Slope Stability Analysis and Design Based On Probability Techniques at Cassiar Mine

CIM Bulletin, Vol. 70, No. 779, 1977

Douglas R. Piteau, Consulting Engineering Geologist, and Dennis C. Martin, Engineering Geologist, D. R. Piteau and Associates Limited, Geotechnical Consultants, West Vancouver, B.C.

This paper describes the open-pit studies relating to slope stability and design of the argillitic waste rocks which form the upper 700 feet of the 1000- to 1100-ft high hanging-wall slope at the Cassiar Mine. Analyses indicated that the likelihood of deep-seated failure in terms of slope angles which are geometrically possible was low. However, wedge failures on benches, which involve either whole or parts of the benches, were found to be of principal importance with respect to stability. Slope stability analyses and related slope design were based essentially on evaluating the geometry of the potential wedge failures in terms of the probability of occurrence of unstable wedge failures which could spill over the berms. Probability theory is developed and applied in such a way that the possibility of benches on the slope becoming ineffective is evaluated and related to over-all slope design. The probability of possible wedge failures spilling over berms of various widths, which are a function of the bench geometry, geometry of the possible failure, over-all slope angle and angle of repose of material involved in the possible wedge failures, is considered. Based on this approach, mine management could evaluate alternative slope designs and decide themselves on what probability of bench failure they feel is acceptable for their particular mining operation.
Keywords: Rock mechanics. Slope stability, Cassiar Mine, Groundwater, Open-pit mining, Pit slopes, Joint sets, Shears, Wedges, Blasting, Control blasting, Probability design.