Active slide in Tertiary bentonitic claystones, Upper Hat Creek, British Columbia

Abstract

Potential exploitation of the 500 m thick coal deposit at Upper Hat Creek, British Columbia, required detailed investigation of the sequence of Tertiary tuffaceous rocks surrounding the coal and the overlying surficial materials. Studies have shown that extensive bentonitic slide deposits partially cover the coal body on the west side of Hat Creek valley; part of this area is currently active. A translational debris slide of approximately 17 × 106 m3 volume and 2 km length is moving across the northwest boundary of the proposed pit into the valley bottom.Detailed field investigations complemented by laboratory testing have defined the basal plane of sliding and permitted a back-analysis to be made. From measured piezometric pressures, the mobilized shear strength on the failure plane is calculated to be [Formula: see text], cr′ = 0, which is largely in accord with laboratory test results.Slope stability in thick montmorillonite-rich claystones/siltstones is known to present difficulties, as evidenced by the Panama Canal excavations. Consideration is given to the engineering significance of bentonitic slides and preventive measures that might be adopted in their control. Key words: active slide, Tertiary, claystone, montmorillonite, open pit, coal, monitoring, residual strength, piezometric pressures.