Evaluating red-bed soft rock as backfill material: Ground deformation at a Tibetan construction site

Red-bed soft rocks are abundant in high-altitude regions but are commonly treated as construction waste because of their rapid softening after wetting, high swelling potential, and low bearing capacity under harsh alpine conditions. In remote mountainous areas, where high-quality aggregates are costly to produce and transport, reusing red-bed soft rock as structural backfill can reduce material costs and land occupation. This study investigates an existing red-bed soft rock backfill site and its overlying building in Mangkang, Tibet, China. High-resolution unmanned aerial vehicle (UAV) close-range photogrammetry, core drilling, geophysical prospecting, in situ and laboratory tests, long-term deformation monitoring, and limit-equilibrium stability analysis were combined to characterize deformation, hydro-mechanical degradation, and stability. The site and building experienced severe differential settlement and structural distress. By March 2025, the maximum relative settlement between adjacent column foundations had reached 677 mm; ground-beam voids were 10–24 cm, and crack widths were 4–5 cm, far exceeding code serviceability limits. Laboratory tests showed that silty mudstone had favorable shear strength in the natural state (c = 61.13 kPa, φ = 26.12°) but was highly water sensitive. After 24 h of immersion, specimens disintegrated into fine particles, and the shear strength of the saturated weathered layer decreased by more than 30%, with cohesion and internal friction angle decreasing to 44.15 kPa and 18.25°, respectively. Stability analysis based on backfill geomechanical parameters and historical hydro-meteorological and seismic parameters showed that the overall site remained stable under natural, heavy-rainfall, and seismic conditions, with factors of safety (Fs) of 2.578, 2.248, and 2.163, respectively. However, the frontal margin was only marginally stable under heavy rainfall, with a local Fs of 1.033. The deformation was governed by the coupled effects of backfill-thickness heterogeneity, inadequate compaction, and water-induced strength degradation, and was dominated by consolidation and creep rather than discrete sliding. These findings indicate that red-bed soft rock can be used as structural backfill in alpine regions if drainage, frontal reinforcement, foundation treatment, and long-term monitoring are implemented. This study provides a mechanism-based and cost-effective approach for using local red-bed materials in similar high-altitude engineering projects.