Tunnel Face Mapping for Empirical Assessment to Determine The Method and Type of Excavation On The Outlet Side of The Budong-Budong Dam Diversion Tunnel

Abstract Determining the method and type of tunnel excavation is important, especially when the tunnel enters the construction phase and there are differences between design and field data. Budong-Budong Dam was designed to use a diversion tunnel, which is currently in the construction phase. This research aims to determine the recommended excavation method and type based on geological data on the outlet side of the diversion tunnel. The methodology used in this research is collecting geological data obtained from surface mapping and tunnel face mapping. The surface condition in the research area consisted of 3 lithological units: Tufa-breccia, Tufa-sandstone, and Silt-cobble deposits. The subsurface condition in the research area observed by face mapping in the Budong-Budong Dam diversion tunnel from the outlet uses the rock mass quality parameters of the RMR and GSI classifications. Based on the RMR classification, the rock mass quality on the outlet side of the diversion tunnel is generally included in the good rock class with an RMR value ranging from 62 to 70. Based on the GSI classification the rock mass on the outlet side of the diversion tunnel is generally classified into the good rock with a GSI value of 70.8. The lithologies on the outlet side of the diversion tunnel comprises of hard and massive tufa-breccia. The RMR classification was used to determine the excavation method, while the GSI classification was used to assess the excavatability of the tunnel. Hence, the recommended excavation method is full-face excavation, and the recommended excavation type based on the excavatability tunnel is blasting.