For the determination of rock mechanical parameters uni- and triaxial compression tests as well as tests for the determination of tensile strength were conducted in the lab. Triaxial compression tests can be conducted as conventional single stage tests, multi-stage tests or continuous failure state tests. The determination of the tensile strength can be performed e.g. by direct tensile tests, Brazilian tests and flexural tension tests. Within a research project the failure behaviour of sandstone is investigated in respect to the different types of triaxial compression tests. The stress-strain behaviour, the fracture pattern including the post-failure behaviour was simulated with PFC3D. This paper presents the results obtained with this research project so far.
Triaxial tests with several limit states or continuous failure states have some advantages: less material is necessary, data scatter is reduced and tests are more economical. On the other hand it is difficult is estimate to what extent the different stress paths influence the obtained failure envelope. Therefore, a research programme was initiated and a homogeneous sandstone (Postaer Sandstone, Elbe-Valley, Saxony) was used to investigate the influence of the different types of triaxial tests on the obtained failure envelope. The lab test programme comprises uniaxial compression and tension tests in different directions to the bedding planes as well as different types of triaxial tests (continuous failure state tests, single stage tests and multi-stage tests) perpendicular to the bedding. In parallel numerical simulations were performed using PFC3D (Itasca 2008), which has the advantage, that different stress paths can be applied to identical samples and deeper inside into the micromechanical damage process in the pre- and post-failure region can be obtained. The aim is to deduce recommendations for the application of the different types of triaxial tests especially in relation to the deduction of the failure envelope.