Observation of the Spatial and Temporal Changes of Microseismic Source Parameters and Locations, Used to Identify the State of the Rock Mass in relation to the Peak and Post-Peak Strength Conditions.

Coulson, Adam L. (University of Toronto ) | Bawden, William F. (University of Toronto )

OnePetro 

ABSTRACT: In this paper the authors examine a region around the Golden Giant destress slot, and specifically a region of a pendant pillar, in which the complete process from microseismic initiation to aseismic behaviour (ultimate failure) was monitored during stress driven failure of a confined region of the rock mass. The principal component analysis (PCA) technique was used to characterize the extent and trend of the yield zone, and significant stages of the failure process by analysis of a cluster of events in the pillar, in which a macrofracture structure is formed. Observations of the event source parameters during the formation of this macrofracture structure, indicate following a gradual increase during loading, a fall in the moment magnitude, seismic moment, radiated energy and apparent stress, associated to facture interaction, which lead to a more substantial drop, at the point of coalescence and localization, during the formation of the macrofracture structure. Conversely to this, the source radius decreases with loading, showing non-self similar behaviour. The source geometric complexity is seen to change from a simple homogeneous model of similar isolated fractures to a more complex inhomogeneous model, starting at fracture interaction and continuing during coalescence and localization.

1. INTRODUCTION

In hard rock mining, at high mine induced stresses the rock mass is often driven not just to the peak strength but often well into the post-peak until ?complete failure? occurs. With the incorporation of microseismic systems into many mines around the world, and the advances in providing inexpensive field instrumentation, there is a unique opportunity to study this stress driven rock mass failure into the post-peak, in the ?field scale laboratory?. Our observations of the failure process in the field presented in this paper are based similar observations from the laboratory testing of intact rocks, in which much has been learned regarding the brittle failure process of hard rock by other researchers [1, 2, 3, 4, 5 and 6]. Additional crucial insight into the fracturing process has been gained with the advent of monitoring and locating the acoustic emission, created during the testing of uniaxial and triaxial rock specimens [7, 8, 9, 10, 11 and 12]. From these studies three key stress related stages in the failure process have been observed (Fig. 1). Following crack closure, and the true linear elastic phase of the material, generally at a stress level of 0.4±0.1 of sf (intact peak strength), crack initiation, (sci) has been observed to occur, associated with an increase in the background level of acoustic emission (AE). This is followed by a stage of stable crack propagation, resulting in damage accumulation, up to the critical damage stress (scd) corresponding to the point of volumetric strain reversal and generally found at a stress level of 0.75±0.1 of sf, after which unstable fracture propagation occurs with a significant increase in AE. This has been identified as the point of true yield, when axial strains become permanent [6], and was termed the long-term strength as defined by Bieniawski [4].