Seismic Imaging of Rock Disturbance in Excavated Slopes

Understanding rock strength is essential when designing and undertaking major excavation projects. The process of excavation reduces the strength of the remaining rock mass, which is often referred to as the disturbance or excavation damage zone, and quantifying the degree of this disturbance is critical when designing slope reinforcement. Currently assessing rock disturbance is relatively subjective; typically it is solely based on a visual inspection of the excavated slope, which provides little information on its extent into the excavated face. In this paper we should results from pico-seismic monitoring and seismic surveys, which are used to image the extent and degree of disturbance within recently excavated slopes. Seismic survey results indicate that the disturbance is primarily confined to the initial 0.5m from the excavated face, while the passive monitoring has detected only a small number of events. This suggests that the rock disturbance at this site is relatively low. From these findings, the level of disturbance was assessed to be lower than the initial value used in the slope design. This allowed a reappraisal of the methodology used during ground excavations, reducing the program timetable and thereby the overall project cost.