摘 要

为保证基坑工程的顺利进行，基坑降水起到了至关重要的作用，可降低基坑处的地下水位，保证基坑处于干燥的施工环境，降水方案的合理性决定了基坑降水工程的经济与安全。然而针对狭长型基坑的降水设计，目前缺少一套相对精确的计算方法。在传统计算方法中，对相关参数过于简化，无法针对地下水进行合理的定量分析，可能导致降水过少而无法达到降水要求或降水过多对周围环境产生不利影响，且传统计算方法不符合狭长型基坑向前推进降水施工的实际情况。针对上述问题，本文依托实际工程，采用理论计算、有限元软件模拟以及现场监测相结合的研究方法，对狭长型地铁车站深基坑的降水设计进行研究，本文主要研究内容及成果如下：

（1）针对狭长型基坑向前推进降水的施工特点，提出了基于降水单元的“大井法”降水理论。将狭长基坑划分为若干矩形降水单元，考虑各单元在降水时的相互影响，按照降水单元施工顺序，逐个计算各单元总涌水量。以工程实例为依托，提出初步降水方案，该方案在理论上可保证地下水位达到降深要求。

（2）针对上述初步降水方案做进一步优化。基于“目标函数法”理论，将基坑总涌水量最小化作为目标，将各单井涌水量作为设计变量，将单井涌水量极限值以及控制点处水位降深作为约束条件，建立数学模型。通过Python软件编写程序对模型进行求解，对上述初步降水方案进行优化，确定最终降水优化方案。结果表明：相比初步降水方案，优化后的总降水井数减少4口，基坑总涌水量减少3815.509m³/d，最终降水优化方案有效减小了降水井数及涌水量，使降水方案更加经济安全。

（3）采用有限元分析软件MIDAS GTS NX对优化方案进行数值模拟，对基坑周围地下水渗流变化及基坑关键位置的水头变化进行分析。研究表明：降水结束后，基坑底部节点水位稳定于-12.4m～-14.0m，可达到要求水位-12.2m以下。此外，各单元所需降水深度随基坑开挖方向依次递减，验证了基于降水单元的“大井法”降水理论的可靠性。

（4）通过现场监测，分析了该基坑地下水位的变化规律，并与数值模拟结果进行对比。研究结果表明：降水施工结束后，基坑底部地下水位介于-12.6m～-14.5m，可达到降水施工要求，且与数值模拟结果基本一致。证明了降水优化方案的可行性及数值模拟方法预测基坑降水效果的有效性，并验证了基于“目标函数法”基坑降水优化理论的可靠性，可为今后基坑降水优化设计提供借鉴作用。

As an important part of foundation pit engineering, foundation pit dewatering plays an important role in reducing water level and ensuring dry land construction. The rationality of dewatering scheme determines the safety and economy of dewatering engineering. However, there is no accurate calculation method for slim foundation pit dewatering design at present. Given the relevant parameters involved in the traditional calculation method are too simplified, to makes a scientific quantitative analysis of groundwater in the slim foundation pit engineering, which may cause too little dewatering to meet the requirements of dewatering or too much dewatering to cause adverse effects on the surrounding environment, moreover the traditional calculation method does not accord with the actual situation of dewatering construction of narrow and long foundation pit. In view of the above problems, based on the actual project, this paper studies the design method of deep foundation pit dewatering in slim subway station by using the research methods of theoretical calculation, indoor test, nondestructive testing and field monitoring. The main research contents and results of this paper are as follows:

Considering the characteristics of slim foundation pit dewatering construction,this paper proposed the “big well method” dewatering theory based on dewatering unit. Slim foundation pit could be divided into several rectangular dewatering units. Given the interaction effect of each dewatering unit, the total water inflow of each dewatering unit is calculated according to the sequence of units. A preliminary dewatering scheme is proposed relying on actual projects and theoretically the groundwater level can be guaranteed to meet the drawdown requirements. 

(2) According to the “objective function theory”, this paper establishes the objective function of the minimum total water output of foundation pit, takes the dewatering depth of control point and the limit value of single well yield as the restraint condition, establishes mathematical model with the water yield of each single well as the design variable, uses Python to solve the model. Then futher the optimized dewatering scheme, and determined a final scheme.The results show that, compared with the first dewatering scheme, the number of dewatering wells in the optimized scheme is reduced by 4, and the total water inflow is reduced by 3815.509 m³/d. The final optimized dewatering scheme effectively reduces the number of dewatering wells and the water inflow,to make the dewatering scheme more reasonable.

(3) Based on the optimization dewatering scheme,MIDAS GTS NX finite element analysis software is used for analyzing the groundwater seepage change near the foundation pit and the water head change at the key position of the foundation pit .The research shows that the simulation results of the final optimized dewatering scheme show that the groundwater level is on -12.4m to -14.0m,below -12.2m, which verifies the feasibility of the final dewatering scheme.At the same time, the required dewatering depth decreases in turn with the excavation direction of foundation pit, which verifies the reliability of the dewatering theory of “large well method” based on dewatering unit.

(4) The variation of groundwater level in the foundation pit was analyzed by field monitoring and compared with the numerical simulation results. The results show that after the precipitation is stable, the groundwater level at the bottom of the foundation pit is between -12.6m and -14.5m.The underground water level can meet the dewatering requirements and is basically consistent with the numerical simulation results. This proves the feasibility of the final dewatering scheme and further verifies the reliability of the dewatering calculation method based on the “objective function theory”, which provides a reference for the optimization of foundation pit dewatering in the future.

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