Characteristics of weathering crust developing on Ben Giang – Que Son granodiorite in Nam Dong, Thua Thien Hue

Abstract

Based on field surveys and indoor analyses, we present major characteristics of weathering crust developing on the Que Son granodiorite in the Nam Dong area, Thua Thien Hue (Central Vietnam), including zonation, chemical and mineral compositions, particle-size distribution and physicomechanical properties. The results show that the granodiorite is highly weathered, generating an 8–10 m thick weathering crust with clear zonation. Chemically, the weathering crust is classified into the SiAlFe type with a high concentration of aluminum (Al₂O₃: 24,5 wt.%) coupled with an extremely low concentration of calcium and sodium (CaO: 0,1 wt.%, Na₂O: 0,1 wt.%); the chemical index of alteration is very high at 91,9. Weathering products are predominantly composed of clay and sandy clay. The weathering products consist of clay and silt with a fraction of clay and dust in the completely weathered zone at 65,7–87,8 wt.%, and this fraction decreases with depth. The clay minerals include kaolinite, illite and allophane/ imogolite. The particle-size composition and physicomechanical properties of the soils (moisture, volumetric soil density, saturated index) tend to gradually decrease in deeper zones. These parameters tend to increase in the rainy season, particularly during and/or after long-time heavy rain, making soils saturated, increasing volume and stability lost, which generates landslide masses in the talus slopes, notably along Road 14B and other minor roads in the area.