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

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.

► Multistep triaxial test to investigate strength parameters avoiding sample variation.
► Linear elastic limit, shear strength and residual strength are determined.
► Dependency from confining pressure is measured on a single sample for any parameter.
► Shear failure envelope could be defined from a single specimen for Opalinus Clay.
► Method suitable for ductile but not for distinctly brittle materials.