Slope mass rating-based analysis to assess rockfall hazard on Yogyakarta Southern Mountain, Indonesia

In the Parangtritis Beach tourism area located in the Southern Mountain of Yogyakarta, karst hills were excavated to build the main accessing road and produce some of long and very steep slopes along the sides of the road. But still, there was none of the slope reinforcement installed along the road. Meanwhile, at several nearby locations within Southern Mountain, rockfall incidents have occurred many times even caused casualties. The potential of rockfall hazard also could be found in the main road of Parangtritis Beach as the study area. The purpose of this study is to determine the rockfall hazard assessment along the main road using Slope Mass Rating (SMR) analysis with the additional parameter of the slope height and the rock block size. The necessary data obtained by direct measurement and laboratory test. Geomechanics analysis, stereographic projection analysis, and hazard parameters weighting were carried out to produce the Rockfall Hazard Zonation Map of the study area. Based on 17 measurement stations, there are 4 (four) rockfall hazard classes in the study area, i.e., very low, low, moderate, and high. The class of very low, which also included road segments without slope, has the largest percentages of 83.83%, followed by the classes of moderate, low, and high with the percentage of 7.16%, 4.28%, and 4,19%, respectively. SMR was assumed as the most significant parameter that influences the rockfall hazard zonation. Historical rockfall points were overlaid over the Rockfall Hazard Zonation Map to validate the predicted hazard zones. Since 91.23% of the rockfall occurred in the moderate and high hazard classes, the zonation map considered reliable to predict future rockfall. This study also identified several landslide potential zones and provides the recommendation of slope reinforcement to be installed in the study area.