Basal-heave stability of narrow foundation pit in soft soil foundation based on hardening soil with small-strain stiffness (HSS) model
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摘要:
随着城市化和综合管廊的发展，狭窄型基坑工况日益增多。其支护形式、抗隆起计算方法及破坏模式与宽大基坑差异显著，在软土地层中坑底隆起尤为明显。为了进一步探究软土地基狭窄基坑支护结构的受力变形规律，抗隆起稳定性及其影响因素之间的敏感性关系，以武汉友谊大道快速化改造工程为实例，基于有限元计算和现场监测，采用小应变硬化土（HSS）土体本构模型和强度折减法研究基坑宽度、支护结构插入比、周边荷载和支护结构刚度等对狭窄基坑抗隆起稳定性的影响。结果表明：在支护结构刚度大、支撑间距小的情况下，狭窄基坑在支撑处和两道支撑中点处的受力变形差异不明显。狭窄基坑的变形土体可以分为地表至地表以下一定深度的基坑外侧土体和基坑底部土体，分别引起坑底隆起和坑外地表沉降；狭窄基坑土体塑性区会在坑底和结构底部处形成，随着塑性区发展，逐渐形成交叉的塑性区，并向两侧土体发展直至整个塑性区贯通，狭窄基坑最终破坏。敏感性分析表明，支护结构插入比和支护结构刚度对狭窄基坑抗隆起稳定性有显著的正面影响，基坑宽度和周边荷载则为负相关，且支护结构插入比的相关性最强。研究成果为软土地基狭窄基坑的支护结构设计和稳定性分析提供科学依据。
Abstract:Objective As urbanization accelerates and utility tunnel projects develop, narrow foundation pits are becoming increasingly prevalent. However, the supporting systems, basal-heave calculation methods, and failure modes of narrow foundation pits differ substantially from those of wider ones, and basal heave is more pronounced in soft soil strata.
Methods To further investigate the force-deformation behavior, basal-heave stability, and the sensitivity relationships among influencing factors of support structures for narrow foundation pits in soft soil foundations, finite element analysis and field monitoring were conducted using the Wuhan Youyi Avenue rapid reconstruction project as a case study. The hardening soil with small-strain stiffness (HSS) soil constitutive model, and the strength reduction method were employed to study the effects of excavation width, support structure embedment ratio, surcharge loads, and support structure stiffness on the basal-heave stability of narrow foundation pits.
Results The results indicated that with higher support stiffness and smaller support spacing, the differences in stress and deformation between the supports and the midpoint between two adjacent supports were minimal. Soil deformation in narrow foundation pits was divided into the soil outside the pit from the ground surface to a certain depth below the surface and the soil at the pit bottom, which induced basal heave and ground surface subsidence outside the pit, respectively. In narrow foundation pits, plastic zones developed at the pit bottom and the base of the structure, with further development, intersecting plastic zones gradually formed and extended into the soil on both sides until the plastic zones coalesced throughout, ultimately leading to failure of the narrow foundation pit. Sensitivity analysis revealed that the embedment ratio and stiffness of the support structure had a significant positive effect on the basal-heave stability of narrow foundation pits, whereas excavation width and surcharge loads exhibited a negative correlation, with the embedment ratio displaying the strongest correlation.
Conclusion These findings provide a scientific basis for the design of support systems and stability assessment of narrow foundation pits in soft soil foundations.
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图 2 基坑宽度对基坑隆起破坏模式的影响
虚线. 传统理论的基坑隆起破坏面；黑色箭头. 土体运动位移方向；D. 支护体系嵌固深度；B1，B2，B3. 不同的基坑宽度；下同
Figure 2. Influence of excavation width on basal-heave failure mode of foundation pit
图 3 基坑隆起破坏滑移面示意图
B，E，I，M，O. 土体端点代号；H1. 假定滑动面圆心O到基坑开挖底面FG的高度；θ1，θ2，θ3. 土体之间夹角，分别为∠GOE；∠GOI，∠GOF；v0. 假定土体的位移速度，下同
Figure 3. Schematic diagram of slip surface of basal-heave failure in foundation pit
图 4 研究基坑位置(a)和监测点布置(b)
Figure 4. Location of studied foundation pit (a) and layout of monitoring points (b)
图 8 基坑变形位移云图（a. 基坑横向位移；b. 基坑纵向位移）
Figure 8. Cloud diagrams of excavation-induced displacements
图 9 基坑土体强度参数折减前后的偏应变增量云图（a. 折减前；b. 折减后）
Figure 9. Cloud diagrams of deviatoric strain increment before and after reduction of soil strength parameters in foundation pit
图 10 基坑土体强度参数折减前后增量位移图
a，b. 分别为折减前、后增量位移云图；c，d. 分别为折减前、后增量位移矢量图
Figure 10. Diagrams of incremental displacement before and after reduction of soil strength parameters in foundation pit
图 12 基坑宽度(a)、支护结构插入比(b)、周边荷载(c)和支护结构刚度(d)对狭窄基坑抗隆起稳定性的影响
通过调整模拟钢板桩的板单元的等效厚度，来反映不同型号钢板桩的规格变化，进而改变支护结构刚度，即等效厚度等效替代支护结构刚度
Figure 12. Effect of excavation width(a), embedment ratio of the support structure(b), surcharge loads(c) and stiffness of the support structure(d) on basal-heave stability of narrow foundation pit
图 13 影响因素的皮尔逊相关系数分析
Figure 13. Analysis of Pearson correlation coefficients of influencing factors
表 1 土体物理力学参数
Table 1. Physical and mechanical parameters of soil
土层 杂填土 素填土 互层土 粉砂 粉细砂 z/m 1.5 2.8 11.7 20 51.8 γ/(kN·m−3) 18.5 18.2 18.2 18.6 18.8 γsat/(kN·m−3) 19 18.7 18.7 19.1 19.3 $E_{50}^{\mathrm{ref}} $/MPa 6.346 5.389 7.5 18.0 28.0 $E_{{\mathrm{oed}}}^{\mathrm{ref}} $/MPa 6.346 5.389 7.5 18.0 28.0 $E_{{\mathrm{ur}}}^{\mathrm{ref}} $/MPa 19.04 16.17 22.5 54.0 84.0 $G_{0}^{\mathrm{ref}} $/MPa 7.933 6.737 62.4 70.94 70.94 γ0.7 7.51×10−4 1.07×10−3 1.80×10−4 4.20×10−4 1.00×10−3 c/kPa 10 12 5 0 0 φ/(°) 18 10 15 33 31 ψ/(°) 0 0 0 3 1 注：z. 土层底面埋深；γ. 天然重度；γsat. 饱和重度；$E_{50}^{{\mathrm{ref}}} $. 参考割线刚度；$E_{{\mathrm{oed}}}^{{\mathrm{ref}}} $. 参考压缩模量；$E_{{\mathrm{ur}}}^{{\mathrm{ref}}} $. 参考卸载-再加载模量；$G_{0}^{{\mathrm{ref}}} $. 参考初始剪切模量；γ0.7. 阈值剪应变；c. 黏聚力；φ. 内摩擦角；ψ. 剪胀角 
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表 2 支护结构物理力学参数
Table 2. Physical and mechanical parameters of support structure
支护结构 型号 等效厚度/m 直径/m 壁厚/m 弹性模量/GPa 钢管 − − 0.325 0.012 30 拉森钢板桩 400×160 0.18 − − 200 400×170 0.19 − − 200 500×200 0.25 − − 200
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表 3 基于不同计算方法的稳定性系数计算结果
Table 3. Stability coefficient calculation results by different methods
计算方法 规范法 本研究改进
公式法数值
模拟法整体稳定性 坑底抗隆起稳定性 稳定性系数 1.596 1.600 1.772 4.926
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表 4 影响因素的重要程度及其排序
Table 4. Importance and ranking of influencing factors
影响因素 皮尔逊相关系数 重要程度 排列顺序 基坑宽度 −0.13 3 周边荷载 −0.24 2 支护结构插入比 0.99 1 支护结构刚度 0.085 4
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