Physical mechanical characterization of a rockslide shear zone by standard and unconventional tests

Abstract

Field and monitoring evidences show that deformation in a rockslide is predominantly localized along a basal shear zone. Mineralogic, grain size, thickness, and fabric characteristics of the shear zone control its behavior and the possible evolution of the rockslide from a slow creeping to a fast moving one. Standard experimental tests can characterize the material from the strength and the deformability, but they are not able to reproduce the in-situ conditions. In this paper, we discuss the results from campaign of experimental tests to characterize the shear zone behavior sampled from a rock-slide. In particular, the shear zone behavior and its evolution are investigated via conventional (direct shear, standard triaxial) and unconventional laboratory testing (with a LHV low-to-high-velocity ring shear apparatus). By LHV tests, it is possible to impose and monitor shearing displacement and velocity and the pore water pressure, simulating in-situ conditions. The tests were carried out on samples collected from boreholes through cataclastic shear zones from the Mont de La Saxe rockslide (Western Italian Alps) containing both phyllosilicates (XRF quantify in about 20%) and graphite (about 10%). Investigations reveal a grain-size reduction in some tests (up to 1%) and a preferential particle alignment in the shear zone. Mechanical investigations evidence a marked creep behavior characterized by two stages after the application of the loading steps. Nevertheless, the stress increments are imposed with a prefixed value, and the viscous displacement rate increases due to the damaging process. Finally, the mean values of the viscous parameters are computed considering Newtonian viscosity law range from 6.40 × 106 to 6.39 × 107 kPa s for the transition phase and from 2.01 × 108 to 5.89 × 109 kPa s for the steady state and according to the tangential stress increment.