Abstract: Ankara – Istanbul high speed railway construction project consists of 32 tunnels. Tunnel 36 is one of these tunnels and water problem encountered during the excavation of this tunnel is studied in this paper. Total length of that tunnel, which is excavated with NATM, is 4096 meters and located between km 239+934,00 and km: 244+030,80. Tunnel is excavated in Porsuk Miocene Formation (sandstone, clay stone, gravel alternation) and Akpinar Formation (limestone-sandy limestone) which have same ages. Excavations started in the tunnel entrance portal in Porsuk Formation and water inflow has happened while passing Akpinar Formation with dip slip fault at km.241+003,80. Water inflow had negative impact on the units of initial support system and it caused deformations in the lipper-bench especially at the locations where construction hadn't been finished. Deformations at these locations are occurred in this form; softening of the formations, which carry the footplates, with the effect of water inflow thus, formations couldn't carry the loads on them and settlements occur. In this study, the problems caused by underground water in the tunnels and solution methods are discussed.
1 INTRODUCTION Groundwater infiltration into tunnels can pose a serious risk during the execution of works and reduce the speed of excavation. The presence of water in a rock massif can induce some difficulties and increase the cost of excavation (Jansson, 1979; Cesano et al., 2000; Day, 2004). Furthermore, the drawdown produced by excavations can cause hydrological, hydrogeological and environmental impacts on groundwater dependent ecosystems (Vincenzi et al., 2009). Among the common hydrogeological impacts in densely populated areas are the drying of private wells or springs close to the tunnel axis due to the water table drawdown (Sjolander-Lindqvist, 2005) and base flow reduction in rivers that drain the basins crossed by the tunnel (Vincenzi et al., 2009).