Evaluation of geomechanical and geochemical properties in weathered metamorphic rocks in tropical environment: a case study from Samanalawewa hydropower project, Sri Lanka

The effect of weathering on changes of physical and mechanical properties of rocks is a prime concern in the perspectives of geology and engineering. These properties have been studied mostly on weathered igneous and sedimentary rocks under humid climates. Studies on weathering of metamorphic rocks, especially under a tropical climate, are rare. This study evaluates change of physical, mechanical, chemical, and mineralogical properties of metamorphic rocks that weather under tropical climatic conditions. Samanalawewa hydropower project area was selected for this study, because rapid weathering of a metamorphic rock (sillimanite garnet gneiss) was observed in the project site. Fresh rocks that are subjected to weathering have reached to completely weathered condition in a time span of less than 25 years in this area. Visually assessed weathering grades were physically and mechanically evaluated using bulk density, equotip hardness, porosity, specific gravity, point load strength, and slake durability tests. Mechanical properties, especially point load strength, change rapidly at the onset of weathering, while chemical properties show significant changes at later stages of weathering. Mineralogical changes such as appearance of secondary minerals are at the latter part of weathering. Physical properties gradually change during weathering. The observed changes in physical, mechanical and chemical properties indicate that their variations during weathering are independent of lithology and climatic conditions.