Soil strength parameters and stability conditions of the slopes of the Tremp basin (Eastern Pyrenees, Spain) derived from back-analysis and field reconnaissance

Continuous instability events take place, within Tremp Basin, causing landslides and earthflows. Translational slides along this region are very common, especially during rainy events were the presence of water in the subsoil seems to assist the instability of some slopes. Some of the factors which are related with mass movements in the area of study are: geometry of slopes, geology, hydrology, and other geotechnical factors. The study consists on analyze characteristic intact slopes and existing landslides to derive their stability conditions by means of the laboratory and back-analysis results. For modeling purposes, stable slopes are analyzed by applying peak parameters, while active landslides are analyzed using residual parameters. A sensitivity analysis of the groundwater changes on the safety factor in order to restrict the variability of said parameters is also included. Once the analysis has been made, such results are corroborated with the parameters resulting from the laboratory in order to establish actual parameters and identify the conditions that lead to the reactivation of active landslides and the possible activation of soil mass movements along stable slopes. Despite we have worked with incomplete information, the analysis has allowed us to obtain consistent results by characterizing, in a non-linear form, the active landslides and intact slopes. Back-analysis reveals that for a higher slope angle, reactivation may take place with relative shallow landslides, and vice versa, and that landslides are very susceptible to low piezometric variations, among others. While in stable slopes there is a relation between slope angle, slope height, and required soil strength parameters in order to achieve the steady state