Sigdel A., Upadhyay S., Acharya S. & Silwal C.B., 2025: Geological and mineralogical controls on landslide formation: A case study from the Birgha Bhanjyang Landslide, Syangja District, Gandaki Province, Nepal. Acta Geologica Slovaca, 17, 2, 51–62.

Geological and mineralogical controls on landslide formation: A case study from the Birgha Bhanjyang Landslide, Syangja District, Gandaki Province, Nepal

Ashok Sigdel, Sushan Upadhyay, Subash Acharya & Champak Babu Silwal

Department of Geology, Tri-Chandra Multiple Campus, Tribhuvan University, Kathmandu, Nepal; ashok.sigdel@trc.tu.edu.np

Abstract

Birgha Bhanjyang landslide, located in the Syangja district of central-west Nepal, is among the most active slope failures in the region. While it is primarily triggered by intense monsoonal rainfall, its evolution is strongly influenced by weak lithology, structural discontinuities, and prolonged rock weathering. Geological investigations reveal that the landslide zone consists mainly of highly weathered shale and phyllite, interbedded with quartzite and minor quartz veins. Geophysical surveys using Electrical Resistivity Tomography (ERT) indicate a thick residual and colluvial cover resting above fractured and weathered bedrock, with distinct shear zones at several depths. These shear zones reflect the presence of the Badi Gad fault, which cuts across the area, contributing to mechanical fracturing and accelerating weathering processes in the rock mass. Mineralogical analysis through X-ray Diffraction (XRD) confirms the presence of clay minerals such as quartz, illite, chlorite, and kaolinite, all products of prolonged chemical and mechanical weathering. Petrographic studies further support these findings, showing feldspar alteration and the branching pattern of biotite decomposition caused by chemical mineral alteration. The geotechnical properties of the soil also show the evidence of ongoing rock disintegration. Altogether, these results highlight that the Badi Gad fault plays a dominant role in driving both mechanical and chemical weathering of the rocks, making it the principal factor behind the instability of the Birgha Bhanjyang slope.

Key words: Birgha Bhanjyang landslide, clay mineralogy, weathering, electrical resistivity tomography, Badi Gad fault

Manuskript doručený: 2025-10-19

Manuskript revidovaný: 2025-12-22