based on case histories in Scandinavia

• Numerical values on a log scale

• Range 0.001 to 1000

Page 32: Rock_Mass

‘‘Q’ Classification SystemQ’ Classification System

(After Barton et al. 1974)

Page 33: Rock_Mass

‘‘Q’ Classification SystemQ’ Classification System

(After Barton et al. 1974)

• represents the structure of the rockmass• crude measure of block or particle size

Page 34: Rock_Mass

‘‘Q’ Classification SystemQ’ Classification System

(After Barton et al. 1974)

• represents roughness and frictional characteristics of joint walls or infill material

Page 35: Rock_Mass

‘‘Q’ Classification SystemQ’ Classification System

(After Barton et al. 1974)

• consists of two stress parameters• SRF can be regarded as a total stress parameter measure of

– loosening load as excavated through shear zones– rock stress in competent rock– squeezing loads in plastic incompetent rock

• JW is a measure of water pressure

Page 36: Rock_Mass

Classification of individual parameters used in Classification of individual parameters used in the Tunnelling Quality Index Qthe Tunnelling Quality Index Q

Page 37: Rock_Mass

Classification of individual parameters used in Classification of individual parameters used in the Tunnelling Quality Index Q the Tunnelling Quality Index Q (cont’d)

Page 38: Rock_Mass

Classification of individual parameters used in Classification of individual parameters used in the Tunnelling Quality Index Q the Tunnelling Quality Index Q (cont’d)

Page 39: Rock_Mass

Q Classification SchemeQ Classification Scheme

Resolves to three parameters

• Block size ( RQD / Jn )

• Interblock shear strength ( Jr / Ja )

• Active stress ( Jw / SRF )

Page 40: Rock_Mass

Q Classification SchemeQ Classification Scheme

Resolves to three parameters

• Block size ( RQD / Jn )

• Interblock shear strength ( Jr / Ja )

• Active stress ( Jw / SRF )

• Does NOT include joint orientation

Page 41: Rock_Mass

Equivalent Dimension DEquivalent Dimension Dee

Page 42: Rock_Mass

Estimated support categories based on the Estimated support categories based on the tunnelling quality index Qtunnelling quality index Q

Page 43: Rock_Mass

Rock Mass Classification SystemRock Mass Classification System

• RMR and Q system or variants are the most widely used

• both incorporate geological, geometric and

design/engineering parameters to obtain a “value” of

rock mass quality

• empirical and require subjective assessment

Page 44: Rock_Mass

Rock Mass Classification SystemRock Mass Classification System

Approach:

• accurately characterise the rockmass ie: full and

complete description of the rockmass

• assign parameters for classification later

• always use two systems for comparison

Page 45: Rock_Mass

Prediction of in-situ deformation modulus EPrediction of in-situ deformation modulus Emm

from rock mass classificationsfrom rock mass classifications

Page 46: Rock_Mass

Geological Strength Index (GSI)Geological Strength Index (GSI)

• Method to link the constants m and j of Hoek-Brown

failure criterion to observations in the field

ie: a possible solution to the problem of estimating

strength of jointed rockmass

• A system for estimating the reduction in rockmass

strength for different geological conditions

• Overcomes deficiencies of RMR for poor quality rock

Page 47: Rock_Mass

Estimate of Geological Strength Index GSIEstimate of Geological Strength Index GSIbased on geological descriptionsbased on geological descriptions

Page 48: Rock_Mass

Estimation of constants based upon rockEstimation of constants based upon rockmass structure and discontinuity surface conditionsmass structure and discontinuity surface conditions

Page 49: Rock_Mass

Plots of cohesive strength and friction angles Plots of cohesive strength and friction angles for different GSI and for different GSI and mmii values values

Page 50: Rock_Mass

Swan Street ShaftSwan Street Shaft

Page 51: Rock_Mass
Page 52: Rock_Mass
Page 53: Rock_Mass
Page 54: Rock_Mass
Page 55: Rock_Mass
Page 56: Rock_Mass

Conceptual Geological StructureConceptual Geological Structure