Abstract: The Aspö Pillar Stability Experiment (APSE) was a full-scale demonstration project at SKB's Hard Rock Laboratory undertaken to examine the failure process in a heterogeneous and fractured crystalline rock mass when subjected to coupled excavation-induced and thermal-induced stresses. A pillar was produced by excavating two 1.8 m diameter deposition holes 1m apart. An ultrasonic acquisition system provided acoustic emission (AE) and ultrasonic survey monitoring throughout the various phases of the experiment to map microcracks and measure changes in rock properties. This paper builds on previous work by performing advanced processing and interpretation methods on the data. The data set of 15,198 AEs shows an intense clustering of events located along the length of the deposition holes. Event clusters are primarily constrained to a damage zone orthogonal to the maximum principal stress, represented by a semi-circle of tightly packed AEs extending from the edge of each hole approximately 20 cm into the pillar. B-value analysis helps to characterise the stress and tectonics of a region globally, but can be used at AE scales to provide an insight into stress build-up and release mechanisms. In the APSE experiment, a close relationship has been found between spalling volume and b-value. The fracture geometry from statistical analysis of AE locations shows the evolution of the dominant structure of the induced fracture network. The results from these approaches increase our understanding of the fracture mechanisms and allow additional information to be gained from the AE catalogue.
1 INTRODUCTION Microseismic, acoustic emission (AE) and ultrasonic techniques are scaled seismic studies that can be used to remotely monitor rock mass deformation and fracturing around engineered structures. Performing b-value analysis and investigating fracture geometry from statistical analysis of AE locations provide additional insights into why the rock responds the way it does.