Deformation characteristics and fluid-solid coupled analysis of a super-deep circular foundation pit in soft soils
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摘要:
软土地区超深圆形基坑工程具有复杂性、危险性和稀缺性等特点，所以对其进行精确的模拟预测具有重要意义。以上海苏州河深隧工程中开挖深度56.3 m的超深圆形基坑为例，建立基于Biot固结理论的水土双向耦合有限元分析模型，结合实际监测数据研究了软土地区超深圆形基坑的受力与变形特性；探讨比较了双向耦合分析与工程中常用的仅考虑渗流场对土骨架作用的单向耦合分析在机理上的区别，以及2种分析计算结果与实际监测数据的差异。结果表明：（1）圆形围护结构受大偏压荷载一侧比未受偏压荷载一侧的侧向位移增大64.7%，表现出明显的空间效应；（2）双向耦合分析因为考虑了随时间的增加，水逐渐从孔隙排出，超孔隙水压力逐渐消散，并伴随着土体体积变化的过程，所以能够对土体以及围护结构的变形进行有效预测；（3）对于围护结构最大侧向位移，单向与双向耦合分析的计算值与实测值的误差分别为42.35%和14.35%；（4）关于最大环向轴力，单向与双向耦合分析的计算值与实测值的误差分别为14.30%和10.27%，双向耦合计算得到的基坑围护结构内力和变形的结果优于单向耦合计算。研究成果可为软土地区圆形超深基坑的设计和施工提供参考。
Abstract:The characteristics of super-deep circular foundation pit project in soft soil area are complexity, dangers, and scarcity. It is of great significance to simulate and predict its deformation accurately. As to the super-deep circular pit with excavation depth of 56.3 m in the deep tunnel project of Suzhou River in Shanghai, a fluid-solid coupled finite element analysis model based on the Biot consolidation theory was established. The deformation and force characteristics of super-deep circular foundation pits in soft soil areas were investigated by combining the actual monitoring data. The difference in mechanism between the fluid-solid bidirectional coupling analysis (BCA) and the unidirectional coupling analysis (UCA) which is commonly used in engineering to consider the single effect of the seepage field on the soil skeleton, as well as the difference between the calculation results of the two analysis and the actual monitoring data were analyzed. The results show that the lateral displacement on the side of the circular retaining structure subjected to large bias load increases by 64.7% compared with that on the side not subjected to bias load, indicating an obvious spatial effect. The fluid-solid coupling analysis can predict the deformation of the soil and the retaining structure effectively because it considers the process that water is gradually discharged from the pore with time, and then the excess pore pressure is gradually dissipated with the change of the volume of the soil. For the maximum lateral displacement of the retaining structure, the errors between measured and calculated values of the UCA and BCA are 42.35% and 14.35%, respectively. Regarding the maximum circumferential axial force, the errors between the measured and calculated values for UCA and BCA are 14.30% and 10.27%, respectively. The results of foundation deformation and internal force obtained from the BCA are better than UCA. This study can provide the basic information for the design and construction of circular super-deep foundation pits in soft soil areas.
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表 1 各层土体HS-Small模型参数
Table 1. Model parameters of HS-Small model for each layer soil
土层号 γ/（kN·m−3） $ {E}_{\mathrm{oed}}^{{\mathrm{ref}}} $ /MPa$ {E}_{50}^{{\mathrm{ref}}} $ /MPa$ E_{\mathrm{ur}}^{\mathrm{ref}} $ /MPa$ {G}_{0}^{{\mathrm{ref}}} $ /MPac'/kPa φ'/（°） $ {\gamma }_{0.7} $ /10−4$ {\mu }_{\mathrm{u}\mathrm{r}} $ Pref/kPa m K0 Rf 厚度/m ① 18.8 4.02 4.54 27.88 102.00 6 36.0 2.7 0.2 100 0.8 0.55 0.9 3.81 ③ 17.0 3.01 3.61 24.05 79.36 3 29.6 2.7 0.2 100 0.8 0.53 0.6 6.30 ④ 17.0 2.41 2.89 19.30 54.03 4 27.9 2.7 0.2 100 0.8 0.53 0.6 8.70 ⑤1 17.6 3.38 4.05 20.25 52.65 5 30.5 2.7 0.2 100 0.8 0.49 0.9 6.70 ⑤3 17.8 4.63 5.55 27.76 72.17 5 31.6 2.7 0.2 100 0.8 0.48 0.9 15.40 ⑤4 19.5 5.74 6.89 34.45 89.58 16 32.4 2.7 0.2 100 0.8 0.46 0.9 3.80 ⑦ 19.5 11.58 11.58 46.32 138.96 0 34.5 2.7 0.2 100 0.5 0.38 0.9 4.60 ⑧1 18.0 4.75 5.70 28.51 67.50 8 32.4 2.7 0.2 100 0.8 0.46 0.9 4.20 ⑧2 18.5 5.63 6.75 33.75 89.59 8 33.1 2.7 0.2 100 0.8 0.45 0.9 18.90 ⑨1 19.4 14.66 14.66 58.64 146.60 0 37.5 2.7 0.2 100 0.5 0.36 0.9 3.50 ⑨2-1 20.2 15.69 15.69 62.76 156.90 0 37.5 2.7 0.2 100 0.5 0.33 0.9 8.60 ⑨2-2 19.1 17.21 17.21 68.84 172.10 0 36.5 2.7 0.2 100 0.5 0.31 0.9 13.90 ⑩ 19.9 10.42 12.51 62.53 135.69 19 31.6 2.7 0.2 100 0.8 0.5 0.9 4.10 ⑩夹 19.4 7.70 9.24 46.22 113.55 16 32.0 2.7 0.2 100 0.8 0.43 0.9 8.54 ⑩A 19.3 12.37 12.37 49.48 138.54 0 37.0 2.7 0.2 100 0.5 0.36 0.9 15.70 ⑪ 21.0 15.40 15.40 61.60 172.48 0 36.0 2.7 0.2 100 0.5 0.32 0.9 26.00 注：γ为重度； $ E_{\mathrm{oed}}^{\mathrm{ref}} $ 为固结试验的参考切线模量；$ {E}_{50}^{{\mathrm{ref}}} $ 为三轴固结排水剪切试验的参考割线模量；$ E_{\mathrm{ur}}^{\mathrm{ref}} $ 为三轴固结排水卸载再加载试验的参考卸载再加载模量；$ {G}_{0}^{{\mathrm{ref}}} $ 为小应变刚度试验的参考初始剪切模量；c'为有效黏聚力；φ'为有效内摩擦角；$ {\gamma }_{0.7} $ 为0.7倍的初始剪切模量时对应的剪应变；$ \mu_{\mathrm{ur}} $ 为泊松比；Pref为参考应力；m为应力水平相关的幂指数；K0为正常固结条件下的静止侧压力系数；Rf为破坏比。
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表 2 基坑施工工况
Table 2. Foundation pit construction step
计算步 工况 开始日期 结束日期 Step0 初始渗流场计算 Step1 初始应力场计算 Step2 基坑周边主要建筑物施工 Step3 地下连续墙施工 Step4 表层土开挖（挖深1.80 m）
并施工顶圈梁2020-10-16 2020-10-29 Step5 开挖第2层土方（挖深9.50 m）
并施工第1道环梁2020-12-16 2021-01-04 Step6 开挖第3层土方（挖深18.00 m）
并施工第2道环梁2021-02-28 2021-03-24 Step7 开挖第4层土方（挖深29.05 m）
并施工第3道环梁2021-04-02 2021-04-24 Step8 开挖第5层土方（挖深39.50 m）
并施工第4道环梁2021-05-07 2021-06-03 Step9 开挖第6层土方（挖深49.50 m）
并施工第5道环梁2021-06-13 2021-07-17 Step10 开挖第7层土方（挖深56.30 m） 2021-07-29 2021-08-10 Step11 底板施工 2021-08-17 2021-09-05 Step12 内衬墙施工 2021-09-22 2022-02-24
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