Evaluation of Rock Bridges Through DFN Models to Improve Pit Slope Design in the Absence of Joint Persistence Data Available to Purchase

Pit slope design relies on appropriate selection of design strength parameters. This selection is not trivial given the inherent heterogeneity of rock masses. In strong rocks, step-path failures may develop through combination of planar shear along joints and failure through intact rock. The presence of rock bridges between discontinuities increases shear strength along potential failure surfaces. However, estimating rock bridge percentages is challenging in the absence of mapping and joint persistence information. The lack of joint continuity data is common to many open pit projects until construction begins and the rock masses are exposed. This paper describes the rock bridge characterization approach used for the proposed Jay Pit area of the Ekati Diamond Mine, for which joint persistence data are not currently available. The approach involved developing a discrete fracture network (DFN) model to estimate the amount of intact rock between discontinuities as a percentage of the total failure surface length. The model used discontinuity orientation and spacing data from borehole drilling at the site, and discontinuity persistence data from wall mapping in a nearby pit. Possible stepped failure paths were determined for various DFN model realizations, and rock bridge percentages were calculated along these paths. Rock bridge statistics were applied in the stability assessment to steepen the recommended pit slopes by up to 6°.