Combination of chemical indices and physical properties in the assessment of weathering grades of sillimanite-garnet gneiss in tropical environment

Abstract

Changes in mineralogical, chemical, and mechanical properties of a rock mass during the weathering process allow calibrating the degree of weathering. Based on those properties, various weathering indices can be used as tools for this purpose. This study focuses on using chemical weathering indices that represent the chemical behaviour of a rock mass and their relationship with other properties for better evaluation of weathering grades. Metamorphic rocks that undergo weathering due to both chemical and operational means at Smanalawewa hydropower station in Sri Lanka, where tropical climate exists, were selected for this study. Among the chemical weathering indices calculated, the product index, Ruxton ratio, silica-titania index, alumina-titania index, chemical weathering index by Sueoka, and S/SAF index change consistently with progressive weathering. They correlate well with the weathering potential index, Miura index, bulk density, and point load strength. Among the six indices mentioned, the product index, Ruxton ratio, silica-titania, and alumina-titania indices have been recognised as better measures to classify metamorphic rocks in Sri Lanka for chemical means. Since there is a good correlation with physical and mechanical properties, the chemical weathering index and S/SAF index also appear to be better indices in the same context. Weathering potential, Parker, and modified weathering potential indices, which are widely used, show minor fluctuations during weathering. On the contrary, the chemical index of weathering and chemical index of alteration fluctuate with progressive weathering. These changes exhibit the influence of bulk mineral composition of the rock, where aluminium rich minerals are present. This study clearly shows that chemical weathering indices of a particular rock mass are more reliable when they correlate well with physical and mechanical properties.