Constraining the critical groundwater conditions for initiation of large, irrigation-induced landslides, Siguas River Valley, Peru

In the last three decades, a series of large landslides have occurred in the Siguas River Valley, Peru, and their causes have been attributed to irrigation input affecting the local groundwater conditions. While the causal relationship of the irrigation water to these landslides is widely accepted, the actual groundwater conditions necessary to initiate these landslides have not been previously quantified. In this paper, we present finite element modeling work to estimate the critical groundwater conditions required to initiate failure for the Zarzal and Pachaqui Chico Landslides, given back-calculated strength parameters, topography, and remote estimates of the likely range of possible groundwater scenarios. Modeling results indicate that elevated groundwater tables are required for landslide initiation and that these elevated groundwater tables are more likely due to perched zones forming from irrigation water input, rather than from elevation of the original deeper water table. In addition, the moisture content outside the saturated zone appears to be a less significant factor in landslide initiation than the position and configuration of the saturated zones. These results compare favorably to groundwater data acquired since the initiation of these landslides. Understanding of the groundwater conditions at initiation is important for designing mitigation to help prevent future landslides, and several mitigation options are briefly discussed.