Benchmark study of undrained triaxial testing of Opalinus Clay - analysis and comparative evaluation

Five different laboratories were selected by Nagra to perform a benchmark study of undrained triaxial testing of Opalinus Clay (OPA). Core material was taken from the Mont Terri Underground Research Laboratory (URL), both from the "shaly" and the "sandy" facies. A total of thirty tests were carried out. The aim of the study was to demonstrate that very similar quantitative results can be achieved by different laboratories testing "identical core material" if strict sample handling and testing procedures were adopted.

In addition, the robustness of testing results was also to be demonstrated by using two fundamentally different procedures to carry out the saturation of the specimens before the shearing phase of the tests.

This report presents a detailed diagnostic analysis of all the experimental stages ("phases") of the performed tests. A comparison of the testing results obtained by the different laboratories is presented, with the aim of assessing and validating the testing procedures for the planning and performance of future testing campaigns.

Good to very good reproducibility was achieved in tests carried out with Opalinus Clay from theshaly facies ("shaly OPA"), in which samples are homogeneous in terms of macroscopic structure and mineralogical composition. The reproducibility of the testing results was demonstrated not just using the same testing procedure by two different laboratories, but also by using the two different procedures to saturate the samples prior to the shear phase.

In contrast to the shaly facies, comparison of test results with core mat erial from the sandy facies ("sandy OPA") is more challenging due to the inherent heterogeneity of the material. Compared to the shaly facies, index properties and the bulk mineralogical composition of tested samples from the sandy facies was much more heterogeneous.

The study demonstrates that robust testing of Opalinus Clay is possible when rigorous testing procedures are adopted and respected. Robust testing also yields greater confidence in the evaluation of the hydro-mechanical response of the material.