深基坑预降水引发基坑变形机理及预测与控制方法研究

More and more excavations are adjacent to operating high-speed railway and subway. Accurately predicting the environment deformation caused by the whole process of excavation construction, and finely controlling the deformation in millimeter level become the important requirement and challenge in excavation. Field measurements show that the deformation of retaining structure and surrounding environment induced by pre-excavation dewatering can reach centimeter level, which can lead to the deformation of operating high-speed railway and subway exceed their allowable values. However, the method to predict and control the pit deformation induced by pre-excavation dewatering have not been yet proposed by the traditional pit design theory and relevant research. Taking the Beizhan Station of Metro Line 3 in Tianjin as the engineering background of this project, based on the laboratory model test and numerical analysis, the mechanism of pit deformation induced by pre-excavation dewatering will be intensively studied. The influence of the construction parameters like dewatering depth, pit width, soil property and retaining wall embedment on the dewatering-induced pit deformation will also be discussed. On this basis, the prediction method of the retaining structure deformation and the surrounding land subsidence will be proposed based on the Winkler foundation model and ground loss method. Finally, based on the mechanism of dewatering-induced pit deformation, the control mechanism of pit deformation under pre-excavation dewatering will be studied, and the method to control the pit deformation induced by pre-excavation dewatering will then be put forward based on the traditional control method of pit deformation.

随着邻近已运营高铁、地铁的深基坑工程越来越多，准确预测基坑施工全过程引起的环境变形并进行mm级的控制成为深基坑建设的重要需求和挑战。工程实测表明，基坑开挖前的预降水可引起围护结构和周边环境cm级的变形，并可导致已运营高铁、地铁的变形超过允许值。然而，传统基坑设计理论和相关研究未提出开挖前预降水引发基坑变形的预测方法与控制方法。拟以天津地铁3号线北站站基坑为工程背景，通过室内模型试验与数值分析，深入研究基坑预降水引发的围护结构及地层变形机理，揭示预降水深度、基坑宽度、土质条件、围护结构嵌入深度等对预降水产生变形的影响规律，在此基础上，采用Winkler地基模型及地层损失法，提出基坑预降水引发围护结构变形及周边地面沉降的预测方法。最后，基于基坑预降水引起变形的机理，开展预降水条件下基坑变形的控制机理研究，并在基坑变形传统控制方法基础上，提出基坑预降水引发变形的控制方法。

目前，基坑领域绝大多数研究均忽略了开挖前抽水引起的围挡侧移，揭示这部分变形机理、提出变形预测与控制方法对于软土区深基坑设计与施工有较大意义。本项目以天津地铁3号线北站站基坑为工程背景，通过室内模型试验及数值模拟，深入研究了基坑开挖前抽水引发的围护结构及地层变形机理与规律，研究表明，开挖前抽水引起的基坑变形存在四个效应，即，时间效应（变形随时间延长而非线性增大）、尺度效应（变形随基坑宽度增大呈双线性增大）、边角效应（存在特定的边角效应变形区）及增透效应（变形随地层渗透连通性增强而逐渐增大）；基于所揭示的变形特性，依托温克勒地基梁理论及地层损失法，提出了开挖前抽水引发围护结构侧移及坑外地面沉降的简化计算方法；最后，开展了开挖前抽水条件下基坑变形的控制机理研究，并提出了基于内隔墙和双回灌井组合回灌的变形控制方法。本研究成果对于完善我国基坑设计理论与方法，提高基坑变形控制理论水平，指导新的城市发展及环境下深基坑设计与施工具有重要的理论与工程实践意义。