The Relevancy of Mineral Composition to Rock Shear Strength in Kaliwadas Hills, Karangsambung, Kebumen

Abstract

The occurrence of landslides on the slopes due to a decrease in shear strength in the area of landslides in the limestone hills of Kaliwadas, Kebumen, Central Java, causes public facilities to be damaged and disrupting the activities of everyday citizens. Factors that influence rock shear strength are cohesion value and internal friction angle, while mineralogy and grain size is one of the factors derived from the environment which also affects the shear strength value. This study aims to determine the effect of mineral composition and grain size on rock shear strength. A number of rock samples were taken from the study site to be prepared, cylindrical samples for direct shear and petrographic strength tests with thin incision samples. Direct shear test results provide cohesion values and deep shear angles in limestone with peak and residual conditions of 0.22 Mpa and 39.73 ° respectively; 36.61 kPa and 29.28 ° while in claystone 67.86 kPa and 25.36°; 13.41 kPa and 16.17 °. Petrographic analysis of rocks shows that limestone contains calcite (88%) and quartz minerals (5%), while claystone contains calcite (91%) and quartz minerals (2%). Based on the mineral content of limestone and claystone and their effect on rock shear strength, it can be concluded that limestones have a quartz mineral composition, cohesion values and deep shear angles that are better and larger when compared to rock shear strength in claystone and believed to be the cause of the slip field in the research location.