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  Preliminary assessment of environmental impact for a high potential water inflow tunnel in Taiwan

  Taiwan is located on a convergent plate boundary between the Asiatic continental plate and the Philippine Sea oceanic plate, strongly folded, faulted and highly fractured rocks are common. The fissures and voids in broken rocks may provide well flow path and storage space for groundwater. In addition, Taiwan also has plentiful rainfall. The approximate annual rainfall of 2500 mm is about 2.6 times of the world’s average value. And the startling rainfall of up to 6000 mm/year was occurred in some windward mountain area. Massive rainfall and broken geological condition cause that the groundwater resources in Taiwan is considerably abundant. Therefore, groundwater flow into the tunnel during construction is not unusual in Taiwan. Some tunnelling projects were even delayed or stopped by the unexpected large water inflow. The most famous case in Taiwan is the New Yungchuen Tunnel, which original tunnel route was abandoned by the disastrous water ingresses (Wang et al., 2011). Except for the hazards of tunnelling instability and construction delay, large water inflow may also give rise to the concerns about the loss of groundwater resources. In recent years, the stability of water supply is attacking by severe climate change. Heavy rainfall and aridity seem to become gradually common. For the duration of dry season, regional groundwater resources would decrease and of course the water supply is insufficient. The insufficiency of water supply is easily blamed on the neighbouring tunnel construction in case of water outflow during tunnelling. Therefore, the variation of the regional groundwater resources induced by the tunnel construction is necessary to be understood to verify the actual reason for the possible shortage of water supply. The science of hydrogeology that study the occurrence, distribution, movement and properties of the water below ground surface would be a proper method to assess the variation of groundwater resources during tunnelling. A high potential water inflow tunnel in eastern Taiwan was introduced in the paper to illustrate the hydrogeological surveys performed in the case tunnel. And the groundwater resources variation during tunnelling was further estimated base on the results of field measurement and hydrogeological modelling. The background and the hydrogeological surveys of the case tunnel were described briefly as flows.

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