[en] The mechanical behaviour of rock materials is largely influenced by the stress state and pore pressure; this needs to be considered in many applications, such as petroleum engineering, tunnelling or stability of underground excavations.
Conventional triaxial tests (2 = 3) are generally used to assess the influence of confining and pore pressure. In this work, those tests are performed on a porous rock named Sorcy limestone, along with more advanced stress paths like triaxial extension (2 = 1) and purely deviatoric tests with independent pore pressure control. Those tests are combined to a true triaxial study on the dry rock in order to investigate the effect of true triaxial stresses and, hence, build three-dimensional limiting envelopes accounting for the influence of pore pressure.
Three-dimensional limiting envelopes of dry Sorcy limestone are built in meridian and octahedral planes (Figure 1). The shape of the octahedral sections changes with the mean stress from triangular to hexagonal and then quasi circular. At even higher mean stresses, the evolution continues towards a triangle with apexes oriented in the direction of triaxial extension stress states. In meridian sections, the deviatoric stress first increases with the mean stress. The transition between this deviatoric mechanism and the cap surface occurs at different mean pressures depending on the Lode angle.
Terzaghi's effective stress is found to govern the yield and plastic behaviour of saturated Sorcy limestone. Triaxial tests show that the limiting envelope does not depend on the loading path (conventional compression and purely deviatoric stress paths). In triaxial extension, the brittle-ductile transition is observed for a higher mean pressure than in triaxial compression, as previously found for true triaxial tests on the dry rock.
Finally, it is shown that upon appropriate normalisation, the dry and saturated yield envelopes come very close one to each other (Figure 2). This suggests that only the size of the yield envelope changes between dry and saturated state (pore pressure = 5 MPa); its shape does not seem to evolve.
