The Prithvi Highway is Nepal's economic lifeline — connecting Kathmandu to Pokhara and carrying the bulk of trade and transport between the country's two largest cities. Yet the highway traverses one of the most geologically hazardous corridors in the Himalayan region, where landslides close the road for weeks each monsoon season.
Overview & Motivation
The Prithvi Highway corridor traverses geologically complex terrain characterised by steeply dipping phyllite, quartzite, and metasandstone of the Lesser Himalayan sequence, active fault zones associated with the Main Boundary Thrust system, and catchments that receive 1,800–2,400 mm of annual precipitation — concentrated in the June–September monsoon window.
This combination of weak rock, steep relief, active tectonics, and intense seasonal precipitation creates persistent and recurring slope instability that threatens both road infrastructure and the settlements that depend on it. Despite the well-documented hazard, subsurface characterisation of individual landslide bodies remains limited — most previous studies have relied on surface mapping alone.
Research Gap
Surface mapping can identify landslide morphology and extent, but cannot resolve the critical subsurface architecture — the depth and geometry of the failure surface, the thickness of saturated colluvium, and the nature of the substrate — that controls failure mechanics and informs stabilisation design. This investigation uses ERT to fill that gap.
Study Area
The study covers a 12 km stretch of highway between Malekhu and Benighat, where field reconnaissance identified seven active and semi-active landslide features of varying size and activity level. The geological sequence comprises Lesser Himalayan metasedimentary rocks — predominantly phyllite and quartzite — with local intrusions of Proterozoic granite.
The regional structure is dominated by NW-SE striking thrust sheets dipping 25–45° to the north, creating a characteristic trellis pattern of drainage and slope asymmetry. Slope angles of 30–60° are common throughout the study area, frequently exceeding the angle of internal friction for weathered phyllite (φ ≈ 22–28°).
Methodology
Phase 1 — Geological Mapping
Systematic geological mapping at 1:5,000 scale documented lithological units, structural measurements (strike, dip, joint orientation, spacing, and roughness), and landslide morphological features including scarps, flanks, toes, and internal deformation structures. Rock mass quality was assessed using the Rock Mass Rating (RMR) system at 24 stations distributed across the study area.
Phase 2 — ERT Geophysical Survey
Six ERT profiles totalling 840 m were acquired across four landslide features using a 48-electrode Wenner-Schlumberger array with 5 m electrode spacing. This configuration provides optimal depth sensitivity to approximately 40 m, well suited to resolving the colluvium-bedrock interface critical to slope stability assessment.
Data inversion was performed using RES2DINV software, applying a robust least-squares inversion algorithm with a damping factor of 0.05. Convergence was achieved within 5–7 iterations with RMS errors of 3.2–6.8% across all profiles.
Phase 3 — Integration & Interpretation
Resistivity cross-sections were compared against surface geological observations to assign lithological interpretations to resistivity zones. Corroborating data from road-cut exposures, stream bank sections, and available borehole logs from previous highway maintenance investigations were incorporated into the final geological model.
Results
| Resistivity Zone | Value (Ω·m) | Interpretation | Depth (m) |
|---|---|---|---|
| Zone A | 8–25 | Saturated colluvium / failure zone | 0–18 |
| Zone B | 25–80 | Moist colluvium / weathered phyllite | 5–25 |
| Zone C | 80–300 | Moderately weathered phyllite | 15–40 |
| Zone D | >300 | Fresh phyllite / quartzite bedrock | >30 |
The most significant finding is the consistent presence of a low-resistivity zone (8–25 Ω·m) at 12–18 m depth across the four profiled landslide features. This zone aligns closely with the shear surfaces visible in exposed road cuts, confirming a translational failure mechanism in which colluvial material slides along the weathered phyllite substrate.
Primary Finding
The lithological contrast between saturated colluvium (ρ = 8–25 Ω·m) and underlying weathered phyllite (ρ = 80–300 Ω·m) at 12–18 m depth is the dominant control on slope failure along the Prithvi Highway study corridor. This interface represents the critical failure surface for slope stabilisation design.
Significance & Implications
These results have direct practical implications for highway maintenance and slope stabilisation. Identification of failure surface depth and geometry allows engineers to design targeted interventions — subsurface drainage at 10–15 m depth, anchored retaining structures, or slope regrading — rather than relying on surface-only treatments that address symptoms without resolving the underlying mechanism.
More broadly, this investigation demonstrates that ERT can serve as a cost-effective, non-invasive screening tool for landslide characterisation along Nepal's mountain road network — providing critical subsurface information at a fraction of the cost of geotechnical drilling while maintaining sufficient resolution for practical engineering application.
Status & Next Steps
Field data acquisition is complete. Analysis and report preparation are ongoing. Planned next steps include: (1) numerical slope stability modelling using the ERT-derived subsurface geometry, (2) seasonal comparison of ERT profiles acquired in pre-monsoon and post-monsoon conditions to quantify saturation-induced resistivity changes, and (3) production of a final hazard zonation map for the 12 km study corridor.