Integrated Analysis of Inclinometer, Rainfall, and DGPS Tension-Crack Data for a Slow-Moving Landslide: A Case Study from Chandmari Village, Sikkim

Slow-moving landslides (SMLs) are governed by a combination of causative factors such as geology, tectonic setting, and climatic regime, along with triggering and conditioning factors including precipitation, complex groundwater dynamics, river toe erosion, seismic activity, and anthropogenic modifications. In this study, we present results from monitoring and analysis of a slow-moving landslide at Chandmari Village, Gangtok, in the eastern Himalaya. The analysis is based on real-world data obtained from an operational landslide early warning system active at the site since 2018. An integrated, multi-instrument dataset combining subsurface deformation measurements, hydro-meteorological observations, and high-resolution surface mapping is used to investigate landslide behaviour.

Slope movement was monitored using in-place inclinometer strings installed to depths of up to 30 m within the deforming slope mass to capture depth-wise displacement profiles. These measurements were analysed alongside continuous rainfall data recorded at 5-minute intervals from a local rain gauge to examine hydrological controls on deformation rates. In addition, Differential GPS (DGPS) surveys were conducted to map the spatial distribution of tension cracks on the slope surface. The combined dataset enabled assessment of both slip-surface depth and the spatial extent of the landslide body, as well as their response to seasonal rainfall.

Preliminary results indicate that periods of sustained rainfall correspond to increased displacement rates at specific depth intervals, suggesting activation of shear zones rather than a single, well-defined slip surface. Prolonged rainfall episodes further indicate the involvement of deeper slip surfaces. DGPS-based mapping reveals well-defined tension cracks at the head of the landslide body, while time-lag analysis between rainfall accumulation and deformation response highlights delayed slope adjustment controlled by groundwater dynamics.

The study demonstrates the temporal coupling between subsurface deformation and seasonal hydrological forcing and illustrates the effectiveness of multi-instrument monitoring for characterizing slow-moving landslides. The insights gained into deformation mechanisms support the development of improved early warning and mitigation strategies for urban hill-slope environments under long-term climatic and anthropogenic stress.