Discussion And Identification On Zhongba Tunnel Karst Groundwater System Numerical Model

Zhongba tunnel, located in the Southwest Sichuan Basin, is a typical karst tunnel which is also a local critical control tunnel connected with major line. It is representative interstream area on the simple structural environment without fracture structure crossing. Still, there are plenty of depressions and avens along the tunnel line on surface as well as underground rivers below ground, so it should be highlighted the problems of water bursting and environmental hydrogeology. All in all, it is essential to analyse and predict the interaction of tunnelproject and groundwater environment.Currently, numerical simulation is one of the main means to solve the problem of tunnel karstwater bursting. However, there is a serious defect about a generalized model on certain research area, because elemental information collected by linear geological survey is finite. Meanwhile, some traditional calibration model methods,such as water balance simulation, level observation fitting and seepage field fitting, is little meaning actually. Above all, adopting a more reasonable method to check the model is of great practical significance.According to the environmental geological karst development characteristics of the section, this paper has divided karst water system and researched each water system characteristics. Then, we used the Visual Modflow to simulate the seepage field in the study area fully and check the preliminary validation. After selecting suitable object in the model, we combined with water measured value to check the model. Finally, we forecasted the groundwater environmental impact on the basis of the follow-up period and operation after the tunnel construction. There is the following several aspects we obtained:(1) The study area is located in the plateau transition of Sichuan Basin and the Yunnan-Guizhou, interstream area on the landscape, and the area is controled by common the river cutting on both sides. The structural environment of the region is simple and have no fault structure cross. The main karst stratum of the area is Trias including T21, T1j and P1m, among which the highest development of karst is P1m, followed by T21and T1j.(2) The study area where there is a karst stripe is mainly bare and the distribution is obvious concentrated and large exposed. Within the stripe, karst phenomenon such as avens, depressions and underground rivers are abundant controled by local structure line, and bedding arranged as a string of beads along the axial distribution, development height more than around800-1000m.(3) According to the different factors of aquifer petrofabric, the study area is divided into two hydrogeologic units:Permian karst water unit and Triassic karst water unit. The Permian karst unit smaller influence on tunnel. Among above, the influence on tunnel of Permian karst unit id smaller than Triassic due to the tunnel position. Then according to the watershed, karst water units is divided into1-1and I-2, and analysis on each unit is conducted.(4) Based on analyzing the characteristics of karst water system in the study area, Visual Modflow is used to established the karst water system of3D dimensional groundwater flow field model. On this basis, according to the feature of water outburst and mud outburst of Zhongba tunnel, selecting high-growth areas and groundwater watershed as an object, combined with the measured water inflow to the model to identify and develop appropriate identification scheme. Eventually when the high-growth area of karst generalization width of200m, the groundwater divide2958m from the first river side, water inflow calculated by model is most close to the measured values, think that the scheme is the most reasonable;(5) On the basis of the above study, to establish model to reveal the corresponding variation in the condition of exposed rivers, unexposed rivers and blocking condition prediction. The results showed that, when the undisclosed groundwater divide position journeyed head river side, the migration distance is about300m. Tunnel biggest drop up to25m, the influence of taking the tunnel line as the center is about1.1km.The underground water level has been returned to a natural state after tunnel closure2years later.(6) The study on the lack of basic data, the conceptual model under the condition of defects is more reasonable approach to identify, and provides an idea reference for the simulation of subsequent similar situation in the region.