Rock mass quality determination of weak rock masses for design of Narogong diversion tunnel in West Java, Indonesia

Narogong weir was planned to be located in West Java, Indonesia. The rock types in the weir location are dominated by alternating siltstone and shale marlstone. The tunnel’s line was designed to be perpendicular to the axis of an anticline. This study aimed to determine the quality of rock masses at the Narogong diversion tunnel and recommend the tunnel excavation method and support system. The Geological Strength Index (GSI) and Central Research Institute of Electric Power Industry (CRIEPI) classifications were used to determine the rock mass classes and the rock mass qualities. Then, the tunnel excavation method and support systems were resolved by the Rock Mass Rating (RMR) table based on the newly developed GSI-RMR correlation. The laboratory testing shows that siltstone and shale marlstone in the research area had low Uniaxial Compression Strength (UCS) values ranging from 4.5 to 25 MPa and fell into a weak rock class. Although the siltstone and shale marlstone had low UCS values, the drill core evaluations indicated that the alternating layers were classified as fair and good quality rock masses based on GSI and CM – CH class based on CRIEPI. The lower GSI values of the rock masses in BT-09 and BT-12 were affected by the presence of the anticline fold. The tunnel excavation method and support system were divided into two segments based on rock mass qualification.