Managing Risks on an Active Haul Road Adjacent to a Propagating Subsidence Zone at Telfer Gold Mine

The Telfer underground sub-level cave (SLC) operations are located approximately 600 to 1000 m below the west wall of Main Dome open pit at the Telfer Gold Mine, Australia. The Telfer SLC commenced caving in 2006 and broke through to the surface in 2009 forming a large subsidence zone that has progressively developed above the SLC operation. Surface deformations within the subsidence zone continue at a rate of 500 mm to 2000 mm per month and are dependent to the rate of underground mining in the SLC operation. Underground and open pit operations have continued concurrently with the progressive enlargement of the SLC subsidence zone. The stage 4 open pit orebody was solely accessible via the West Ramp, which is located adjacent to the SLC subsidence zone with notable steady state ductile deformation occurring on ramp. Risks were managed by understanding the cave propagation mechanism through modelling and monitoring, and the use of comprehensive TARPs (trigger-action-response-plans) and an array of real-time surface and sub-surface deformation monitoring instruments. This paper discusses the monitoring systems used to track the cave subsidence and monitor ground deformation. These include slope stability radar, automatic total stations with survey prisms, shape-accel arrays, time-domain reflectometers, seismicity monitoring arrays, networked SMART markers, surface extensometers, automated crackmeters and monthly aerial surveys. These monitoring systems were used to determine the cave subsidence zone shape, location, rate of propagation and the direct response in the slopes adjacent to the West Ramp. The monitoring systems and TARPs are critical for identifying approaching hazards and taking proactive risk mitigation measures.