Discontinuity survey and brittle fracture characterisation in open pit slopes

SRK News | Issue 54: Rock Engineering and Slope Stability

A4   |   Letter

Modern photogrammetry surveys can produce highly detailed 3D models of a rock slope. Using a high end DSLR camera with a long focal length telephoto lens, we can construct a model of an open pit slope with resolution in the order of a few centimetres from distances of 1km or more. Despite the continuing technological advances in remote sensing technology and numerical modelling tools, predictive modelling of rock slope stability is still limited by the intrinsic inability to observe the hidden interior structure of a rock mass, and by computational limitations restricting the explicit simulation of small scale material heterogeneity and localisation phenomena inherent to brittle fracture.

We have identified that using photogrammetry provides abundant scope to improve the workflow for rock mass mapping, and we are continually seeking to maximise the data that we extract from our photogrammetry models. By using a multi-scale approach to photogrammetry surveys, we characterise the influence of survey scale and resolution on key rock mass parameters such as discontinuity persistence, spacing, and intensity. In addition to long range surveys, we use close range photography to comprehensively characterise the intensity of small scale damage, induced by brittle blasting, and to identify incipient discontinuities, which end in intact rock and retain cohesion and tensile strength from rock bridges.

For one Australian open pit mine, we used the statistical information on joint spacing, trace length, and intensity gathered using photogrammetry, to develop a conceptual finite-element model using discrete fracture network techniques. The modelling results help to highlight the role of progressive brittle fracture in developing multi-bench scale-slope instability. We based the model sequence on the first principles understanding of progressive rock slope failure: brittle fracture is characterised by a two-stage process involving, first, the destruction of cohesion and tensile strength along incipient structures, followed by developing shear displacement and mobilisation of frictional strength. The results help to constrain critical values of cohesion and tensile strength for adversely dipping incipient discontinuities, which can be directly related to discontinuity persistence, spacing, and intensity measurements taken from photogrammetry surveys.

Zack Tuckey: ztuckey@srk.com.au

SRK Turkey