Multistep triaxial strength tests: Investigating strength parameters and pore pressure effects on Opalinus Clay

Physics and Chemistry of the Earth 36 (2011) 1898–1904

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Multistep triaxial strength tests: Investigating strength parameters and pore pressure effects on Opalinus Clay W. Gräsle Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg 2, D-30655 Hannover, Germany

a r t i c l e

i n f o

Article history: Available online 31 July 2011 Keywords: Strength test Opalinus Clay Poroelastic coupling Shear strength Residual strength Anisotropy

a b s t r a c t Natural variability between rock samples often hampers a detailed analysis of material properties. For the investigation of strength parameters the concept of multistep triaxial strength tests was developed to avoid the impact of sample variability. The limit of linear elastic behavior, shear strength and residual strength were measured at different confining pressure on a single specimen. Appropriate tools for near real time data analysis were developed to facilitate a precise and timely control of the test procedure. This is essential to minimize the problem of sample degradation during the test. The feasibility of the test concept was proven on three samples of Opalinus Clay from the Mont Terri rock laboratory. Each investigated strength parameter displayed a distinct deviation from a linear dependency on confining pressure or mean stress respectively. Instead, curves consisting of two linear branches almost perfectly fit the test results. These results could be explained in the framework of poroelastic theory. Although it is not possible to determine Skempton’s B-parameter (Skempton, 1954) and the Biot–Willis poroelastic parameter (Biot and Willis, 1957) separately from multistep strength tests, the product of both parameters can be derived from the test results. Although material anisotropy was found by the test results, numerous simple strength tests (Gräsle and Plischke, 2010) as well as true triaxial tests (Naumann et al., 2007) provide a more efficient way to investigate anisotropy. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Any assessment of short and long term stability and safety of prospective underground repositories for radioactive waste requires a well-founded numerical modeling. Therefore, reliable strength and elasticity parameters of potential host rocks are of urgent need. In case of argillaceous rocks the hydro-mechanical coupling is by far more pronounced than in other potential host rocks like crystalline rocks or rock salt. Thus, due to interaction between pore pressure effects inside the rock matrix (Tarantino, 2010) resulting from different saturation depending on the quality of sampling the experimental data base regarding THM-properties is still unsatisfactory. The distinct anisotropy of argillaceous rocks, particularly in indurated clays (Bock, 2009; Popp and Salzer, 2007), even complicates the investigations. Furthermore, the lithological material scattering hampers the identification and quantification of influencing parameters on the rock mechanical properties, as data variation arising from heterogeneity between samples often obscures details of material E-mail address: [email protected] 1474-7065/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.pce.2011.07.024

behavior. Besides efforts to reduce this statistical noise by careful selection and treatment of samples, there are essentially two approaches to overcome this problem: – To generate very large data sets for better statistics. – To avoid the impact of natural variability by obtaining an extensive data set from a single sample. The multistep strength test presented here follows the second approach to characterize the mechanical behavior of Opalinus Clay from Mont Terri and the possible impact of pore pressure effects. While reducing the impact of natural variability, this test concept bears a serious risk of introducing additional disturbances into the data resulting from progressive sample alteration during the long and complex test procedure. Thus, avoiding a too fast sample alteration is a major challenge in multistep strength tests. 2. Experimental approach Tests are performed on cylindrical samples (Table 1) from the shaly facies of Opalinus Clay from the Mont Terri rock laboratory (Bossart and Thury, 2008). As Opalinus Clay exhibits a distinct

1899

Table 1 Samples from the shaly facies of Mont Terri Opalinus Clay investigated in undrained multistep strength tests. Sample ID

02075

09001

08021

Orientation (P = parallel, S = normal to bedding)

P

P

S

Drill hole Depth (m) Length (mm) Diameter (mm)

BHE-B1 7.82 – 8.02 200.01 99.57

BLT-16 2.96 – 3.16 201.60 100.26

BLT-15 6.02 – 6.21 190.20 101.70

Density (kg/m3) vp (m/s)

2392 2909

2471 3363

2454 No signal

Applied confining pressure (MPa)

1 – 21

1 – 35

1 – 30

deviatoric stress σ1- σ3 [MPa]

W. Gräsle / Physics and Chemistry of the Earth 36 (2011) 1898–1904

data deviating from linear elastic behavior

6 5 all data data used for fit linear fit

4

"spike" attributable to a stick slip transition of piston friction

3 2

data affected by an imperfectly compensated hysteresis of piston friction and load frame deformation

1 0.15

0.16

0.17

0.18

0.19

0.2

deformation ε1 [%] Table 2 Elastic and strength parameters measured in multistep strength tests on Opalinus Clay. In case of sample 02075 the covered range of confining pressure was insufficient to determine the second branch for linear elastic limit and shear strength. Geometry Sample ID Linear elastic limit

Residual strength

r0 (MPa) c (MPa) u (°) capp (MPa) uapp (°) a B (-) r0 (MPa) c (MPa) u (°) capp (MPa) uapp (°) a B (-)

Elasticity

P 09001

S 08021

26.6 1.74 2.6 2.95 0.0 1.00

18.2 0.90 3.6 1.83 0.7 0.81

16.4 4.05 19.5 9.72 0.5 0.98

11.4 3.66 16.7 6.81 1.4 0.92

13.6 0.72 26.7 4.59 12.4 0.57

14.1 1.01 24.4 6.74 2.8 0.89

10.9 0.74 24.2 5.17 2.5 0.90

8.6

9.6

5.5

r0 (MPa) c (MPa) u (°) capp (MPa) uapp (°) a B (-)

Shear strength

P 02075

E (GPa)

1.16 4.2

4.92 24.9

anisotropy, there are samples drilled parallel to the bedding (P-samples) as well as normal to the bedding (S-samples). Since Opalinus Clay is very susceptible to damage by desiccation (Schnier, 2004) or the impact of oxygen samples are sealed in gas-tight foils immediately after drilling and stored in liners filled with nitrogen (3 bar) as a protective gas (for technical details see Gräsle and Plischke, 2010). The tests are carried out in compression mode in Karman-type triaxial cell. Undrained boundary conditions are applied throughout the tests. More details about the experimental setup are given in Gräsle and Plischke (2007). Shear strength as well as residual strength are affected significantly by strain rate, displaying higher values at higher strain rate (Hoteit et al., 1999; Jung and Biscontin, 2006). As conventional load controlled strength tests result in rapidly increasing strain rates when approaching failure, they are prone to overestimate material strength at low strain rate. To avoid this problem, tests are performed in strain controlled mode. Considering the very low strain rates expected in the context of long term stability of underground repositories, strain rate should be chosen as low as possible to avoid an overestimation of strength. Therefore, the applied strain rate is 107 s1, which is close to the lower limit for the used test apparatus. The concept of the multistep strength test comprises three test sections, each focused on the investigation of one mechanical char-

Fig. 1. Example of a loading phase performed with a confining pressure r3 = 19 MPa. The data show various types of common disturbances as well as the onset of non-linearity. Obviously, the sensitivity of this type of plot is insufficient to determine the deviation from linearity shown at the upper end of the curve.

acteristic of Opalinus Clay. Any test section is composed of a series of strain controlled load cycles at various levels of confining pressure: 1. The linear elastic limit, i.e. the onset of non-linearity in the stress–strain-relationship rdev(e1) during strain-controlled triaxial loading, is determined in Section 2.1. 2. Section 2.2 is focused on a multiple determination of shear strength at various confining pressures. 3. Test Section 2.3 is a conventional test of residual strength. Unfortunately, the triaxial apparatus available for the tests is not equipped with a pore pressure sensor. Therefore, pore pressure effects can only be detected indirectly. 2.1. Test section A – limit of linear elasticity Obviously, the onset of damage must be detected very carefully, if it should be investigated multiple at several levels of confining pressure. To avoid significant damage of the sample in the sequent steps, thus changing its properties significantly and impeding further investigations of an ‘‘undisturbed sample’’, a rather strict and well detectable criterion for the beginning of damage is required. Since sample deformations are fully reversible within the range of linear elastic behavior, the limit of linear elasticity might be the very first evidence for incipient damage (if one does not regard compaction occurring during loading and unloading cycles as damage). Although the linear elastic limit may be met far below any level of relevant material damage, it nevertheless characterizes the transition to another deformation regime, either to a non-linear elastic (i.e. nonlinear but reversible) behavior or to an irreversible inelastic (i.e. plastic) alteration of material properties. To investigate the limit of linear elasticity test Section 2.1 consists of a sequence of loading cycles. Each cycle comprises three phases: first, the sample is loaded with a constant rate of deformation de1/dt = 107 s1. As soon as a deviation of the axial stress from a linear path can be detected the loading is stopped. The sample is unloaded at a rate of dr1/dt = 0.1 MPa/min to an axial stress 0.5 MPa above the confining pressure of the subsequent loading cycle. In the third phase the confining pressure is increased while keeping the axial stress constant. The main challenge of this concept is the reliable and ‘‘near real time’’ detection of the deviation from linearity (usually within a few minutes, with lowest tolerance at low confining pressure). There are several instances that make this task difficult:

1900

W. Gräsle / Physics and Chemistry of the Earth 36 (2011) 1898–1904

2.5

exact slope noisy, Δε=5e-6

12

noisy, Δε=1e-5 noisy, Δε=4e-5

linear elastic limit σdev.lin

6 ideal deviatoric stress noisy deviatoric stress linear elastic dev. stress

4

1.5 1.6

1.8

8 onset of non-linearity ε lin

2

slope [GPa]

10

onset of non-linearity εlin

deviatoric stress [MPa]

noisy, Δε=2e-5

2

2.2

1.6

2.4

1.8

deformation [10 ] Fig. 2. An artificial data set built with parameters and measurement inaccuracies similar to those found in the performed tests. The determination of the linear elastic limit rdev.lin from this type of plot would lead to an overestimation of rdev.lin. This systematic error would increase in case of increasing stochastic data variation. Note that rdev.lin and elin are much lower than those shown in Fig. 1 because the artificial data set reproduces the behavior at a much lower confining pressure of r3 = 1 MPa.

2

2.2

2.4

deformation [10 -4]

-4

Fig. 3. In a dr1/de1-plot a rather large smoothing window with De P 2  105 is required for the artificial data set to reduce data noise sufficiently. Due to the smoothing of the curve, a tendency to underestimate elin has to be expected when using this type of plot.

Actually it seems to be too difficult to develop an algorithm for an automatic determination of the linear elastic limit. Consequently, tools are required to assist the operator to recognize at right time when the loading has exceeded the linear elastic limit.

0.05

0

-0.05

ideal reduced stress noisy reduced stress

-0.1

smoothed, Δε=5e-6 smoothed, Δε=2e-6 smoothed, Δε=1e-6

-0.15 1.6

1.8

onset of non-linearity εlin

– The slope of the deviatoric stress vs. deformation relationship will usually depart from the constant Young’s modulus continuously, thus to some extent the determination of the linear elastic limit is a question of measuring accuracy. – Due to measurement noise, observations will always fluctuate a bit around an idealized linear relationship. This smears the onset of a systematic deviation from linear elasticity and particularly hampers the development of an algorithm for automatic detection. – Any disturbance of the process control or data acquisition can interfere with the detection of the linear elastic limit. As shown in Fig. 1, abrupt drops (‘‘spikes’’) in deviatoric stress (0.5–1.5 MPa, typically lasting 2 min) accidentally occurred in the measurements. They result from stick–slip transitions of piston friction. Although they do not affect the sample behavior, they sometimes disturb the determination of the linear elastic limit. Spikes were eliminated from the data by an appropriate filter. – Piston friction displays a distinct hysteresis. This usually leads to an initial slope of the deviatoric stress vs. deformation relationship which is much steeper than the representative value of Young’s modulus. Then the curve bends asymptotically towards the linear relationship. Sometimes, this makes it difficult to determine a linear part of the curve unambiguously and may also lead to an overestimation of the Young’s modulus. In extreme cases, the hysteresis effects might not fade sufficiently until the linear elastic limit is reached. – The loading frame of any triaxial apparatus exhibits an elastic deformation depending on the axial load. This frame deformation has to be considered as a correction term in the acquisition of deformation data because strain gauges are mounted outside the cell. Any error in the quantification of the non-linear frame deformation will result in a non-linear error of the calculated sample deformation and might conceal linearity. A small but distinct hysteresis of the frame deformation even aggravates this problem.

reduced axial stress [MPa]

0.1

2

2.2

2.4

deformation [10 -4] Fig. 4. When using a reduced stress plot a smaller smoothing window with De P 2  106 is sufficient to overcome the impact of data noise for the artificial data set.

Obviously, it is not possible to accomplish this task by means of a simple deviatoric stress vs. deformation plot (Fig. 1). A self-evident approach is to use a dr1/de1-plot. Unfortunately, even a moderate noise in a data set can disturb the calculation of a derivative dramatically. An artificial data set resembling typical material and test parameters can demonstrate the problems arising from data noise: the assumed material behavior is given by E = 10 GPa, a linear elastic limit of rdev,lin = 2 MPa (corresp. to a deformation elin = 2  104), and a decreasing stiffness dr/ de = E(1104(e elin)) for elin < e < 3  104. A stochastic normal distributed noise with standard deviations de = 107 and drdev = 0.03 MPa was added to the ideal rdev(e) data (Fig. 2). When the derivative dr1/de1 at the point e is calculated by a linear fit to all data points within a smoothing window [e  De, e + De], a rather large smoothing window with De P 2  105 is required to reduce data noise sufficiently in case of this artificial data set (Fig. 3). A broad smoothing hampers the goal of near real time detection of the linear elastic limit since it delays the data availability by a half width De of the smoothing window, thus increasing the risk of sample damage.

1901

W. Gräsle / Physics and Chemistry of the Earth 36 (2011) 1898–1904

linear elastic limit σdev.lin

0.2

5 all data data used for linear fit running average (13 values) deviatoric stress (right axis)

-0.2

-0.4

4

3

deviatoric stress σ1-σ3 [MPa]

0

onset of non-linearity ε lin (derived from the green dots)

reduced stress σred [MPa]

6

2

-0.6

0.15

0.16

0.17

0.18

0.19

0.2

deformation ε1 [%] Fig. 5. The reduced stress plot of the loading cycle already presented in Fig. 1. The sensitivity of this plot for detection of the linear elastic limit, i.e. the deviation from the rred = 0 line, is enhanced considerably compared to the rdev(e1)-plot (cf. Fig. 1).

A ‘‘reduced stress’’ plot, i.e. a plot of the deviation of axial stress from a pure linear elastic behavior (rred(e1) = r1(e1) r1.linear(e1)), turned out to be the most efficient tool for the detection of the linear elastic limit. The linear elastic curve is determined from an obviously linear section of the measured r1(e1) data. As for the drdev/de1-plot, smoothing of rred (by running average) is required in case of noisy data. Because there is no calculation of differential quantities the width of the smoothing window can be reduced by approximately one order of magnitude (Fig. 4) compared to the use of a drdev/de1-plot. Fig. 5 illustrates the suitability of the reduced stress plot for the data set already shown in Fig. 1. A software for near real time data analysis has been developed. It facilitates an easy configuration of the spike-filter and it displays r1(t)- as well as r1(e1)-plots to aid a rough selection of the linear section. Based on this rough selection, a reduced stress plot is displayed to allow for a fine tuning of the selection of the linear section and the determination of the linear elastic limit. The number of test cycles in Section 2.1 has been increased from 21 in the first test (Fig. 6) to more than 100 in the subsequent tests without finding any indication of relevant damage. 2.2. Test section B – shear strength Overall test Section 2.2 is a multiple strength test. It is based on the attempt to measure the shear strength of a single sample in repeated turns at different confining pressures r3. Of cause, some sample damage due to initiation of local microcracks is unavoidable when approaching the shear failure condition. Therefore, it must be expected that only very few measured peak stresses rdev.peak approximately represent the properties of the undamaged material (if the material behaves very brittle the development of a pervasive shear failure would destroy the sample immediately once the failure condition is met; but in practice it was found that the clay behaves very moderate). After a few loading cycles, the progressive damage of the sample will change its properties gradually towards the residual properties. To obtain as many representative peak stress values as possible, any load cycle is terminated immediately when a peak is detected. For any investigated material the number of peak stress values approximately representing properties of the undamaged material must be determined. For the shaly facies of Mont Terri Opalinus Clay a first test demonstrated reliability of the initial four cycles by their conformity (R2 = 0.997) with a linear Mohr–Coulomb-type relationship (see sample 02075 in Fig. 8). Later on, rdev.peak rapidly

falls below this strait line declining towards residual strength. Degradation is particularly obvious for mean stress in the range of 19 MPa < roct < 21 MPa, where strength decreases despite an increasing confining pressure. Load cycles were carried out with small intervals of r3 in the first test to analyze the number of representative data obtainable from one sample. In contrast, the initial four load cycles were performed covering a wide range of r3 in subsequent tests to allow for the detection of pore pressure effects. 2.3. Test section C – residual strength Test Section 2.3 focuses on the determination of the residual strength. This can be repeated for various levels of confining pressure r3. Within any level of r3 the development of a well defined plateau requires a significant axial deformation in the order of 1% (Fig. 6). Hence, samples undergo an axial deformation of 10–20% till the end of the test (Fig. 7). No particular problems have to be expected as long as the deformation does not become too large and the sample integrity is not destroyed. In one case (sample 08021), the data set from Section 2.3 remained incomplete, because the rubber tube sealing the sample failed due to large deformation. 3. Results and analysis 3.1. Strength parameters as functions of mean stress Actually, results of three undrained tests are available, two on P-geometry and one on S-geometry. The analysis is performed in octahedral stress space, where mean stress is given as

1 3

roct ¼ ðr1 þ r2 þ r3 Þ

ð1Þ

and octahedral shear stress as

soct ¼

1 3

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðr1  r2 Þ2 þ ðr2  r3 Þ2 þ ðr3  r1 Þ2

ð2Þ

A shape of two linear branches appears to be characteristic for the soct(roct)-relationship of the investigated strength parameters linear elastic limit, shear strength, and residual strength (Fig. 8). A reasonable explanation of these finding results from poroelastic theory:

W. Gräsle / Physics and Chemistry of the Earth 36 (2011) 1898–1904

deviatoric stress σdev [MPa]

1902

6

section A

4

2

0 0

0 .05

0 .1

0 .1 5

1

1 .5

deviatoric stress σdev [MPa] confining pressure σ3 [MPa]

deformation ε1 [%]

section A + B 20

10

0 0

0 .5

deviatoric stress σdev [MPa] confining pressure σ3 [MPa]

deformation ε1 [%]

section A + B + C 20

10 section A section B section C confining pressure

0 0

5

10

15

deformation ε1 [%] Fig. 6. The stress–strain-path of the first multistep strength test (sample 02075). Any test section requires its own diagram since the typical deformation increases by one order of magnitude from any section to the subsequent one. Note that ‘‘spikes’’ have not been eliminated in this plot.

The investigated specimens have been protected against desiccation during drilling, storage, and specimen preparation as good as possible. Therefore, their water content is close to saturation at the beginning of a test. Consequently, sample compression might result in saturated conditions when mean normal stress surpasses a critical value r0. Beyond this threshold further increase of mean stress causes a buildup of pore pressure p according to Skempton (1954):

 @p  B¼ @ roct undrained

)

p ¼ B ðroct  r0 Þ

ð3Þ

Note that for roct = r0 pore pressure becomes p = 0. Furthermore, for simplicity Skempton’s B-parameter is assumed to be constant in the investigated range of stress. At the same time Biot’s equation on effective stress r0 applies (Biot, 1935): 0

r ¼rap

ð4Þ

Merging both equations presuming a constant Biot–Willis poroelastic parameter a leads to

r0oct ¼ r0 þ ð1  a BÞðroct  r0 Þ

ð5Þ

Given a Mohr–Coulomb failure line

soct ¼ c þ r0oct tan u

ð6Þ

For roct P r0 this results in an apparent Mohr–Coulomb line described in total stresses:

soct ¼ c þ ½r0 þ ð1  a BÞðroct  r0 Þ tan u ¼ capp þ roct tan uapp

ð7Þ

Where the apparent cohesive shear and apparent friction angle are given as

uapp ¼ arctanðð1  a BÞ tan uÞ capp ¼ c þ r0 a B tan u

ð8Þ

W. Gräsle / Physics and Chemistry of the Earth 36 (2011) 1898–1904

before test

after test

08021 (S)

09001 (P)

02075 (P)

sample

1903

octahedral shear stress τoct [MPa]

l l l l

l l l l

An analogous consideration applies to the two-branch-relationships for linear elastic limit and residual strength. Presuming this theoretical framework, Skempton’s B-parameter and the Biot–Willis poroelastic parameter cannot be determined separately from multistep strength tests, but it is possible to calculate the product a B according to Eq. (8). Table 1 summarizes the strength functions obtained from three samples.

sample ID and orientation

lin. limit

peak

residual

Fig. 7. The investigated samples before and after the test. Although little variation of friction angle u is found for residual strength (Table 2), the orientation of shear planes varies significantly depending on sample orientation (P-geometry for samples 02075 and 09001, S-geometry for sample 08021). Differing from the angle expected from u in case of an isotropic material (indicated by red lines), the shear planes are rotated towards the weakest planes (i.e. the bedding planes) in the anisotropic Opalinus Clay. Sample 08021 after test is shown while still wrapped in Teflon foil. It disintegrated while removing the Teflon.

l08021 (S) l09001 (P) l02075 (P) l02075 damaged

10

3.2. Anisotropy For any investigated strength parameter indications of material anisotropy can be found. All parameters obtained when loading parallel to the bedding planes (P-geometry) surpass those measured perpendicular to the bedding (S-geometry) (red1 points always fall below the corresponding green and blue points in Fig. 8). Same applies to Young‘s modulus also determined in the multistep strength test (Table 2). Regarding the considerable variation between the investigated P-samples, this finding is of cause not statistically significant. Nevertheless, it is in good agreement with results of comprehensive strength tests carried out on Mont Terri Opalinus Clay (Gräsle and Plischke, 2010; Bock, 2009; Popp and Salzer, 2007; Naumann et al., 2007).

5

0 0

10

20

30

40

mean normal stress σoct [MPa]

4. Summary and conclusions

Fig. 8. Results of three multistep strength tests displayed in octahedral stress space.

If suction effects are negligible for roct 6 r0, the Mohr–Coulomb equation in total stresses becomes

soct ¼ c þ roct tan u

ð9Þ

Thus, under the mentioned requirements a failure line linear in effective stress space results in a function consisting of two linear branches in total stress space:

(

soct ¼

c þ roct tan u

for

capp þ roct tan uapp

for

roct 6 r0 roct > r0

ð10Þ

A new concept of multistep triaxial strength tests was developed to investigate strength parameters avoiding uncertainty by sample variability. The main challenge of this concept is a sufficient limitation of sample degradation during the test to obtain data almost representing properties of an undisturbed material. Tools for near real time data analysis and immediate detection of linear elastic limit respectively peak stress were developed to achieve this objective. 1 For interpretation of color in Figs. 1-8, the reader is referred to the web version of this article.

1904

W. Gräsle / Physics and Chemistry of the Earth 36 (2011) 1898–1904

Tested on three specimens of Mont Terri Opalinus Clay (shaly facies), the concept proved to be appropriate for this material. The linear elastic limit, shear strength and residual strength could be determined at various levels of confining pressure on a single sample. This allows for an analysis of the dependency of these strength parameters from mean stress without the blurring impact of sample variability. A function consisting of two linear branches appears to be an adequate description of this dependency for any of the investigated strength parameters. Poroelastic theory provides a reasonable explanation of this finding. At the current stage of work only indirect indications of pore pressure effects at higher confining pressure are available. Therefore, an improvement of the test concept is projected by implementing a pore pressure sensor into the triaxial apparatus to facilitate direct prove. Due to the complexity and long duration of the multistep triaxial strength test the accomplishable number of tests will always be rather limited. Therefore, they are less suitable for the investigation of questions essentially requiring the comparison of different specimens. Thus, although indications of material anisotropy could be derived from multistep strength tests, a large number of simple strength tests is a more appropriate approach to deal with anisotropy. Alternatively, true triaxial test to some extent allow for the investigation of anisotropy on a single rock sample (Popp and Salzer, 2007; Naumann et al., 2007; Naumann and Plischke, 2006). References Biot, M.A., 1935. Le problème de la consolidation des matières agileuses sous une charge. Ann. Soc. Sci. Bruxelles, Serie B 55, 110–113. Biot, M.A., Willis, D.G., 1957. The elastic coefficients of the theory of consolidation. J. Appl. Mech. 24, 594–601.

Bock, H., 2009. RA Experiment: Updated Review of the Rock Mechanics Properties of the Opalinus Clay of the Mont Terri URL based on Laboratory and Field Testing. Mont Terri Project, Technical Report TR 2008-04. Bossart, P., Thury, M. (Eds.), 2008. Mont Terri Rock Laboratory. Project. Programme 1996 to 2007 and Results. Reports of the Swiss Geological Survey No. 3. Swiss Geological Survey, Wabern, Switzerland. Gräsle, W., Plischke, I., 2007. LT Experiment: Strength and Deformation of Opalinus Clay. Mont Terri Project, Technical Report TR 2007-05, Federal Institute for Geosciences and Natural Resources (BGR), Hannover, Germany. Gräsle,W.,Plischke,I.,2010.LTExperiment:MechanicalBehaviorofOpalinusClay,Final report from Phases 6–14. Mont Terri Project, Technical Report TR 2009-07. Federal Institute for Geosciences and Natural Resources (BGR), Hannover, Germany. Hoteit, N., Ozanam, O., Su, K., François, O., Heitz, J.F., Nguyen Minh, D., 1999. Geomechanical Research for Radioactive Disposal in Deep Clays – First Results and Prospects. WM’99 Conference. Published online . Jung, B.C., Biscontin, G., 2006. Modeling of Strain Rate Effects on Clay in Simple Shear. Proceedings of GeoCongress 2006: Geotechnical Engineering in the Information Technology Age. Published online . Naumann, M., Hunsche, U., Schulze, O., 2007. Experimental investigations on anisotropy in dilatancy, failure and creep of Opalinus Clay. Phys. Chem. Earth: Parts A/B/C 32, 889–895. Naumann, M., Plischke, I., 2006. Laboratory Temperature Testing (LT) Experiment: Phases 9 and 10 – True Triaxial Experiments on Cubic Specimens. – Technical Note TN 2005-71. Federal Institute for Geosciences and Natural Resources (BGR), Hannover, Germany. Popp, T., Salzer, K., 2007. Anisotropy of seismic and mechanical properties of Opalinus clay during triaxial deformation in a multi-anvil apparatus. Phys. Chem. Earth: Parts A/B/C 32, 879–888. Schnier, H., 2004. Postdismantling Laboratory Triaxial Strength Tests. Deliverable 8b, WP3/Task 32 Postdismantling Rock Mechanic Analysis – Heater Experiment (HE). Federal Institute for Geosciences and Natural Resources (BGR), Hannover, Germany. Skempton, A.W., 1954. The pore pressure coefficients, A and B. Géotechnique 4, 143–147. Tarantino, A., 2010. Basic Concepts in the Mechanics and Hydraulics of Unsaturated Geomaterials. In: Laloui, L. (Ed.), Mechanics of Unsaturated Geomaterials. Wiley, London/Hoboken NJ, pp. 3–28.