Ground support loading monitored with Rock Bolt Sensor (RBS™) and relationship to nearby production and seismicity

Abstract

The National Research Council of Canada has developed a novel ultrasound based Rock Bolt Sensor (RBS™) to monitor load, deformation, and integrity of rock bolts. Several RBSs were deployed to monitor bolts in the roof of an intersection of two drifts and with another region of monitoring tens of metres away in the roof of the drift. The intersection spanned two rock types with different stiffness characteristics. The region was also covered by an extensive seismic system with a mixture of triaxial geophones and both triaxial and uniaxial accelerometers. Using three weeks of recorded data, the loading on 12 bolts was monitored during the production of several nearby stopes and analysed alongside the recorded seismicity. A significant increase in loading on the bolts in the roof was recorded when production occurred within 40 m of the instrumented intersection despite several other stopes producing more significant seismicity following blasting. The load was also taken up asymmetrically, with more substantial changes in load occurring in the bolts in the greywacke. During the study period, several large seismic events occurred within 500 m of the study area. Seismic moment tensor analysis was conducted on several of the seismic events to estimate the mechanism of failure and, using seismic stress inversion, the orientation of the observed fractures was estimated and the directions of principal stress. There was a strong increase in the load on the bolts located in the intersection, across the two different formations, several hours after a large seismic event occurred on that contact several hundred metres away. This work showed a connection between loading on the bolt and large changes in the local stress field due to nearby production and significant seismicity along structures in the region. In addition to monitoring the loading of the rock bolts, the Rock Bolt Sensor can be utilised to better understand the coseismic and postseismic deformation within the mine and improve models of the local stress field.