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Benchmark Study of Undrained Triaxial Testing of Opalinus Clay: Results and Implications for Robust TestingNormal access

Authors: S.B. Giger, A. Minardi, A. Ferrari, L. Laloui, R.T. Ewy, R. Stankovic, J. Stenebråten, M. Soldal, E. Pimentel and H. Baumgartner
Event name: Sixth EAGE Shale Workshop
Session: Shale Multiphysics I
Publication date: 28 April 2019
DOI: 10.3997/2214-4609.201900267
Organisations: EAGE
Language: English
Info: Extended abstract, PDF ( 299.59Kb )
Price: € 20

We have performed a comparative study of undrained triaxial testing with five different laboratories, to explore the reproducibility of test results. Opalinus Clay was sourced as testing material from a borehole at the Mont Terri URL. Cores were vacuum-sealed, and resin impregnated immediately after recovery. Systematic determination of basic properties such as water content, grain density and bulk mineralogy of specimens after testing assisted in diagnostic test evaluation. A detailed testing protocol was requested to avoid specimen damage during initial loading («swelling») and to verify specimen saturation. A balanced pore fluid was used for testing, and a consolidation phase was performed to reach specific target effective stress levels prior to the shear phase. One laboratory deviated from these protocols, as it did not use an external pore fluid. Instead, specimens were brought to variable saturation levels in a desiccator prior to assembling them into the rig. For specimens with almost identical basic properties, the test results were indeed found to be in very good agreement, despite the different procedures applied. Differences in test results can be attributed to material heterogeneity. The study provides compelling evidence that robust triaxial testing can be achieved with shales.

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