Main Hydrogeological Question Of Chengdu Subway 1, 2 Line Project

The Chengdu subway project is the key project to implement western development strategy and promote the city image. It has vital significance to alleviate the condition of urban traffic, impel and promote the urban economic development. Chengdu is situated at middle and upper reaches of the Minjiang River .The water in Chengdu area is very rich because of the Dujiang weirs hydraulic engineering. The area main distributes Quaternary loose piled sand gravel stratum pore phreatic water, strong permeability, aquifer thickness from 110 meters to several meters. The subway project constructs in rich water area, the hydrogeological question is main controllability geological question in the engineering construction.The city groundwater is general flow from northwest to southeast. The subway 1 line is north-south direction; the subway 2 line is east-west direction. After the subway completed, the underground station and tunnel in fact form a underground anti- water building like "+". The article main studies whether this building can change the city original groundwater flow field, and creates the correlation hydrogeological question. Because the city groundwater quality is originally good, the subway project may has a little influence to it with the requirements strictly. Therefore, this article emphatically analyze two aspects: interfering flow field in operation period and station precipitation in construction period.This article relied on "Chengdu the second metro line project environmental effect appraisal" research project, and simultaneously unified the first metro line project which was under construction. This article system calculated precipitation influence scope and water inflow, and had carried on the Chengdu groundwater flow field simulation forecast. The concrete content and the achievement were as follows:1. According to the Chengdu urban district and the regional geological environment, the hydrogeological environment, Chengdu divided into three hydrogeological districts. It was: west strong water-rich area; middle medium water-rich area and east poor water area.2. In operation period, the article analyzed the relations between subway trend and groundwater runoff, computing decrease extent of groundwater flow sections after subway constructed. The groundwater flow sections of 28 underground stations were 97110.7 square meters in natural state. When subway constructed, they decreased to 53853.7 square meters, decrease extent was 44.5%. The groundwater flow sections of 30 underground tunnel were 448480.8 square meters in natural state. When subway constructed, they decreased to 321809.8 square meters, decrease extent was 28.2%. Through the groundwater flow field simulation and prediction, it might see in the subway construction pre-and post:â‘ The direction of groundwater runoff in Chengdu urban land was not fundamentally changed; it had small influence to the region flow field after the subway completed.â‘¡When groundwater flowed after subway line, the phreatic water surface produced certain raising level, especially in west strong water-rich area.Simulation and prediction two situations groundwater raising level after the subway project completed, the maximum value was 0.248 meters when just considering 2nd line in Niu Wangmiao station. The maximum value was 0.298 meters when simultaneously considering 1st and 2nd lines in east gate bridge station.3. In construction period, the article calculated each underground station influence scope of foundation pit dewatering: station influence scopes in west strong water-rich area were in 620 meters; station influence scopes in middle medium water-rich area were in 460 meters except south train station south three surrounds administration square road section. Because of unwater length oversized, influence scope in that section was 1711 meters.Because the station far and near from the rivers was different, the article calculated foundation pit water inflow in the different situations. It was under 14000 cubic meters per day of 1st line far from the river. It was over 10000 cubic meters per day of 2nd line in west strong water-rich area; it was under 5000 cubic meters per day in middle medium water-rich area. There were 4 stations near the river. Maidenhair tree station had the biggest foundation pit water inflow because deep fall and too close to the river. The east gate bridge station had the minimum value.4. In view of near the river area subway station cut and cover construction technique, simulated supply situation between the river water and the groundwater. To protect the pollution when river water inflow groundwater, it proposed the optimization design proposal. This article take the traditional Chinese medicine college station as an example, established the ground water transfusion model. In order to make drawdown required standards, it obtained pumpage of 4 pumping wells: Q_c1=3000 cubic meters per day, Q_c2=Q_c3=2600 cubic meters per day, Q_c4=3100 cubic meters per day. After pumping,, the water level whole dropped in the area, the water near foundation pit became an underground funnel. At that time the river water level was higher than the groundwater level, the river water supplied groundwater, 2 rivers became discharge area to recharge area. After joining 2 Q=2000 cubic meters per day water-injection wells between the Modi river and the traditional Chinese medicine college station, joining 1 Q=1000 cubic meters per day water-injection well between the traditional Chinese medicine college station and the Xijiao river, there became groundwater divide between the rivers and the station pumping wells, the river water could not flow backward the ground water. At the same time to make the station base water level reaching standard, it must enlarge foundation pit water inflow, and Q_c1=3500 cubic meters per day, Q_c2'=Q_c3'=3000 cubic meters per day, Q_c4'=3600 cubic meters per day.