INFLUENCE OF TRANSIENT PORE PRESSURES ON THE PROGRESSIVE FAILURE OF ROCK SLOPES

The concept of strength degradation and progressive failure represents an important advancement in our understanding of deep-seated rock slope failure. 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 slope failure as seen in monitoring data. Data shows that slope movements are intermittent, correlating with seasonal precipitation patterns. This presentation will discuss several case studies involving monitored natural and engineered rock slopes, and analyses that consider progressive failure driven by transient pore pressures related to seasonal precipitation and groundwater infiltration patterns. These show that transient pore pressures cause localized reductions in effective stresses along fractures promoting slip, which in turn triggers slip of adjacent fractures and/or the rupture of intact rock bridges. Over time, this repeated pattern gradually weakens the slope. Occasionally, acceleration episodes involving extreme slope movements occur but these do not necessarily scale with the size of the precipitation event(s). This introduces significant uncertainty in early warning monitoring; acceleration events may either be an early warning of impending failure, or a false alarm related to localized movements that eventually lock up and subside. The results presented will demonstrate 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 in turn can be used to establish and constrain early warning alarm thresholds.