Impacts of road construction on landsliding in Nepal

Abstract

Rainfall-induced landslides occur extensively across the mountainous terrain of Nepal during the monsoon. Anthropogenic factors such as slope cutting for local road construction has been reported to have further aggravated the risk of rainfall-induced landsliding in Nepal. Although roads are vital infrastructure for development, the increased rate of landsliding due to road construction has reversed the anticipated benefits from improved road access. For the prevention and mitigation of such landslides, it is imperative to assess how the two key factors- road and rainfall interact to cause landslides. This study aims to better understand this interaction by conducting research that integrates reconnaissance surveys, in-situ tests, laboratory testing, field monitoring and numerical modelling using concepts of unsaturated soil mechanics.

In this study, reconnaissance surveys were conducted along a newly widened local road (Daklang-Listi) in Sindhupalchok, Nepal to identify the issues related to local road construction that are conducive to landsliding. The field observations revealed that the non-engineered road widening involving slope cutting at steep angles without suitable slope protection, drainage and control over spoil disposal increased the susceptibility to rainfall-induced landsliding. For the investigation of the physical process of landsliding caused by the road-rainfall interaction using numerical modelling, the scope of this study was narrowed down to a case study of Kanglang landslide which occurred during the monsoon in July 2018 after road widening. In-situ testing and trial pit excavations were undertaken for the evaluation of subsoil conditions and for soil sampling at the selected location. Laboratory testing was performed to determine the physical, mechanical and hydraulic properties of the soil samples. To understand the hydrologic response of the soil during atmospheric drying and wetting, a field-monitoring station was installed near the Kanglang landslide, which consisted of a rain gauge, 10HS water content sensors and a data logger. The temporal variation of volumetric water content due to soil-atmospheric interaction was evaluated and the factors influencing the volumetric water content response were identified. The monitored data showed that the progressive wetting of soil during wet periods could lead to soil approaching saturation at shallow depths, which can reduce the soil suction and ultimately cause shallow slope failures. A back-analysis of the Kanglang landslide was performed using fully coupled flow-deformation analysis and safety analysis in PLAXIS 2D to evaluate the effect of the road cut and rainfall on the failure mechanism. Before the back-analysis, the numerical model was calibrated against the field-measured volumetric water contents. The calibration results confirmed that the model was capable of capturing the in-situ changes in soil water content due to infiltration and drying. The back-analysis results demonstrated that the ingress of rainwater caused a reduction of soil suction at shallow depths and the presence of the road cut promoted the failure leading to accelerated displacements at the road cut. To reinforce this observation, the back-analysis was replicated in a model without a road cut. The results of this analysis showed that the presence of the road cut decreases the initial factor of safety, thereby, increasing the susceptibility to failure. In addition, it was also demonstrated that the rainfall event that triggered the Kanglang landslide would not have triggered a failure in the absence of the road cut, hence, confirming that the presence of road cut in the hillslopes can be detrimental to slope stability during rainfall.