Underground Microseismic Monitoring of Pillar Systems to Evaluate Mechanism of Failure

Portable underground monitoring systems equipped with uniaxial and triaxial geophones were installed at two underground sites. The Microseismicity recorded at these sites was analysed in terms of spatial distribution of seismic events, peak particle velocities supported by numerical modelling of the wave-field around the pillar, as well as the source mechanism of seismic events. A novel method for studying in situ pillar behaviour was developed and helped identify the mechanism of failure. Numerical models were created to simulate the wave propagation for possible failure mechanisms. The synthetic seismograms were compared with the observed seismograms in an attempt to find the greatest similarity. The moment tensor was obtained from the weighting coefficients used in the correlation. This methodology differs from the traditional inversion approach, and takes into account a far more complex geometry and the influence of this geometry on the waveforms. Footwall slip along the narrow side of the pillar was found to be the probable mechanism for failure at both sites. Shear fractures of the pillar were also identified in support of this solution. The pillar fracturing was found to occur during and shortly after blasting, while the pillar was being cut.