Study On The Impact Of Tunnel Construction On The Groundwater Environment Of The Partition Style Karst Water-rich Anticline

With China's economic development and accelerating urbanization process, the cities demand and depend on more and more transport facilities. Although greatly developed, the transportation industry is often limited by nature conditions in many regions, such as southwest mountain areas of China. In such places, bridges and tunnels have to set up to meet the requirement of railway or highway routes.During the construction of tunnels, especially in the southwest karst regions of China, many tunnels are facing groundwater hazard. On the one hand, a lot of water inrush accidents cause great danger to the safe and operation of the tunnels; on the other hand, it is also a destruction to the groundwater resources with a large number of groundwater discharged from tunnel sites.The groundwater hazard has always drawn attention for serious casualties and economic losses during the tunnels construction. But in recent years, with the improved technology and the ability to deal with water hazard, the sensitivity of water inrush has decreased gradually. The destruction to groundwater resources and the impact on the local residents caused by the construction of previous tunnels is increasingly revealing. The focus of tunnel construction is gradually altering from the hazard of groundwater on the tunnel construction to the hazard of the tunnel construction on groundwater, and from the direct impact on the surface landscape to the impact on the groundwater environment. With the enforcement of environmental protection policies in our country, it has become an essential process to evaluate and predict the impact on groundwater resources caused by tunnels construction.The study zone is located in ZhongLiang Mountain in the west of Chongqing city; the construction of a series of tunnels has serious effect on groundwater resources. This paper analyzes and studies the possible impact caused by proposed Chengyu passenger special line, Gele mountain tunnel, and draws following conclusions, which play a reference part in the construction of the tunnels in similar hydrogeological conditions areas.1. Guanyin gorge anticline in the ZhongLiang Mountain is mainly divided into east and west valleys which are two nearly north-south independent T2l and T1j karst hydrogeologic units. Groundwater mainly discharges in deep-cycle to the Jialing River in the north and the Yangtze River in the south. The first and third water-rich sections in the T1f formation are independent hydrogeological units, groundwater also discharges to Jialing River in the north and the Yangtze River in the south, the exposed area is small and the recharge is very limited, the amount of groundwater stocked is limited, and the relationship between the units and the tunnel is not close.In addition, the gullies in west and east sides of the mountain, which cut through T₃xj to T₂l + T₁j formations, discharge part of the groundwater as a part datum level of drainage, which forms a local runoff system.2. By analyzing the effect of the existing tunnels on groundwater environment, it is obvious every tunnel has hydraulic connection with the local groundwater flow system, and the dewatering and the impact on the groundwater are limited. It shows that the connection between the tunnel in the east valley and shallow local flow system is closer than west valley. It also shows that two groundwater flow systems in the east valley and the west valley are independent.3. As the tunnel is more sensitive to the groundwater in the east valley than the west and the tunnel section in the east valley locates 800m north of Xiangyu railway tunnel, the impact range in the north of the east valley expands to north. Since the university town tunnel of Yusui highway have been completed, the impact range in the south of both valley has connected with the impact range of Chengdu-Chongqing Expressway, and the groundwater has been dewatered, so it is expected that Chengdu-Chongqing Passenger special line tunnel does not impact the south side not obviously.Because the tunnel trunk of the proposed tunnel in the west valley is nearly in the center of the cone, it makes little change to the groundwater in the west valley and the expanding effect range is small.4. If the drainage occurs during the construction, and the timely and effective plugging measures are taken after then, the restoration effect of groundwater should be obvious after taking plugging measures in proposed Gele mountain tunnel, according to the plugging practice of Gele mountain tunnel of Yu-Huai railway.5. Use the numerical simulation, Visual Modflow, to simulate the impact of the tunnel excavation on the groundwater in the tunnel site under different conditions. The results are as follows:(1) When drainage is the main part in the construction, in the case that the predicted normal inflow is 18952m³/d, the expanded range of the pumping cone is to extend about 800m to the south and the north of the original pumping cone.(2) In the case that the tunnel is pre-plugged (considering the inflow of 1890m³/d) and the groundwater runoff reaches stability, the boundary of the original pumping cone of the groundwater is unchangeable, except the core region in the east valley occurs a certain degree of expansion.