experimental and numerical modelling of the weathering effects on shaft lining material

8/12/2019 Experimental and numerical modelling of the weathering effects on shaft lining material

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Experimental and numerical modelling of the weathering effects on shaft

lining material

W. Yang, Y. Jia, A.M. Marshall, D. Wanatowski, and R. Stace

Abstract

The collapse of abandoned and often hidden mine shafts is a serious problem in the UK and

many parts of Europe. The collapse of these shafts is often related to the failure of the shaft

lining. To analyse shafts for stability, the properties of the lining need to be well defined.

Many UK shafts are lined by multiple rings of brickwork. Brickwork is a complex structure

that is relatively poorly understood in this context. This paper presents an experimental and

numerical study of the effects of weathering on the brickwork. The influence of harsh

environmental conditions and time were particularly examined. The test samples examined

consisted of mortar and brick cylinders, as well as brickwork beams. To reproduce the

weathering process in the lab, the samples were placed into three water baths of potable water,

an artificial mine water and an aggressive acidic solution, for about one year. Four phases oflaboratory tests were conducted throughout the time period in order to assess the degradation

of mechanical properties. A corresponding numerical model of the brickwork beams was

built using FLAC-3D and validated against experimental data.

Introduction

Masonry was used extensively for mine shaft construction in the UK up to the 1950s. Many

shafts were lined by multiple rings of brickwork (constituting brick and mortar). This form of

construction is no longer routinely used and its structural analysis is not covered by modern

design codes. To analyse the stability of these older abandoned shafts, it is important to

simulate realistically the behaviour of the brickwork lining. The properties of the lining needto be well defined. Various researchers have sought to understand the behaviour of brickwork

including experimental tests [1-5] and numerical modelling [6-11]. However, the effects of

time and environmental conditions on the brickwork are particularly important in the context

of shaft linings since the strength of the brickwork could be considerably degraded with time

and chemical attack. Very limited literature has been found on the subject of weathering

effects on brickwork. Therefore, in order to have a better understanding of the time and

environmental effects on shaft lining brickwork and to collect realistic parameters necessary

for numerical modelling of shaft linings, weathering tests were conducted.

This study was undertaken as part of the Mine Shafts: Improving Security and New Tools

for the Evaluation of Risks (MISSTER) project, funded by the European CommissionResearch Fund for Coal and Steel (RFCS).

Experiment study of weathering effects on brickwork

Test Material

The aim of this study was to determine harsh-mine water and time effects on brickwork.

Brick, mortar and brickwork were tested. The bricks used were Mellowed Red Sovereign

Stock supplied by Wienerberger. The mortar samples were prepared based on BS PD 6678

[12]. The compressive strength class of the mortar was M6 and the cement: sand ratio was

155:710 (mixed by mass). The mortar was cast as cubes and cured for 28 days and then cored

into cylinders for later laboratory tests. The dimensions of the cylinders were 37mm diameter


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and 74mm height. Cylinders of brick were also prepared to the same specifications. In order

to test the brickwork lining as a structure, brickwork beams were designed and prepared for

testing according to BS EN 1052-2 [13]. The dimensions of the brickwork beams were:

89025095 mm (length height width). The test materials are shown in Figure 1.

Figure 1: Test material a) brick, b) mortar and c) brickwork beam.

Test method and Experiment setup

The weathering tests involved first immersing the samples in water baths of varying chemical

composition and then performing laboratory tests at different time intervals. The immersion

process was used to replicate the environmental conditions which cause degradation of

shaft/fill/capping materials. Laboratory tests were used to evaluate the weathering effects on

mechanical parameters of the test materials. The test samples were placed into potable water,

mine water and an aggressive acidic solution for about one year. Four phases of laboratory

tests were conducted during this time. Phase 0 tested the initial material properties of the

samples. Every 16 weeks,degradation of the structural parameters of the test materials was

evaluated by uniaxial compressive strength tests and four point bending tests.

Immersion

The setup of the immersion process is shown in Figure 2. The expected concentration of the

main chemical components and PH of the mine water and aggressive acidic solutions are

given in Table 1. The properties of the mine water were based on data obtained from the

RFCS PRESIDENCE project report (RFCR-CT-2007-00004) [14]. The aggressive acidic

solution was used to accelerate the weathering process in order to gain insight into the longer-

term weathered conditions of the materials. The PH and concentration of the solutions were

kept as constant as possible during the tests by refreshing the solution every 3 weeks. The PH

of the mine water and aggressive acidic solution was monitored during the tests by a PH

meter. Water pumps were used in the containers to circulate the water to ensure uniform

distribution of the chemicals in the solution.

a) b)

c)


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Laboratory testing

The samples were tested at four stages throughout the weathering process (Phase 0 baseline

test, followed by Phases 1 to 3 at approximately 16 week intervals). The uniaxial compressive

strength test (UCS) was conducted to measure Youngs modulus () and compressive

strength (c) of the cylinder samples and the four-point bending test (FPB) was used tomeasure the flexural strength of the brickwork beams. By comparing the results obtained

from different phases, the water and time effects on the material were studied. The

degradation of material properties with time and chemical attack were examined. The

measured parameters of the material were then used in numerical models which were

developed to further analyse the effect of weathering on overall brickwork stability.

Table 1: Concentration of the main chemical components for the immersions

Mgmg/l

Namg/l

Clmg/l

SO4mg/l

PH

Mine water (PRESIDENCE report) 31 15.7 13 360 6.0Mine water (used in this study) 40 14.5 12 353 5.2Aggressive acidic solution 400 724 600 8182 1.3

Figure 2: Immersion baths.

Test results

Uniaxial compressive test

The uniaxial compressive test is arguably the most popular and fundamental method for rock

testing. In this study, the cylindrical samples (brick and mortar as shown in Figure 1) were

tested. The test machine and experiment set-up is shown in Figure 3. During the test, vertical

load was applied on the samples and gradually increased until the samples failed. The loading

rate for the test was 0.02mm/min. The load applied was measured by a load cell and the

vertical deformation of the samples was measured by two linear variable differential

transformers (LVDTs). The average results from two LVDTs at each side of the samples

were used to calculate axial strain. For each material from each solution, 5 samples were

Brickwork beam

Mortar and brick cylinders


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tested. The test procedure followed the International Society for Rock Mechanics: testing

measure [15].

Figure 3: Set-up of theuniaxial compressive test.

The stress-strain relationship of the brick and mortar are shown in Figure 4. According to the

results of Figure 4a/c/e, the stress-strain curves of brick do not show a clear trend when

comparing Phase 0 through to Phase 3 for any of the immersion conditions (potable water,

mine water, or aggressive acidic solution). The reason for this could be that the brick samples

showed considerable variability in results at any given stage of testing (due to the variability

of brick composition and build quality) and therefore it is difficult to distinguish the

weathering effects on the samples.

For Mortar (Figure 4b/d/f), the effect of both water and the acidic solutions is clear. It can be

seen in Figure 4b/d that at Phase 3 (after 48 weeks), the stiffness (given by the slope of thecurves within the initial linear range) of the mortar is clearly less than phase 0, 1 and 2 from

both solutions. The maximum strength, however, does not appear to have been significantly

affected during this time for these samples. The effect of the aggressive acidic water on the

mortar is observed to be considerable in Figure 4f. A significant reduction of stiffness of the

mortar samples from each phase is observed. At the onset of loading, the stiffness is noted to

be very small (given by a shallow line in the stress-strain data). This is likely due to the fact

that the acid attack opens many micro cracks in the samples (especially at the surface),

making the surface of sample much weaker and resulting in a low overall stiffness. The inner,

less weathered regions of the cores eventually have the effect of increasing the stiffness of the

samples during tests. The overall effect of the varying degree of weathering is a bi-linear

stiffness relationship. In Figure 4f, the Phase 3 results for the mortar in the aggressive acidicsolution are not given because the material was deteriorated to a state that could not be tested.

The strength and stiffness of the mortar at this stage had effectively reduced to zero.

The results of UCS tests show that mortar is very sensitive to the acidic attack; a clear

reduction of both strength and stiffness was observed. For the brick, taking into account the

considerable variability of test results at a given phase of testing, there was no clear evidence

of significant weathering taking place for the duration of the tests. Explorations of several

mine shafts in the UK undertaken as part of this projectalso showed that the state of

deterioration of the mortar was generally greater than that of the bricks (though evidence of

considerable brick deterioration was also found). In general, the results indicate that in terms

of the weathering process, the mortar is likely to be more susceptible to degradation andtherefore play the dominant role in reducing the overall stability of mine shafts over time.

a) b)

LVDTLoad cell


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Figure 6: Four-point bending test results (a) force versus displacement; (b) flexural strength.

Numerical modelling of the four-point bending test

A macro-modelling approach [8] was adopted for the numerical modelling in order to be able

to scale the brickwork lining model up to the size of a real shaft lining. In this method, the

composite brickwork material is modelled as a uniform material. Adopting a micro-modelling

approach in which each brick and mortar element is modelled individually is very

computationally expensive and would prohibit the modelling of full-scale shafts. The

objective of this modelling exercise was therefore to determine a set of equivalent uniform

material properties that could be used effectively to replicate the behaviour of the composite

brickwork beams. Further work will consider a full-scale mine shaft and the effect of

reducing the mechanical properties of the lining (to simulate weathering) on overall stability.

Figure 7b shows the FLAC-3D model that was built to simulate the four point bending tests.

The boundary conditions and loading were designed to match the experimental conditions. In

order to obtain the equivalent uniform material properties, the constitutive Strain-Softening

(SS) model was used for the uniform brickwork material in the 3D model. This constitutive

model is based on the Mohr-Coulomb model in which the cohesion, friction angle and tensile

strength are assumed to remain constant. In the SS model, these strength properties may

soften (reduce) after the onset of plastic yield.

0 1 2 3 4 5 6

x 10-3

0

1

2

3

4

5

6

displacment [mm]

Force[

kN]

Phase 1 Phase 3Phase 2Phase 0

Phase 0 Phase1 Phase 2 Phase 30

0.5

1

1.5

Potable water

Aggressive acidic solutionFlexuralstrength

[N/mm

2]

(a)

(b)


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Figure 7: (a)Flexural strength test of brickwork beam (BS EN 1052-2);(b) FLAC-3D model of the brickwork beam

Figure 8a shows a comparison between results obtained from the 3 experimental tests

(labelled LabFPBT-#) and the numerical simulations. The methodology adopted to obtain the

equivalent material properties for brickwork was to adjust the material properties to make the

force-deflection curve from the numerical model match that of the laboratory tests. Based on

the results of a parametric study, it was found that the stiffness and tensile strength had the

dominating effect on the deformation of the beam. The values of cohesion and friction angle

were therefore kept the same as those of brick (Table 2). An upper and lower bound of

equivalent material properties for brickwork was found based on the experimental data.

Figure 8a shows that the match between the numerical results and the experimental data isgood. The detailed equivalent material properties (upper bound and lower bound) are shown

in Table 2 and Figure 8b. Note that friction and cohesion values reported in Table 2 are based

on a series of triaxial tests performed on the brick cores at Phase 0 (not reported here) and

that the tensile strength was taken as 1/10th the measured UCS.

Figure 8:(a) Comparison of experimental and numerical FPBT results, (b) strain-softening

model values of tensile strength.


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Table 2: Brickwork properties used in numerical models.

Material Density

(kg/m3)

Youngsmodulus

(GPa)

Poissonsratio

Cohesion

(MPa)

Frictionangle

(degrees)

Tensilestrength (MPa)

Brick 1960 2.96 0.15 2.39 46 1.18Upper bound model 1960 1.56 0.15 2.39 46 See Figure 8b

Lower bound model 1960 1.36 0.15 2.39 46 See Figure 8b

Conclusions

A programme of weathering tests was undertaken to study the effect of harsh environmental

conditions that are characteristic of flooded mine shafts on the mechanical properties of

brickwork lining constituent elements, namely brick and mortar. Three different immersion

solutions were used: potable water, mine water and an aggressive acidic solution. Four phases

of laboratory tests (with a fixed time interval of 16 weeks) were conducted to assess thedegradation of the mechanical properties of the materials.

Results from UCS tests on brick did not show conclusive trends of deterioration over time.

The effects of weathering on brick were difficult to discern due to the inherent variability in

the brick material. However, the weathering process had a pronounced effect on the mortar.

A considerable decrease in the stiffness and strength of mortar samples was measured. For

mortar, the Youngs modulus was found to be more sensitive than compressive strength to

acid attack. Variable degrees of weathering resulted in a bi-linear trend of mortar stiffness

during loading.

The results of four-point bending tests on brickwork beams did not show a clear trend of

degradation of flexural strength due to weathering. The results of the tests showed aconsiderable degree of variability which made it difficult to make conclusions regarding the

weathering effects on strength. The load-deflection response did show an effect of weathering

and, as for the mortar samples, showed a bi-linear trend which may indicate a partial degree

of weathering.

A FLAC-3D model was built to simulate the four-point bending test. A set of equivalent

uniform material properties was determined which gave a reasonable prediction of the load-

deflection behaviour of the composite brickwork beams tested in the lab. The advantage of

the uniform material mesh is that it can be effectively scaled up to model full-sized shaft

linings without incurring impractical model run times. The effect of weathering on mine shaft

stability will be explored using this numerical model in a future paper.

Acknowledgements

The authors would like to acknowledge the financial support provided by the European

Commission Research Programme of the Research Fund for Coal and Steel (RFCS). The

work described in this paper was undertaken as part of the Mine Shafts: Improving Security

and New Tools for the Evaluation of Risks (MISSTER) project, funded by the RFCS.


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References

[1] Vermeltfoort, A., Martens, D., Kleinman, C. &Zijl, G. (2005) Brick-mortar interaction in

masonry under compression. PhD Thesis, TU Eindhoven.

[2] Mohammed A.G. (2006) Experimental comparison of brickwork behaviour at prototype

and model scales. PhD thesis.Cardiff University.[3] Lenczner, D. (1972) Elements of load bearing brickwork. Elsevier.

[4] Asteris, P. G. (2003) Lateral stiffness of brick masonry infilled plane frames. Journal of

Structural Engineering, 129 (8),1071-1079.

[5] Sutcliffe, D. (2003) Masonry shear walls: a limit analysis approach. PhD Thesis, The

University of Newcastle.

[6] Rots, J. G. (1991) Numerical simulation of cracking in structural masonry. Heron, 36 (2),

49-63.

[7] Lourenco, P. B. (2000) Anisotropic softening model for masonry plates and shells.

Journal of structural engineering, 126 (9), 1008-1016

[8] Lourenco, P. B., Rots, J. G. &Blaauwendraad, J. (1995) Two approaches for the analysis

of masonry structures: micro and macro-modelling. Heron, 40 (4), 313-340.[9] Creazza, G., Matteazzi, R., Saetta, A. &Vitaliani, R. (2002) Analyses of masonry vaults: a

macro approach based on three-dimensional damage model. Journal of Structural

Engineering, 128 (5), 646-654

[10] Idris, J., Verdel, T. & Al-Heib, M. (2008) Numerical modelling and mechanical

behaviour analysis of ancient tunnel masonry structures. Tunnelling and Underground

Space Technology, 23, 251-263.

[11] Lee, J. S., Pande, G. N., Middleton, J. &Kralj, B. (1996) Numerical modelling of brick

masonry panels subject to lateral loadings. Computers & Structures, 61 (4), 735-745.

[12] British Standard PD 6678:2005 - Guide to the specification of masonry mortar.

[13] British Standard EN1052-2: Methods of test for masonry Part 2: Determination of

flexural strength.[14] RFCS PRESIDENCE project report, RFCR-CT- 2007-00004, Prediction and Monitoring

of Subsidence Hazards above Coal Mines.

[15] The International Society for Rock Mechanics: testing measure (1985).

experimental and numerical modelling of the weathering effects on shaft lining material

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