Open Cut » Rock Mechanics
Since discontinuities, such as joints, faults, and bedding planes in a rock mass are generally weaker than the intact rock, they often govern the strength of the rock mass and thus control the stability of pit walls. Reliable and efficient estimation of the shear strength of the discontinuities is therefore critically important to the optimal design and ongoing management of pit walls in surface mining.
The shear strength of a rock discontinuity is affected by its intact rock strength, and the features of the discontinuity surface, such as roughness and shape, degree of weathering, and type of infilling material.
The roughness of a discontinuity is a key parameter in determining its shear strength (ISRM, 1978). Measurements of rock surface roughness have conventionally been made using methods that require direct contact with the rock. Thus, to date, fieldbased measurements can be made only on rock surfaces that are accessible by mine personnel. Such measurements may require rock climbing equipment or mechanical lift devices. The requirement for physical access to the pit wall means that these manual measurements expose mine personnel to significant safety hazards.
This project focused on remote characterisation of geometric properties of large scale rock discontinuities, the estimation of the roughness, and the shear strength using 3D models of the surface geometry together with readily available intact rock strength (measurable at laboratories).
The feasibility of remote mapping of the infill of a rock discontinuity was also examined. It was concluded that it is technically feasible to remotely map and characterise the type and geometry of the infill of a rock discontinuity with coupled high resolution hyperspectral and photographic techniques. However, a suitable device may have to be developed to allow imaging at closer range to reach the resolution required. The development of devices such as small, remotely controlled rotary wing platforms capable of carrying suitable cameras has made the remote characterisation of discontinuity infill at sub-millimetre resolution possible.
The project illustrates that combined in-situ high resolution 3D imaging technique and numerical modelling of the strength of rock discontinuities and rock mass offers a promising approach that helps overcome the difficulties involved in using some of the conventional intermediate steps in strength estimation of rock discontinuities and rock mass, pitwall geotechnical characterisation, and stability analysis.
The techniques developed in this project for remotely acquiring high-resolution 3D images of exposed surfaces of rock discontinuities, creating 3D models of the surfaces, estimating 3D surface roughness and shear strength, provide a safe, efficient and accurate way for site geotechnical characterisation, pit wall instability assessment, and optimal mine design.