Shear Strength Assessment of a Footwall Slab Using Photogrammetry and Finite Element Modelling

CIM Edmonton 2008
Navid Bahrani, Dwayne D. Tannant,
Abstract Mountainous open pit coal mines are susceptible to footwall slab instabilities due to the development of high unbenched footwall slopes. In this study, the results of photogrammetry surveys were utilized to analyze a footwall slab failure that occurred at a surface coal mine located on the eastern slope of the Rocky Mountains. Photogrammetry surveys conducted before and after failure provided the pre and post failure profiles of the footwall slope as well as the roughness of the failure surface. Roughness profiles were used to determine the Joint Roughness Coefficient, from which values of the dilation angle were calculated.

A 2D finite element program was used to back-calculate the dilation angle by comparing results from a model that incorporated the actual sliding surface geometry (small scale roughness) with a model that used a smooth sliding surface. In each model, combinations of cohesion and friction angle that resulted in yield through all the joint elements were determined. The estimated effective dilation angle determined using the finite element modelling was consistent with that determined from empirical approaches using the roughness profiles. Further numerical modelling making use of a Shear Strength Reduction approach was used to back-calculate the shear strength parameters of cohesion and friction angle applicable for the footwall failure.
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