An investigation of the combined effect of rainfall and road cut on landsliding

Abstract

The reduction of soil suction and consequent loss of shear strength due to infiltration is known to trigger shallow landslides during periods of concentrated rainfall. In the mountainous terrain of Nepal, the risk of shallow rainfall-induced landsliding is further exacerbated by the non-engineered hillslope excavation for the construction of local roads. To better understand the combined effect of rainfall and road cutting on landsliding, a detailed investigation of a shallow landslide was conducted, on a site with a steep road cut that failed due to rainfall infiltration in July 2018. An integrated investigation approach was adopted, combining field and laboratory testing, and field monitoring with a series of coupled hydro-mechanical analyses with the finite element code PLAXIS 2D. The field and laboratory tests were performed to characterise the subsoil condition and determine the soil parameters for the hydro-mechanical analyses. Further, a field monitoring program was set up to obtain real-time measurements of rainfall and volumetric water content of the soil. The monitored data was used for calibration of the numerical model to assess the reliability of its predictions. Results of the numerical back-analysis suggest that the landslide was triggered by rainfall infiltration causing a gradual reduction of soil suction at the shallow depths of ≤1.7m and the presence of the steep road cut promoted slope failure by allowing larger displacements to occur in the hillslope. Without the road cut, the slope was found to have ~35% greater initial factor of safety and under the landslide- triggering rainfall, the slope was found to remain stable with ~170% greater factor of safety than that in the slope with the road cut. This indicates that the presence of a road cut increases the likelihood of landslide during rainfall and lowers the minimum level of rainfall needed for its initiation. Hence, rainfall-induced roadside slope failures could become more frequent and extensive if roads continue to be built using informal slope excavation, without adopting suitable interventions, some examples of which are presented in this study.