Excavation method and support systems using empirical design at Pasir Kopo dam diversion tunnel, Lebak, Banten Province, Indonesia

Abstract The design plans for the 433-meter-long Diversion Tunnel in the BBWS Cidanau Ciujung Cidurian area, situated on the right side of the Pasir Kopo Dam in Lebak, Banten, involve the application of the Rock Mass rating (RMR) method for rock mass classification. This approach is also utilized to determine the excavation method for the tunnel, stand-up time, and support systems. In addition, the tunnel support system uses the Q Systems method to address potential geological uncertainties. Surface and subsurface geological mapping, drill core evaluation, and laboratory tests were conducted to collect essential data. Specifically, surface geological mapping and drill core assessment were conducted to evaluate the rock mass quality along the tunnel’s path. Laboratory testing provided details on the physical and mechanical characteristics of the entire rock mass based on drill core samples. The tunnel section consists of a Tuff rock unit and a Lapilli Tuff rock unit, both categorized as having poor (39,83) to fair (42,8 – 48,71) rock mass quality. The drilling and blasting method is recommended, and Excavation techniques differ based on rock mass classification, with the top heading and bench method varying from 1.00m to 1.50m for poor rock (1 zone) and from 1.50m to 3.00m for fair rock (6 zone). The support system for poor rock mass classification includes No steel set required; Rock bolts 3 m and shotcrete 50 mm, and No steel set required, and for fair rock mass, Rock bolts 4 m and shotcrete 50-100 mm in the crown, 30 mm insides. For zones 1,2,5,7 with very poor quality based on Q–Systems support systems using Shotcrete Rock Fibers, Sprayed concrete E = 500 with a 6 cm - 9 cm thickness and bolting length 2.70 m with space 1.5 m – 1.7 m and For zone 3,4,6 using Shotcrete Rock Fibers, Sprayed concrete E = 500 with a 9 cm - 12 cm thickness and bolting length 2.70 m with space 1.3 m – 1.5 m.