Towards Mine Closure: Assessing the Long-Term Stability of Open Pit Mine Slopes

ABSTRACT: The long-term stability of engineered slopes is becoming a critical focus point as the number of open pit mines anticipated to close in the coming years is increasing, and governments, regulators, and society are collectively placing more emphasis on sustainable management of mineral resources and land use (IISD, 2021). However, there are currently few guidelines on assessing long-term slope stability. Of central importance is the recognition that rock mass properties are not constants, and therefore, open pit slopes that are presently stable may not remain so in the future. Conventional engineering analyses generally assume the strength of a rock mass to be constant and, in doing so, fail to explain the temporal nature of rock slope behavior seen in monitoring data. Data shows that pit slope movements are intermittent, correlating with benching and seasonal precipitation patterns. These initiate episodic damaging events that, in the closure context, control strength degradation and impact long-term slope performance through progressive failure (Eberhardt et al. 2004). This talk will summarize the author's research over the last 20 years into progressive failure, its advancement of our mechanistic understanding of deep-seated rock slope failure, and recent results and guidance in applying it to open pit slope stability assessments and mitigation efforts to aid mine closure designs. Empirical data will be presented to show the evidence for progressive failure, with a focus placed on transient pore pressures driven by seasonal precipitation. Upon closure, changes to the slope geometry (i.e., benching) cease, but seasonal precipitation continues. Progressive failure posits that transient pore pressures in response to infiltration or groundwater recharge act to locally decrease effective stresses, promoting slip along non-persistent discontinuities, which in turn may cause the slip of adjacent fractures and/or the failure of intact rock bridges. Such repeated fluctuations in pore pressures and effective stresses thus are a key driver of progressive failure and can be equated to fatigue, where the rock slope experiences a slow weakening through repeated load cycles. Results from mine closure analyses will be presented, demonstrating how slope displacement monitoring and modeled groundwater fluctuations can be used to calibrate numerical models and establish the degree of criticality present in a slope. The modeling of seasonal variations further enables reference to be made to time in calculations that are otherwise limited to stress-strain behavior. This provides a means to assess displacement rate thresholds at which behavior change may occur for a given failure mode, which can be used to establish and constrain early warning alarm thresholds and trigger action response plans (TARPs). Examples will also be provided incorporating allowances for the development of a pit lake post-closure and for long-term stability improvement through engineered buttress designs.