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Rock Mechanics.pdf - Mining and Blasting

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Figure 18.15 Longitudinal section<br />

looking north at the epicentre of the<br />

3.0 Mn rockburst <strong>and</strong> associated damage<br />

on the footwall haulage levels,<br />

Williams Mine, Ontario, Canada (after<br />

Bawden <strong>and</strong> Tod, 2003).<br />

MONITORING ROCK MASS PERFORMANCE<br />

haulage drives (Bawden <strong>and</strong> Jones, 2002). Following the 29 March 1999 event, a<br />

portable 8 channel microeismic system was installed around the sill pillar to monitor its<br />

response during rehabilitation. Subsequently, a 64 channel mine-wide microseismic<br />

monitoring system was installed. This system has its highest resolution in the sill pillar<br />

area.<br />

Rehabilitation after the 29 March 1999 event was based on a re-design of the support<br />

<strong>and</strong> reinforcement to accommodate a future dynamic event of similar magnitude.<br />

The requirements for this system included initial stiffness, load capacity, displacement<br />

capacity <strong>and</strong> energy absorption capacity. The system chosen used weldmesh<br />

reinforced shotcrete <strong>and</strong> 7 m long plan twin str<strong>and</strong> cable bolts, anchored over a 2 m<br />

length <strong>and</strong> de-bonded over the remaining5mtoprovide displacement capacity before<br />

cable yield. In the areas having the highest seismic risk, rings of connectible high capacity<br />

Swellex bolts were interspersed between the rings of de-bonded cables. It was<br />

recognised that the high stress concentrations in the sill pillar would be exacerbated<br />

over time through stress transfer from the surrounding active mining areas, progressively<br />

consuming the capacity of the support <strong>and</strong> reinforcing system. Accordingly, a<br />

comprehensive monitoring program was established in order to evaluate when <strong>and</strong> if<br />

additional support <strong>and</strong> reinforcement would be required to maintain safety <strong>and</strong> the<br />

operational availability of this high risk area (Bawden <strong>and</strong> Jones, 2002).<br />

In addition to the microseismic monitoring system <strong>and</strong> regular visual checks, the<br />

major elements of the monitoring system were instrumented cable bolts (SMART<br />

cables). Occasional MPBXs were installed on each level to depths exceeding the cable<br />

lengths to check for possible deformation of the rock mass beyond the cable reinforced<br />

zone. The SMART (Stretch Measurement to Assess Reinforcement Tension) cable<br />

bolt is used to measure the displacements <strong>and</strong> calculate the loads to which long<br />

cable bolts are subjected over time. The SMART cable uses a six wire miniature<br />

extensometer incorporated within the cable, thus avoiding interference with the cable –<br />

grout bond. The six wires are attached at user-specified locations along the length of<br />

the cable. The other end of each wire is attached to a spring-loaded wiper that passes<br />

across a potentiometer at the read-out head. As the cable stretches, the displacements<br />

at the anchor points are measured through the movement of the wiper across the<br />

potentiometer. The cable strains <strong>and</strong> loads may be calculated from the displacement<br />

data (Bawden <strong>and</strong> Tod, 2003).<br />

564

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