Forensic Evaluation and Condition-Based Rehabilitation of A High Rainfall Mountain Highway: Samdrup Jongkhar–Trashigang Road, Bhutan

Premature distress on high-rainfall mountain highways is rarely attributable to a single factor. This paper presents a forensic case study of Km 5.0-72.8 of the Samdrup Jongkhar-Trashigang Primary National Highway in eastern Bhutan, where two contrasting distress regimes were observed during July-September 2026: widespread cracking, potholes, and localized slippage in Km 5-40, and recurrent slippage/shoving concentrated in the loaded downhill lane, particularly at curves and gradients, in Km 40-72.8. Pavement history, distress morphology, pavement-condition indexing, falling-weight-deflectometer (FWD) response, core/material properties, interlayer bond indicators, drainage, geometry, and directional axle loading are integrated into a mechanism-based rehabilitation framework. Five homogeneous performance sections are delineated. The evidence indicates that HS-2 (Km 20-40) warrants structural strengthening, whereas HS-3 to HS-5 is governed primarily by interface/shear and moisture risk and should not be treated by uniform deep overlay. The paper proposes a staged response: remove the failed shear plane, correct moisture paths, restore interlayer bond, use dense bituminous reconstruction, and reserve polymer-modified bitumen or stone mastic asphalt for verified high-shear locations. Instrumented trial sections and quality-control hold points are recommended before corridor-wide treatment. The case demonstrates why surface condition, structural capacity, interlayer integrity, and directional traffic demand must be diagnosed separately before rehabilitation is selected.