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Seismic shaking induces stress changes in rock and soil slopes, which when combined with existing static stresses, may exceed the available strength and cause failure. Although one of the greatest hazards during strong earthquakes is associated with consequences of triggered landslides and natural slope failures, existing data seem to indicate that earthquakes are not a credible stability hazard for open pit slopes. Because regulations require investigation of slope stability under earthquake loading, relatively simple quasi-static analyses are typically carried out. The quasi-static analysis of the effect of seismic shaking on stability of slopes is usually too conservative. Thus, the mines primarily rely on empirical evidence when considering seismic hazard in the design of the open pit slopes. However, it is important to understand the reasons for relatively good performance of open pit slopes compared to multiple evidences of landslides during historical earthquakes, but also to determine, using a proper methodology, the conditions when open pit slope stability can be at risk during seismic shaking.
This paper provides, based on mechanical principles (using numerical models), a rationale to explain field observations that indicate relatively small effect of earthquakes on stability of open pit slopes, and also investigates the level of conservatism in the predictions of the quasi-static analyses as a function of important ground motion parameters. First, the increased demand in terms of dynamic stresses is quantified as a function of pit/slope geometry (topographic amplification) and amplification by wave trapping due to difference in stiffness of geological layers. It is shown that open pits have an advantage due to their three dimensional circular/elliptical geometry and less amount of weathered material close to the surface as compared to natural slopes. Next, a typical slope is subjected to suite of ground motions covering a wide range of peak ground accelerations and frequency contents. The dynamic Factor of Safety is calculated based on displacement criteria. After comparing the reduction in dynamic Factor of Safety with different earthquake intensity parameters, peak ground velocity (PGV) appears to be the parameter that best correlates with seismic risk for slopes. Finally, it is shown that the conventional approach of pseudo−static analysis using earthquake-magnitude-based seismic coefficients results in Factors of Safety that are almost always conservative, and often too conservative.
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