EVALUASI SISTEM PENGAMANAN BERDASARKAN NILAI ROCK MASS RATING (RMR) PADA TEROWONG PENGAMBILAN BENDUNGAN DI LOMBOK BARAT, NUSA TENGGARA BARAT

The construction of the dam in West Lombok is one of the National Projects of the Ministry of Public Works and Public Housing (Kemen PUPR) located in West Nusa Tenggara. The tunnel excavation process for this dam entails blasting and digging, which could potentially disrupt the stability of the surrounding area and lead to rock mass movement, including rock falls. The aim of this study is to analyze the quality of the rock mass related to the design of the support system in the tunnel segments. The results of this study show that the tunnel segment STA 0+290.704 - 292.804 exhibits good rock mass quality (RMR 61-80), where, based on the support recommendations (Bieniawski, 1989), the use of rock bolts on the roof with a length of 3m, spacing of 2.5m, and combined with wire mesh, is advised. Additionally, shotcrete with a thickness of 50mm should be applied to the roof. For the segments STA 0+285.309 - 288.104, STA 0+288.104 - 290.704, STA 0+416.068 - 414.864, and STA 0+417.134 - 416.068, which exhibit fair rock mass quality (RMR 41-60), the recommended support system (Bieniawski, 1989) includes the use of rock bolts on both the walls and roof with a length of 4m, spacing of 1.5–2m, combined with wiremesh, and shotcrete application with thicknesses of 50-100mm on the roof and 100 mm on the walls. For the segment STA 0+420.144 - 417.134, which has poor rock mass quality (RMR 21-40), the support system recommendations (Bieniawski, 1989) suggest using rock bolts on both the walls and roof with a length of 4m, spacing of 1.5–2m, combined with wiremesh, and shotcrete with a thickness of 100-150mm on the roof and 100mm on the walls. Additionally, the installation of steel ribs/steel sets is recommended.