The handling of tunnel collapse and settlement (case study: Jragung Dam diversion tunnel, Indonesia)

A diversion tunnel is a crucial part of dam construction because of its function to derivate the river during earth dam filling works. The Jragung Dam diversion tunnel is located in the Kalibeng Formation, which is dominated by claystone and sandstone lithology, and passed by an anticline fold structure. The geological conditions provide challenges in tunnel construction. The collapse of rock material in the tunnel overburden and settlement on the slopes hill requires a comprehensive approach and handling by prioritizing factors of safety, cost efficiency, and speed of time. Field observation including rock mass quality analysis, reviewing the design, also monitoring of discharge and deformation was conducted to find the source of collapse. The groundwater flow and deformed soft rock were accused of being the tunnel collapse determinant. The rock mass improvements used pore-filling of sodium silicate mixture and fore-polling. Then dewatering was carried out using drainage pipes and deep wells. Because of the steep rock’s dip in the tunnel, the release wells and observation wells location need to design effectively. Geoelectric investigations correlated with bore log analysis aimed to find effective locations for groundwater control. The settlement was refilled with plastic concrete because of its lightweight and durability.