Influences of Stress Ratio and Stress Path on Failure Behavior of Intact Completely Decomposed Granite

This paper presents the results from two series of laboratory tests on intact and completely decomposed granite (CDG) with reference to slope engineering consisting of (1) anisotropically consolidated undrained shear (CU) tests with pore pressure measurement; and (2) drained constant deviator stress shear (CQD) tests. Each series includes both triaxial compression and extension loading modes for investigating the influence of stress ratio and stress path on the failure behavior of intact CDG. It is found from CU tests that under compression, soil specimens show slightly contractive tendency at the early stage of shearing, followed by strong dilative tendency. On the contrast, soil behavior under extension is firstly dilative and then contractive. The opposite trends under compression and extension are likely attributed to stress path and stress-induced anisotropy (i.e., the stage of anisotropic consolidation used to reach different stress ratios prior to undrained shearing). On the other hand, the failure envelope of the intact CDG is found strongly affected by the stress path. Under compression, the mobilized friction angle and apparent cohesion are 34° and 14 kPa, respectively. Under extension, these two strength parameters change to 42° and 10 kPa. The failure envelopes suggest that the use of strength parameters from compression CQD tests only may be less conservative for analyzing the stability of shallow slope, where effects of cohesion are likely dominant. When the depth of failure surface is deep, where the shear strength of soil is mainly contributed by friction, the analysis using the results of compression CQD tests could be more conservative.