Assessment of slope deformation and deep seated instability in the Cassiar open pit

CIM Bulletin, Vol. 86, No. 972, 1993

Dennis C. Martin, Piteau Associates Engineering Ltd. North Vancouver, British Columbia, and Eugene F. Mehr, British Columbia Ministry of Mines and Petroleum Resources, Smithers, British Columbia

Development of the final slope for the 340 m (1200 ft) high hanging wall at Cassiar Mine precipitated an area of deep seated instability involving an estimated 17.6 million tonnes. The deformation mechanisms involved squeezing of soft deformable serpen-tinite rocks in the toe which caused planar sliding and graben toppling of the stronger sedimentary rocks in the middle and upper sections of the slope. Although movement rates up to 150 mm/day were recorded, by understanding the controls on failure, the mine was able to install a suitable monitoring system and complete the open pit with only minor loss of the mineable ore. Accelerated mining was required in the last full year of operation. This paper describes the deformation behaviour, failure mechanisms and back analysis results for the failure. The monitoring systems and operational aspects related to the failure are also presented.