Rock Mass Classification on Opak Fault Impacted Zone in Mengger Hill, Bantul, Yogyakarta
Abstract
This study aims to evaluate the geotechnical conditions of the rock masses in Mengger Hill that previously has been affected by the Opak Fault. Mengger Hill has some outcrops that show an offset between the bedding planes that indicated it as one of the impacted zone of the Opak Fault that was caused by a 5,9 Richter scale earthquake in Bantul, Yogyakarta. Because it located close to residential areas and and also used as Geosite and tourist attractions because its beauty scenery, rock stability analysis is essential for risk mitigation. This study will be focused on determining the types of constituent rocks, rock strength, and rock mass classifications using RMR (Rock Mass Rating) and SMR (Slope Mass Rating) methods to determine the rock mass quality and support recommendation for the slope of Mengger Hill as an initial development for a sustainable Geoheritage.
The study began with a rock sampling to determine the rock characteristics and scanline method to measure the orientation set of the discontinuities along the slope. The rock samples that has been taken is done for a Point Load Test in Laboratory. The results show that the constituent rock of the rock mass is dominated by massive tuff from the Semilir Formation and has Is(Point Load Index) values between 0.27 to 0.78 MPa that converted into σc (Uniaxial Compressive Strength) using Kramadibrata’s Equation (1992) with a range between 3.22 to 9.25 MPa. The orientation set is indicating a wedge sliding potential with RMR value of 65 (Good Rock) and SMR value of 80 (IIa - Good). The support recommendation is a spot bolting, fence, and toe ditch (optional) to handle debris in the form of blocks failure.
