Study on the causal factors for rapid rock weathering and stability of the Samanalawewa dam : a case study

Abstract

When utilizing rocks for a rockfill dam factors such as rate of rock decomposition have to be considered very seriously for long term stability in addition to shear strength governed engineering properties. Generally the rates of natural decomposition of high grade metamorphic rocks are comparably very low; this process can be accelerated due to several reasons such as influence of climatic conditions, lithology and structure. Especially the metamorphic rocks in the regions of tropical climatic conditions tend to weather more rapidly than the rocks in other climatic conditions. This process occurs basically due to the continuous chemical alterations of minerals with the time under heavy rainfall and temperature conditions, which in turn influences the mechanical properties of rocks. This study aims to investigate the causes of rock weathering and to determine the shear strength parameters of rockfill by estimating uniaxial compressive strength of rocks utilized for the rockfill of Samanalawewa dam to study the present stability of the dam. Department of Geology, University of Peradeniya supplied test results on geochemical analysis and petrographic study from site investigations in the Samanalawewa dam and appurtenant areas for rocks. Those data had been very valuable for the purpose of this study. Other investigations such as finding of geotechnical data were carried out by the laboratory tastings. Those tests were carried out myself at Faculty of Engineering, University of Moratuwa and Central Engineering Construction Bureau. The study begins with a literature review and an interaction matrix, to determine the degree of rock weathering and its relation to the different Physical and Engineering properties of rocks. The predicted parameters are then analyzed based on the compiled data from aforementioned organizations. The final results show that the values of factor of safety of the slope stability of the dam have been reduced with grade of weathering. When rockfill of the dam becomes moderately weathered, most of factor of safety values derived are below permissible level. Hence it is recommended to carry out remedial measures against rock weathering as an urgent requirement.