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WASM bolt overcoring

An urgent and overwhelming need to examine the entire length of fully encapsulated rock bolts in-situ has been identified by a number of mining companies.

This is due to the fact that conventional pull testing programs for quality control can only be meaningfully applied to point anchored or friction bolts. In addition, non-destructive ultrasonic testing methods are either still in development or have not been widely used in Australia and demonstrated to be reliable. Consequently, WASM has developed a purpose-built bolt overcoring drill rig. This versatile overcoring system is capable of drilling at any orientation (360°) and overcoring reinforcement lengths up to 3m. Overcoring of in-situ bolts can be undertaken in the walls and backs to a collar height of 5-7m.

Research papers

In-situ corrosion assessment of galvanised friction bolts using overcoring techniques - International Conference on Ground Control in Mining Hassell & Villaescusa

The effects of corrosion upon the load bearing capacity and ultimate tensile strength of galvanised friction bolts were determined. Corrosion rates were measured by overcoring of the reinforcement elements in situ. Overcoring was achieved at several sites using a specifically designed overcoring drill rig. This rig has the capacity to drill up to 3.0m (140mm diameter core) in any orientation and up to a height of 7.0m. The recovered core and friction bolt elements were pull tested in the rock mechanics laboratory at the WA School of Mines (WASM) in order to determine their load displacement characteristics for various embedment lengths. The friction bolt elements were then analysed for rates of corrosion and determination of the ultimate tensile strength. Relationships were formed between this data and the environmental conditions the friction bolts were exposed to, in order to determine the major factors affecting the corrosion process and how they affect bolt performance.

A methodology for selection of resin grouted bolts Varden & Villaescusa

A new methodology has been developed to quantify the performance of fully encapsulated resin bolts. Investigations were undertaken at the Bullant mine near Kalgoorlie, Western Australia. A number of different resin anchored rock bolts were installed. The resin encapsulated bolts were subsequently overcored and analysed. Analysis consisted of push tests to obtain load-displacement characteristics. Additional observations were used to assess the mixing ability of different bolt designs and the effects that the bolts’ design and borehole size had on the load-transfer . The investigations identified a number of problems that occurred during installation. These problems cause variations in load transfer along the length of the bolt. It was found that the majority of the problems were at the anchor region of the bolts.

Quantifying the performance of resin anchored rock bolts in the Australian underground hard rock mining industry Villaescusa, Varden & Hassell

The purpose of rock support and reinforcement is to ensure excavations remain safe and open for their intended life span. The effectiveness of a reinforcement strategy is important for two main reasons: these being safety to personnel and equipment, and achievement of the most economical access to extract ore. For a particular rock mass, a stabilization scheme capable of matching the expected behaviour is selected based on an assessment of the likely failure modes predicted from the interaction of an excavation (geometry and purposes), the network of geological discontinuities, weathering and the loading conditions from stress and blast damage.