上海北横通道中山公园工作井超深基坑的减压降水设计与验证

Design and Verification of Decompression and Dewatering for Super Deep Foundation Pit of Zhongshan Park Working Shaft in Shanghai North Cross Passage

上海北横通道中山公园工作井基坑开挖深度约31.0 m,坑底下伏⑦1层承压含水层,地下连续墙墙底位于⑧2层承压含水层,且⑧2层与⑨层连通,开挖过程中面临⑦1、⑧2层承压含水层突涌风险。以此为例,依据按需降水的原则对基坑开挖过程中降水井的布设进行了合理设计,同时进行了抽水试验。抽水试验结果表明:⑦层承压水减压降水井布置能够满足降水需求;⑧2层承压水降水井开足时能满足1.05倍安全系数的抗突涌稳定性要求,但无备用井,应考虑在坑外增加2口应急备用井;⑧2层停止抽水后,承压水水位5 h内基本回弹到了初始水位,为保证施工安全,⑧2层降压井启动后现场应配备双电源供电,并确保5 min内自动切换电源。

The excavation depth of the working shaft foundation pit of Zhongshan Park in Shanghai North Cross Passage is about 31.0 m. There are ⑦1 confined aquifer under the pit bottom, the bottom of diaphragm wall is located in the bearing aquifer of layer ⑧2, and Layer ⑧2 is connected with layer ⑨. In the process of excavation, it is faced with the risk of inrush in the confined aquifer of layer ⑦1 and layer ⑧2. Taking this as an example, according to the principle of dewatering on demand, the layout of dewatering wells in the process of foundation pit excavation is reasonably designed, and the pumping test is carried out at the same time. The results of pumping test show that: the layout of pressure reducing wells for confined water of layer ⑦ can meet the demand of dewatering;the pressure reducing wells for confined water of layer ⑧2 can meet the stability requirement of anti-surge stability requirement of 1.05 times of safety factor, but there is no standby well, and so it should be considered to add two emergency spare wells outside the pit;the confined water level basically rebounds to the initial water level within 5 hours after stopping pumping from layer ⑧2. In order to ensure construction safety, double power supply shall be provided on site after starting the depressurization well of level ⑧2, and the power supply shall be automatically switched within 5 minutes.