Unloading-induced instability of a simulated granular fault and implications for excavation-induced seismicity

Abstract

Deep excavation in rock masses has the potential to break the frictional equilibrium of nearby faults, resulting in induced seismicity. We conducted the experimental and numerical studies on a simulated granular fault to uncover the mechanism of excavation-induced seismicity. A series of laboratory experiments were used to investigate the effects of initial shear stress, initial normal stress and its unloading rate on the frictional instability of the fault, and a numerical simulation was carried out to interpret stress variation and particle evolution during the unloading process. Our results show that both normal and shear stresses sharply drop when the fault is approaching a critical stress state. The stress reduction is due to interparticle force decrease and particle contact breakage. The evolution of fault state depends on the initial stress condition and excavation process. A greater initial normal stress and a lower initial shear stress provide a favorable environment to accumulate higher strain energy in adjacent rock, leading to larger slip displacement. A larger normal stress unloading rate can also cause higher strain energy and larger slip displacement. Understanding unloading-induced instability of a simulated fault allows us to interpret the seismic events occurred during the excavation of the Gotthard Base Tunnel in Switzerland. The reduction of normal and shear stresses associated with the excavation work decreases the differential stress applied to a natural fault zone, and subsequently results in the occurrence of induced earthquakes.