True Triaxial Experimental Investigation of Rock Response Around the Mine-By Tunnel Under an In Situ 3D Stress Path

Underground excavation causes rock damage, affecting its stability and fluid transport properties. Traditional experiments usually use loading regime (compressive tests) to damage the sample. However, they cannot mimic realistic in situ conditions where excavation unloading also plays a significant role. In the study, we conducted a true-triaxial acoustic emission (AE) experiment to simulate excavation-induced damage by applying in situ 3D stress path to a sample. The path originating 1.0 cm from the Mine-by tunnel crown at the Underground Research laboratory (URL) was determined using an elastic numerical model as the tunnel advanced. The computed stress path was applied to a cubic granite sample using a state-of-art true-triaxial facility to simulate rock damage in the roof. AE activity was intensively monitored, and a damaged plane was determined by fitting a plane through the localized AE events. A source parameter analysis for cluster events near the damaged plane was undertaken via a spectral fitting method. The source type was determined using the moment tensor inversion method. The source size and stress drop were also estimated. The results indicated that stress unloading plays an important role in AE activity. Temporal characteristics of AE events were converted to a spatial distribution relative to the tunnel advancement; this exhibited a good agreement with the microseismicity recorded in the tunnel roof. Source parameters (e.g., corner frequency and moment magnitude) were consistent with the field recordings. Stress drop of the experimental events ranged from 0.6 to 6 MPa, which was comparable with the higher seismicity recorded at the Mine-by tunnel roof. The stress rotation and low confinement that may cause disagreement between the laboratory and field observations are critically analyzed.