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<p>Rainfall-induced landslides frequently disrupt the transport network across the tectonically active mid-Nepal Himalaya. This study examines the relationship between residual shear strength (in terms of the angle of residual internal friction, ϕᵣ) and the mineralogical composition of roadside landslide soils collected from fault and thrust zones along the Kanti National Roadway. Fifteen representative soil samples were analyzed using ring shear tests and X-ray diffraction (XRD, powder method) to quantify primary framework minerals (e.g., quartz, feldspar) and secondary phyllosilicates (e.g., chlorite, mica). The ratio of secondary to primary minerals, defined as the Secondary to Primary Peak-Area Ratio (<i>SPR</i>), was proposed as a new mineralogical index to predict <i>ϕ</i>ᵣ. Results show a strong inverse correlation between <i>SPR</i> and <i>ϕ</i>ᵣ, indicating that phyllosilicate enrichment markedly reduces residual strength through frictional weakening and significant realignment of platy particles during shear. Samples with similar clay content but differing mineralogy exhibited distinct <i>ϕ</i>ᵣ values, confirming that mineral composition, rather than total clay fraction, controls post-failure behavior. Spatial trends indicate maximum phyllosilicate enrichment within 2 km north of the Main Boundary Thrust (MBT), consistent with low-grade metamorphism and hydrothermal alteration. The <i>SPR</i> index provides a practical and cost-effective proxy for estimating <i>ϕ</i>ᵣ from XRD data, useful for preliminary slope stability analysis and hazard assessment in data-scarce Himalayan terrain. Although limited to low normal stresses (≤200 kPa) and a modest sample size, the findings establish a clear link between tectonic deformation, mineralogical transformation, and geotechnical weakening in the Lesser Himalaya.</p>

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Effect of Phyllosilicate Enrichment on the Residual Shear Strength of Thrust Zone Landslide Soils in the Mid-Nepal Himalaya

  • 期刊论文

  • 发表日期：

    2026年3月21日

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  • Suresh Neupane,
  • Netra Prakash Bhandary,
  • Saurav Sharma

摘要

Rainfall-induced landslides frequently disrupt the transport network across the tectonically active mid-Nepal Himalaya. This study examines the relationship between residual shear strength (in terms of the angle of residual internal friction, ϕᵣ) and the mineralogical composition of roadside landslide soils collected from fault and thrust zones along the Kanti National Roadway. Fifteen representative soil samples were analyzed using ring shear tests and X-ray diffraction (XRD, powder method) to quantify primary framework minerals (e.g., quartz, feldspar) and secondary phyllosilicates (e.g., chlorite, mica). The ratio of secondary to primary minerals, defined as the Secondary to Primary Peak-Area Ratio (SPR), was proposed as a new mineralogical index to predict ϕᵣ. Results show a strong inverse correlation between SPR and ϕᵣ, indicating that phyllosilicate enrichment markedly reduces residual strength through frictional weakening and significant realignment of platy particles during shear. Samples with similar clay content but differing mineralogy exhibited distinct ϕᵣ values, confirming that mineral composition, rather than total clay fraction, controls post-failure behavior. Spatial trends indicate maximum phyllosilicate enrichment within 2 km north of the Main Boundary Thrust (MBT), consistent with low-grade metamorphism and hydrothermal alteration. The SPR index provides a practical and cost-effective proxy for estimating ϕᵣ from XRD data, useful for preliminary slope stability analysis and hazard assessment in data-scarce Himalayan terrain. Although limited to low normal stresses (≤200 kPa) and a modest sample size, the findings establish a clear link between tectonic deformation, mineralogical transformation, and geotechnical weakening in the Lesser Himalaya.