地铁车站基坑“先浅后深”施工的影响分析

Analysis of the Influence of the Excavation of the Working Shaft on Adjacent Structure by Shallow-First-and-Then-Deep Sequence

摘要: 以地铁六号线某车站端头井基坑(B2区)为例，该B2基坑采取“先浅后深”施工工序，存在一定的施工风险。为此，通过有限元软件建立二维模型模拟基坑施工过程，获得了基坑施工过程中附属结构(B3区)的位移、应变、应力情况，进一步评估附属结构的安全性。研究表明，车站附属结构在施工过程中最大水平和竖向发生在靠近东端头井的位置，分别为3.01 mm和7.20 mm，越远离端头井观测点的位移越小；施工过程中附属结构的最大倾斜率为0.046%，小于规范规定的0.2%限制；深基坑施工后附属结构底板最大拉应力约为0.27 MPa，小于混凝土的设计抗拉强度，满足抗裂要求。建议：为了确保底板不开裂，必须在浇筑底板期间，停止邻近深基坑开挖，即只有B3整体结构达到设计强度后才能进行深基坑施工。

Abstract: Taking the excavation of the working shaft of a station of the Metro Line No.6 for the case study, the construction of the shaft adopts a shallow-first-and-then-deep sequence, i.e. first constructing the auxiliary structure (AS), the structure buried shallowly and then the working shaft, the one buried deep, but this sequence risks damaging the AS. Therefore, a two-dimension model is established by finite element method to simulate the excavation process, and the displacement, strain and stress of the AS due to the excavation are then obtained to evaluate the safety of the AS. The result shows that the maximum horizontal and vertical displacement of the AS during construction is 3.01mm and 7.20mm respectively, all located near the shaft. The farther away from the shaft, the smaller the displacements are. The maximum inclination rate of AS during construction is 0.046%, which is much less than 0.2%, the allowance specified in the National Building Code (GB50007-2011); When the shaft is completed, the maximum tensile stress of the mat of the AS is about 0.27 MPa, which is less than the concrete design tensile strength and meets the requirements against cracking. It is hence suggested that the excavation of the shaft only be carried out after the completion of AS lest at the early phase after placement the concrete tensile strength should be lower than the stress induced by excavation, and lead to cracking of the components of the AS.