Quantification of rock mass damage in underground excavations from microseismic event monitoring

Rock mass damage assessment is required for many applications in rock engineering practice including support design, contamination transport control, stope design, amongst others. While various methods such as displacement measurement, seismic refraction, and direct observation using borehole camera have been used, relatively few efforts have been made to use microseismic monitoring to quantify the rock mass damage. From laboratory tests, it is well known that microseismic events are indicators of fracturing or rock damage as the rock mass is brought to failure at high stress. By capturing the microseismic events, underground excavation induced rock mass degradation or damage can be located but can the amount of damage in terms of changes to strength or deformation properties be measured? In the present study, a method of characterizing rock mass damage near excavations based on microseismic event monitoring is developed and a damage-driven numerical model is presented that takes the microseismic data as input to determine the damage state described by fracture density. The approach is built on the discovery that a realistic crack size corresponding to a seismic event can be established by applying a tensile cracking model instead of the traditional shear model, commonly used in earthquake analysis. The rock mass is softened by the introduction of cracks and this is simulated by a micro-mechanics based constitutive model. The material property input for the model are Young's modulus, Poisson's ratio of the intact rock, and information obtained from the monitoring of microseismic events such as the location and the source size of each event calculated from source parameters. Using data from the Atomic Energy of Canada Limited Mine-by Experiment, this model has been verified by investigating the linkage between microseismicity, rock mass damage and ground deformation. It is found that when damage related softening based on microseismic data is considered, predicted rock mass displacements are in good agreement with extensometer measurements. (C) 2002 Elsevier Science Ltd. All rights reserved.