Surficial mass movements on cut slopes of Prime National Highway 1 in Bhutan

This paper presents the characteristics of surficial mass movements after cutting a slope through Prime National Highway 1 (PNH-1) in Bhutan. A surficial mass movement, in the form of slope failures, affects the ability of traffic to move along PNH-1. The geology of the area of PNH-1 consists of metamorphic rocks. There are several types of surficial mass movement in slopes – for instance, rock/soil slide, debris flow and rockfalls. Slope failure occurs due to geological discontinuities, the boundary of strata, joints and so on. From the observation of more than 100 slope failures along PNH-1, the authors classified the surficial mass movements into three types: ‘denudation’, ‘flows’ and ‘slides’. Among these three types, denudation and flows destabilise the surface layer of the slope a while after cutting, while slides tend to collapse on the slope within a short time after cutting. According to Varnes’s landslide classification, surficial mass movements such as denudation and flows are categorised as flows, and slides as slides.