软弱土地基基坑开挖与支护的有限元分析
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摘要
水泥土搅拌桩支护结构是软土地区常用的一种基坑支护型式,具有止水效果较好、无噪音、污染小等诸多优越性,且具有较好的经济效益,在开挖深度小于7m的软土基坑中得到广泛的应用。软土地区基坑中,水泥土挡墙的变形往往过大,如何有效的控制基坑变形,使基坑工程安全又经济,是人们一直研究的课题。目前主要的计算方法有极限平衡法、弹性抗力法以及有限元法,其中有限元法能从整体上分析支护结构及周围土体的应力与位移性状。本文总结了前人的研究成果,并结合工程实际,针对广东佛山某软弱土地基基坑开挖与支护工程,建立了二维有限元模型,对水泥土搅拌桩支护结构,做了较为全面的有限元分析,研究了影响基坑变形的多种因素,本文的主要研究内容和成果如下：
1.总结了目前几种常用的计算基坑受力与变形的方法,分析各种方法的特点及适用范围；随后探讨挡墙水平荷载计算的水土合算与分算问题；简要介绍格栅式水泥土挡墙的设计方法：分析了基坑变形现象和机理,并介绍基坑变形控制标准。
2.采用岩土工程分析软件Plaxis建立了二维有限元模型。在该模型中,土体和水泥土挡墙采用摩尔—库伦弹塑性模型,考虑了土的弹塑性力学性能；采用接触面单元模拟桩土之间的接触作用；较详细的介绍了该模型中各种材料参数的确定方法；讨论了基坑开挖模拟中的初始应力场,开挖荷载的处理接触面单元等问题。
3.以佛山某软弱土地基基坑开挖与支护工程为研究对象,运用该模型进行数值模拟,有限元计算结果论证了该基坑施工方案的合理性,表明有限元方法可应用于深基坑开挖与支护工程验算、指导工程设计与施工。
4.以工程实例为对象,运用该模型分析水泥土挡墙宽度、墙体嵌固深度、墙体变形模量、被动区土体加固、组合支护、地面超载等设计施工因素对基坑变形和整体稳定性的影响,并得出墙体变形、基坑坑底隆起以及墙后地表沉降的变化规律。从设计和施工两方面提出了水泥土挡墙支护基坑变形控制的具体措施,为今后类似工程的建设提出有益的建议。
Soil-cement mixing pile support structure is a kind of forms which is widely used in the soft-clay areas. With the merits of good waterproof, low noise, low pollution and low cost, it came into use broadly in soft soil foundation pit which is less than 7m in depth. In soft soil foundation pit, the large deformation of soil-cement retaining wall often occurs. How to effectively control the deformation to ensure the foundation pit security and economy is a subject of study which people having been always pursued. At present, the mainly computation methods include limit equilibrium method, elastic resistance method and the finite element method, the finite element method which can analyze the traits of stress and displacement of the support structure and the surrounding soil in all. This paper summarizes the results of previous studies, combined with engineering practice, directed to a soft soil foundation pit in Foshan, established 2-D finite-element model and analyzed the property of soil-cement mixing pile support structure roundly, and studies the impact of the design and construction of deformation factors. The main research content and results are as follows:
1. In the first part of this paper, the current calculation theory in common use of foundation pit engineering are introduced, and analyze the characteristics of various methods and the scope of application; then the method of estimating water and earth pressures on supporting structure separately and together is analyzed, and make a briefly introduction of cement-soil-type grille retaining wall design; finally, the phenomenon and mechanism of deformation is explored, and brings up the deformation control standards.
2. This paper adopts Plaxis, a geotechnical engineering analysis software, to establish a two-dimensional finite element model. In this model, the soil and cement retaining wall all use elastic-plastic Mohr-Coulomb model, taking into account of the elastic-plastic mechanical properties of soil:adopts interface element to model the contact influence between stake and soil; makes a detailed presentation of the model parameters of various materials methods; discusses a number of issues of excavation simulation, such as the initial stress field and excavation load handling.
3. Taking a supporting engineering in Foshan, makes a simulation analysis with Plaxis. Finite element calculation results demonstrates that the rationality of the construction program and the finite element method may apply to soil-cement mixing pile support structure.
4. Using this model explores the influence of a number of factors affecting the deformation and stability of foundation pit, such as wall width, wall depth, deformation modulus of wall, soil reinforcement, combination support and ground load. The general rules of wall displacement, excavation base heave and surface settlement are obtained. Then, the detailed deformation control measures of cement-soil retaining wall are proposed at the point of design and construction, brings forward up useful recommendations for the future construction.
1.总结了目前几种常用的计算基坑受力与变形的方法,分析各种方法的特点及适用范围；随后探讨挡墙水平荷载计算的水土合算与分算问题；简要介绍格栅式水泥土挡墙的设计方法：分析了基坑变形现象和机理,并介绍基坑变形控制标准。
2.采用岩土工程分析软件Plaxis建立了二维有限元模型。在该模型中,土体和水泥土挡墙采用摩尔—库伦弹塑性模型,考虑了土的弹塑性力学性能；采用接触面单元模拟桩土之间的接触作用；较详细的介绍了该模型中各种材料参数的确定方法；讨论了基坑开挖模拟中的初始应力场,开挖荷载的处理接触面单元等问题。
3.以佛山某软弱土地基基坑开挖与支护工程为研究对象,运用该模型进行数值模拟,有限元计算结果论证了该基坑施工方案的合理性,表明有限元方法可应用于深基坑开挖与支护工程验算、指导工程设计与施工。
4.以工程实例为对象,运用该模型分析水泥土挡墙宽度、墙体嵌固深度、墙体变形模量、被动区土体加固、组合支护、地面超载等设计施工因素对基坑变形和整体稳定性的影响,并得出墙体变形、基坑坑底隆起以及墙后地表沉降的变化规律。从设计和施工两方面提出了水泥土挡墙支护基坑变形控制的具体措施,为今后类似工程的建设提出有益的建议。
Soil-cement mixing pile support structure is a kind of forms which is widely used in the soft-clay areas. With the merits of good waterproof, low noise, low pollution and low cost, it came into use broadly in soft soil foundation pit which is less than 7m in depth. In soft soil foundation pit, the large deformation of soil-cement retaining wall often occurs. How to effectively control the deformation to ensure the foundation pit security and economy is a subject of study which people having been always pursued. At present, the mainly computation methods include limit equilibrium method, elastic resistance method and the finite element method, the finite element method which can analyze the traits of stress and displacement of the support structure and the surrounding soil in all. This paper summarizes the results of previous studies, combined with engineering practice, directed to a soft soil foundation pit in Foshan, established 2-D finite-element model and analyzed the property of soil-cement mixing pile support structure roundly, and studies the impact of the design and construction of deformation factors. The main research content and results are as follows:
1. In the first part of this paper, the current calculation theory in common use of foundation pit engineering are introduced, and analyze the characteristics of various methods and the scope of application; then the method of estimating water and earth pressures on supporting structure separately and together is analyzed, and make a briefly introduction of cement-soil-type grille retaining wall design; finally, the phenomenon and mechanism of deformation is explored, and brings up the deformation control standards.
2. This paper adopts Plaxis, a geotechnical engineering analysis software, to establish a two-dimensional finite element model. In this model, the soil and cement retaining wall all use elastic-plastic Mohr-Coulomb model, taking into account of the elastic-plastic mechanical properties of soil:adopts interface element to model the contact influence between stake and soil; makes a detailed presentation of the model parameters of various materials methods; discusses a number of issues of excavation simulation, such as the initial stress field and excavation load handling.
3. Taking a supporting engineering in Foshan, makes a simulation analysis with Plaxis. Finite element calculation results demonstrates that the rationality of the construction program and the finite element method may apply to soil-cement mixing pile support structure.
4. Using this model explores the influence of a number of factors affecting the deformation and stability of foundation pit, such as wall width, wall depth, deformation modulus of wall, soil reinforcement, combination support and ground load. The general rules of wall displacement, excavation base heave and surface settlement are obtained. Then, the detailed deformation control measures of cement-soil retaining wall are proposed at the point of design and construction, brings forward up useful recommendations for the future construction.
引文
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[16]刘忠玉,李永振.土体非线性对水泥上挡墙水平位移的影响[J].郑州工业大学学报,2003,24(1)：20-23
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