深基坑降水作用下远场土层变形机理研究

Large-scale geotechnical engineering activities have become one of the most significant causes of land subsidence in urban area, and accelerating effect of ground settlement induced by large area dewatering of deep excavation is increasingly prominent under the complicated geological condition. Furthermore, both the desynchrony of the seepage and soil deformation during temporary dewatering and the cutoff effect of the excavation retaining structures will make the soil-water coupling effect be more complex, which increases the difficulty to predict the ground surface settlement and to control the land subsidence in urban area. In order to evaluate the land subsidence caused by the deep excavation, the mechanism and analysis method of dewatering-induced soil deformation will be investigated in this project by using model tests, field monitoring and numerical analysis. Firstly, experiment devices and relevant monitoring system for laboratory dewatering simulations are developed, and lab model tests are conducted to study the deformation responses of soil layers using the proposed testing system. With the obtained test results, a suitable constitutive model is obtained by improving the small strain constitutive model and the corresponding parameters are determined with the laboratory test results. Secondly, numerical analysis methods are studied based on the soil-water coupling theory and the developed constitutive model. Based on the proposed numerical method and the results of laboratory model tests and field monitoring, parametric studies, including different stratum structures and different forms of excavation retaining structure, are carried out to investigate the effects and influences laws of dewatering on surrounding environment. Finally, deformation mechanism of soil layers during the dewatering in deep excavation is revealed and a method used to predict dewatering-induced soil deformation is established based on the above results. The findings in this research are significant to illustrate the disaster mechanism of land subsidence due to dewatering in deep excavations and provide basic theories for studying the control measures of the land subsidence caused by deep excavations.

大规模岩土工程活动已成为城市地面沉降的重要诱因，复杂地质条件下深基坑工程大面积降排水对地面沉降的加速作用日益突出。基坑临时性降水的渗流变形不同步和止水帷幕的遮拦效应导致土层流固耦合作用复杂化，使得地面沉降的预测和控制更为困难。本项目针对深基坑引起的工程性地面沉降问题，采用模型试验、现场测试和数值分析等方法研究基坑降水引起的远场土层变形机理及分析方法。在研制室内降水模拟试验系统的基础上，通过模型试验研究组合土层试样在减压降水作用下的变形响应特征，确定合理的本构模型和参数；研究基于流固耦合理论的降排水数值模拟方法，结合模型试验和现场测试，探讨不同地层结构和止水形式下降排水的环境效应及其影响规律，揭示基坑降水过程中远场土层受力变形和渗流响应机理，建立土层变形预测方法。研究成果对于阐明基坑降排水引起地面沉降灾害及其控制机理具有重要意义，将为深基坑减压降水引发地面沉降的防治措施研究提供理论基础。

在富含地下水的地区进行地下空间开发时，需对地下水进行处理，特别是涉及承压含水层时需进行减压降水。降水会改变周边土体的受力条件，影响土体的变形，给基坑的安全施工和环境保护提出新的难题。因此，研究深基坑中减压降水引起的地下水渗流场及土体变形发展规律，并提出相应的环境保护措施具有重要意义。本项目针对沿海软土地区基坑工程，采用模型试验、现场测试和数值分析等方法，研究了深基坑减压降排水引发土体变形与沉降机理。首先开展上海典型软土室内试验，获得了典型土层的基本物理力学参数。采用改进的GDS三轴试验仪对上海黏土土样进行室内降压回灌模拟试验，研究降压回灌的渗流作用对黏土渗透特性的影响规律。试验通过设置围压和初始孔压来控制土样应力水平，利用反压变化模拟孔隙水的降压和回灌过程，测试土样的渗流变化与变形，从而分析土样孔隙比、渗透系数等参数的变化规律。基于典型土层变形室内试验结果，推导出考虑降水影响的本构模型及参数，进一步提出了基于流固耦合理论的深基坑降水环境影响分析方法，开展了三维流固耦合数值分析，研究获得了开挖降水耦合作用下地面沉降规律，揭示了基坑卸荷影响下减压降水引起的坑外土体变形规律。研究发现，渗透系数在降压阶段因孔隙比减小而降低，并存在非线性关系；土体孔隙比在回灌阶段逐渐增大，但渗透系数因堵塞效应而呈现进一步降低趋势；可用e-lgk拟合曲线建立孔隙比与渗透系数之间的非线性关系。减压降水会加剧坑外地表沉降，影响范围超出了4倍最大开挖深度（简称为He）。卸荷影响下降水引起的地表沉降最大值出现在坑外2He处。坑外2He以外区域，降水引起地表沉降的分布与承压含水层水头降低分布特征一致；坑外2He以内区域，地表沉降分布与水头降低分布差异显著。卸荷影响下降水引起的地表沉降主要为承压含水层的压缩变形，同时，由于墙土相互作用和卸荷效应的影响，上覆土层在坑外2He内出现了膨胀变形。根据上述研究，提出深基坑施工中减压降水引起坑外地表沉降的分布模式。研究成果阐明了基坑降排水引起地面沉降灾害及其控制机理具有重要意义，为深基坑减压降水引发地面沉降的防治措施研究提供了理论基础。