Journal of Geology & Geophysics
Open Access

Landslides in Nepal: Causes, Consequences and it’s Adopted Methodology

Artem R Oganov^{* *}Correspondence: Artem R Oganov, Department of Geosciences, State University of New York, New York, USA, Email:

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Description

Common causes of shallow landslides during times of intense rainfall include a decrease in soil suction and a resulting loss of shear strength due to infiltration. In Nepal's mountainous terrain, non-engineered hillslope excavation for the construction of local roads increases the risk of shallow rainfall-induced landslides.

Around the world, landslides brought on by rainfall pose a major hazard to people's lives and property. Every year, intense monsoonal rains between June and September cause several landslides in the mountainous nation of Nepal. Approximately 80% of the yearly rainfall falls during the monsoon season, and >90% of the deadly landslides in Nepal take place during this time. Landslides in Nepal during the past ten years have resulted in over 1200 fatalities and over $14 million in economic losses.

This has been attributed to the non-engineered or informal method of slope excavation for road construction, which frequently crosses terrain with steep angles and lacks suitable planning, design, drainage, and protection measures. The proliferation of unofficial local roads in Nepal has further increased the risk of rainfall-induced landslides. Based on the spatial distribution analysis of landslides and roads in Sindhupalchok district, Nepal, rainfall-triggered landslides may happen there almost twice as frequently as they do elsewhere. Landslides were more frequent during high rainfall events in the Phewa watershed in western Nepal in locations where the roadways crossed or were nearby.

Understanding how a transient atmospheric phenomenon like rainfall affects the behavior of unsaturated soil close to the ground surface is necessary for evaluating the physical process of landslides caused by rainfall. It is well known that rainfall infiltration into the unsaturated zone increases soil water content and decreases soil suction and shear strength. Changes in pore water pressure brought on by infiltration alter soil stresses, which in turn alter soil deformation, which alters the seepage process by altering the soil's important hydraulic properties like permeability, porosity, and water retention capacity. Therefore, in order to analyze the physical mechanism causing rainfall-induced landslides, it is necessary to use powerful numerical modeling techniques that can simulate both the interdependence between seepage and stress-deformation in unsaturated soils as well as the changing atmospheric conditions.

The simultaneous computation of the hydraulic and mechanical responses of variably saturated soils provided by the coupled hydro-mechanical analysis efficiently captures this dependency and is required for the appropriate assessment of rainfallinduced landslides. The coupled hydro-mechanical analyses have been reported to provide a better prediction of the factor of safety compared to the uncoupled analyses, where the hydraulic and mechanical aspects of soil behavior are treated separately. Additionally, the pore water pressure predictions from coupled analyses have been found to be in good agreement with fieldmeasured pore water pressures. In addition to effectively predicting seepage and stress distribution, coupled analyses can accurately forecast actual failure timeframes.

Geologically, the Kanglang landslide is located close to the middle of a hillslope and is underlain by a thick colluvial deposit that may represent an old landslide deposit. It is made up of a mixture of gravel, cobbles, and boulder-sized rocks. These deposits that are found on mountain slopes are created by the gradual downslope flow of materials from processes like hill wash or gravity-driven mass movement. Shale, sandstone, limestone, dolomite, slate, phyllite, schist, and quartzite are the predominant sedimentary and metasedimentary rocks in the area south of the Main Central Thrust (MCT), which is located about 7.4 km away. The majority of the Daklang-Listi road passes through the Lakharpata formation, which is composed of the Upper Nuwakot unit's Malekhu Limestone, Benighat Slate, and Dhading Dolomites. This includes the Kanglang landslide. The Nepalese district of Sindhupalchok was heavily impacted by many co-seismic landslides during the 2015 Gorkha earthquake.

The adopted methodology consists of three essential components. The first step is a thorough geotechnical field investigation, which includes trial pit excavation, Standard Penetration Tests (SPTs), and borehole drilling close to the landslide crown to examine and reconstruct the slope's subsurface condition before the landslide. To characterize the soil's geotechnical stability and estimate its mechanical (shear strength parameters) and hydraulic soil qualities, soil samples were taken from the trial pit and borehole (SWRCs and saturated permeability). The second is the field monitoring programme, which uses in-the-moment measurements of rainfall and the volumetric water content of the soil to evaluate the in- situ hydrologic changes in the soil. Finally, in the third stage, a series of hydro-mechanical analyses (or completely coupled flowdeformation assessments) were performed utilizing the estimated soil parameter estimates, the unsuccessful slope reconstruction, and the finite element code PLAXIS 2D.