Establishing the in situ rock bolt behaviour underground in order to model and design improved rock bolt support systems

The Challenge

Rock bolts are subjected to complex loading conditions in underground mines with axial, shear and combined loads (axial and shear). Establishing the behaviour of in-situ rock bolts by testing and monitoring with fibre optic sensors to develop improved models and design of rock bolt support systems.

Key Findings

• Under combined load conditions: 80% of the rock bolt axial load capacity should be assumed for the rock bolt design.
• However, in situations with large shear, this is further reduced to only 50%. For both conditions, if the displacement percentage is higher than 80% then the rock bolt selection should be reconsidered (i.e. low-stiffness bar).
• FLAC3D pile elements underestimate shear deformation in the case of localised high shear stress as found in a rock bolt installed across a joint. The modified pile model can simulate the correct shear response of rock bolt and combined axial and shear load failure of the rock bolt at joints and discontinuities.
• Optical instrumented rock bolts provide a very high-resolution strain profile of the rock bolt installed in-situ.

Benefits to WA

Continuous improvement in the design of rock bolts and associated support system may lead to improved safety outcomes in underground mining operations in Western Australia. A new and improved instrumented rock bolt was designed for underground testing and software development which enables strain values to be monitored.

Keywords: rock bolt; underground; shearing