Detailed Investigation and Integrated Slope Stabilization of a Progressive Deep-Seated Landslide on the Damchu-Chukha Highway (Km 19.800), Bhutan: A Case Study

A progressive landslide at Km 19.800 on the Damchu-Chukha section of the Phuentsholing-Thimphu highway (AH-48), Bhutan, evolved from pavement cracking, embankment settlement and gabion-wall distress into complete loss of the original carriageway and subsequent deformation of an upslope temporary diversion. The failure history indicated that the instability was not confined to the road embankment but was consistent with a deeper soil-weathered-rock mass movement. A scientific investigation was therefore undertaken using field reconnaissance, one borehole, laboratory characterization, seismic refraction tomography (SRT), rock-mass classification using RMR and GSI, and two-dimensional limit-equilibrium analyses under static, rainfall-saturated and pseudo-static seismic conditions. The interpreted profile comprised approximately 6.0-6.5 m of soil/fill over thick highly and moderately weathered rock of poor rock-mass quality. Unreinforced analyses indicated inadequate stability, with rainfall saturation producing the most adverse response. The proposed mitigation combines a Shored Reinforced Soil (SRS) wall with soil nails/rock bolts, mechanically connected geosynthetic reinforcement, high-tensile mesh, erosion-control treatment, and integrated surface and subsurface drainage. Final analyses reported overall factors of safety of 1.709, 1.300 and 1.241 for static, rainfall and seismic cases, respectively, satisfying the case-specific acceptance criteria adopted for the respective loading conditions. The case demonstrates a diagnostic transition from symptom-based retaining-wall repair to mechanism-based stabilization of the complete slope-road system. It further illustrates the application of a mechanically coupled SRS-soil-nail system where conventional reinforced-soil geometry is constrained by steep Himalayan terrain. The limited analytical reserve under saturated conditions makes drainage performance, construction-stage verification and post-construction monitoring integral to the design philosophy.

deep-seated landslide; Himalayan highway; slope stabilization; shored reinforced soil wall; soil nailing; seismic refraction tomography; rainfall-induced instability.

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