Engineering Geological Characterization of DGSDs in the Himalaya

The Himalayan region is characterized by complex tectonic activity, steep terrain, and highly variable soil and rock mass conditions. Among the main geomorphic processes shaping Himalayan slopes, Deep-Seated Gravitational Slope Deformations (DGSDs) is a fundamental component of engineering geomorphology that directly and/or indirectly possessing the engineering geological and geotechnical challenges. Many researchers investigated many mountain slopes globally, which have distinct geomorphic signatures due to DGSDs movement. These deformation processes are generally classified into three main types: sackung (or sagging), lateral spreading, and rock block sliding. In many cases, these DGSDs are commonly associated with active fault systems. In the Himalayan context, DGSDs are often expressed as Large-Scale Landslide Topography (LLT), with inferred ages ranging from approximately 100 years BP to 10,000 years BP. Evidence of DGSDs can also be evaluated through the presence of ancient landslides and landslide dams. These large-scale slope deformations are most likely initiated following major Himalayan earthquakes and are driven by a combination of gravitational forces and progressive weakening of slip zones. This encourages to use DGSDs for study of paleo seismicity in Nepal. The weakening of slip zones is usually enhanced by clay mineralization along rock joints and shear zones resulting from hydrothermal alteration associated with Main Central Thrust (MCT).

Based on present activity in the Himalaya, DGSDs can be broadly classified into four types: active, dormant, reactivated, and extinct. Each category presents different levels of risk and civil engineering relevance. The engineering geological and geotechnical challenges associated with DGSDs can be effectively assessed through combining topographical maps, satellite and remote sensing data, engineering geological mapping and detailed field investigations.

After few extensive observations along roadside slopes, tunnel portals, and tunnel alignments in the Nepal Himalaya, a strong relationship between DGSDs and adverse engineering geological conditions are well identified. DGSD-prone terrain presents specific challenges, including difficulty in identifying, quantifying, and extrapolating deformation zones, limited working areas for investigations, elevated construction risks, increased engineering geological uncertainties in tunnels portals and roadside slopes.

Through selected case studies, this paper highlights key engineering geological and geotechnical challenges encountered in DGSD-affected areas in the Himalaya and demonstrates how proper geomorphological site evaluation can support optimal alignment selection for roads, hydropower projects, and tunnels. Ultimately, improved understanding of DGSDs is essential for reducing landslide risks and minimizing the tendency to attribute project difficulties to undefined “geological problems” in the Himalayan region.