Unexpected Cavern in Double Shield TBM Water Transfer Tunnel Construction Project in Northern Thailand
During TBM-excavation of water transfer tunnel in Thailand, the “Fault 26 Incident” appeared at km. 11+675 on Dec. 20th, 2018. The unexpected large cavern was encountered by the cutter head of the TBM. The cavern was inspected and surveyed. A ground area of about 1,800m² (50x40m), was partially filled with huge blocky rock boulders reaching up to about 10m above tunnel invert. The length of the tunnel axis crossing the cavern is about 30m. A method for driving the TBM through this cavern is as follows: 1st step: bypass tunnel excavation to the cavern at the right side of TBM. 2nd step: foam sealing in a depth -10m below tunnel invert to provide a bottom barrier. 3rd step: application of self-compacting concrete up to elevation +10m (5m above tunnel roof) to consolidation of the rock boulders. 4th step: down stage grouting in case of open voids. 5th step: drive TBM passed through the treated breakdown pile of rock. Program for monitoring and observation the tunnel displacement during and after TBM pass through the treated cavern is also provided. The incident could be managed in a controlled way without serious impact to human health & safety and the project.