Laboratory assessment of the effect of vibro-replacement ground improvement on the mechanical response of natural silt

The effect of vibro-replacement ground improvement on the mechanical response of a low-plastic silt was examined using constant-volume monotonic and cyclic direct simple shear (DSS) testing. The silt samples representing "pre-improvement", "post-improvement", "post-improvement-aged" phases were retrieved from the field for testing. The pre- and post-improvement silt specimens consolidated to in-situ stress state, under constant-volume monotonic DSS shearing, displayed no strain-softening. Cyclic-mobility type strain accumulation with a stress-history-normalizable response was noted when pre- and post-improvement silt specimens were tested under constant-volume cyclic DSS loading, in spite of the ground improvement imparted in the latter case. In contrast, the post-improvement-aged silt specimens at in-situ stress state displayed a dilative strain hardening response. Under similar cyclic stress ratios, the rate of build-up of excess pore water pressure with increasing number of loading cycles was found to be least for the post-improvement-aged silt specimens when compared with pre- and post-improvement cases. As such, the cyclic resistance ratio of the post-improvement-aged silt specimens were significantly higher when compared with those from pre- and post-improvement silt specimens. These findings also highlight the importance of understanding the effects of vibro-replacement on the soil fabric/ microstructure especially considering the time elapsed since the application of ground improvement.