Empirical design for the excavation method and support system at Budong-Budong Dam Diversion Tunnel, Central Mamuju, West Sulawesi Province

Budong-Budong Dam, located in the West Sulawesi Province of Indonesia, was designed to use a tunnel to divert river flow during the dam construction. The Rock Mass Rating was used to examine the tunnel design with a less detailed drill core investigation. The rock strength parameter is only based on the field estimation. The entire diversion tunnel will pass through Breccia with weak strength. Evaluation is required using integrated and precise data to guarantee safety during and post-construction. The RMR classification was used to determine the excavation and preliminary tunnel support, while the Q-System was used to compare the support of the tunnel. Those classifications were used to anticipate the geological conditions uncertainty. Geological data were collected by geological mapping, evaluation of subsurface drilling, and laboratory analysis. The tunnel will pass through a Tuffaceous Breccia and Tuffaceous Sandstone insert of Tuffaceous Siltstone. In addition, a poor-quality rock was found around the inlet and outlet, and a fair-quality rock in the middle of the tunnel. Hence, drilling and blasting were recommended for the excavation. The tunnel’s proposed support system included shotcrete, wire mesh, and rock bolts in the middle of the tunnel. Steel sets are added around the inlet and outlet.