Influence of Foundation Pit Excavation and Pile Foundation Construction on Maintenance Structure Deformation
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摘要: 以北京通州某深基坑工程为例，分析基坑开挖卸荷、基坑锚杆施工、基底CFG桩和抗拔桩施工对基坑围护结构和周边环境的影响。结果表明：基坑围护结构及基坑周边地表随着基坑的开挖、围护锚索和基坑内工程桩的施工出现典型的先上浮后沉降的趋势，应力重新分布现象明显。具体表现为基坑开挖卸荷初期引发围护结构及基坑周边地表上浮，随着基坑开挖深度的增加，在基坑侧向位移和基坑锚索竖向分力作用下，围护结构及基坑周边地表开始下沉；在基坑槽底施工CFG桩和抗拔桩削弱了护坡桩嵌固区被动土压力，基坑降水导致土体有效应力增加，产生附加固结沉降，在基坑地下水渗流的联合作用下，围护结构及基坑周边地表呈现二次加速下沉；基坑开挖和基础桩施工对桩顶水平位移和锚索轴力影响较小。根据分析结果，建议类似基坑增加嵌固深度、调整被动土压力区打桩顺序，将有利于围护结构及基坑周边环境变形控制。Abstract: A deep foundation pit in Tongzhou, Beijing was studied as an example, the effects on the foundation pit maintenance structure and the surrounding environment were discussed including foundation pit excavation and unloading, maintenance anchor construction, CFG, and uplift pile construction. The analysis results show that: the typical maintenance structure and the surrounding surface settlement of the foundation pit float first and then decrease with the excavation of the foundation pit, maintenance of anchor cable, and construction of pile foundation. The specific performance is that the excavation unloading of foundation pit will cause the maintenance structure and the surrounding surface floating; With the increase of the excavation depth of the foundation pit, the maintenance structure and the surrounding surface begin to sink under the condition of the lateral deformation of the foundation pit and the vertical component force of anchor cable; The construction of CFG and uplift pile in the foundation pit weakens the passive earth pressure in the embedded area of the slope protection pile, dewatering in the foundation pit leads to an increase in effective stress in the soil, resulting in additional consolidation settlement, under the combined action of groundwater seepage in the foundation pit, the maintenance structure and the surrounding surface show secondary accelerated subsidence; Foundation pit excavation and foundation pile construction have little effect on horizontal displacement of pile top and axial force of anchor cable. According to the analysis results, it is suggested that increasing the embedded depth of a similar foundation pit and adjusting the piling sequence of the passive earth pressure zone will be beneficial for deformation control of the maintenance structure and surrounding environment.
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表 1 地层物理力学参数
层号 土类名称 重度
/（kN·m−3）孔隙比 黏聚力
/kPa内摩擦角
/（°）② 黏质粉土 19.0 0.76 18～22 17.2～20.9 ③ 黏质粉土 19.3 0.71 15～19 12.6～16.4 ④ 黏质粉土 20.1 0.63 15～24 10.5～18.8 ⑤1 黏质粉土 19.7 0.62 22～30 12.7～16.6 ⑥ 黏质粉土 19.3 0.76 14～21 11.9～18.4 注：各土层的黏聚力和内摩擦角为天然快剪试验所得的范围值。 
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表 2 基坑土方开挖和锚索施工时间表
2018年 2019年 第一步土方 6.15–7.2 第一道锚杆 6.29–7.4 第二步土方 11.15–11.27 第二道锚杆 11.24–11.29 第三步土方 11.30–12.15 第三道锚杆 12.14–12.19 第四步土方 12.20–1.10 第四道锚杆 1.9–1.11 第五步土方 1.11–1.21，2.21–3.7 第五道锚杆 3.6–3.14 第六步土方 3.14–4.7
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