Surficial Mass Movements on Cut Slopes of the Prime National Highway 1 (PNH-1) in Bhutan

The article presents characteristics of surficial mass movements after cutting a slope through the Prime National Highway 1 (hereinafter abbreviated as PNH-1) in Bhutan. The surficial mass movement, in the form of slope failures, affects the ability of traffic to move along the PNH-1. The geology in the area of the PNH-1 consists of metamorphic rocks. There are several types of surficial mass movement in slopes, for instance, rock/soil slide, debris flow, and rock falls. Slope failure has occurred due to the geological discontinuities, the boundary of strata, joints, and so on. From the observation of more than 100 slope failures along the PNH-1, the authors classified the surficial mass movements into three types: ‘Denudation’, ‘Flow’, and ‘Slide’. Among these three types, ‘Denudation’ and ‘Flow’ destabilize the surface layer of the slope after a while after cutting, while ‘Slide’ tends to collapse on the slope within a short time after cutting. According to Varnes’ landslide classification, surficial mass movements such as ‘Denudation’ and ‘Flow’ are categorized as ‘Flows’, and ‘Slide’ are categorized as ‘Slides’.