柔性基础下桩体复合地基性状与设计方法研究
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摘要
近年来公路和铁路路堤、土石坝、堆场、储罐等工程中桩体复合地基的应用日益增多,人们发现其性状与建筑工程中桩体复合地基的有较大差异。在这类柔性基础下的桩体复合地基设计中,目前仍采用刚性基础下桩体复合地基的设计理论,造成计算值与实测值差距较大,其结果偏于不安全。因此,开展柔性基础下桩体复合地基性状与设计方法的研究不仅具有理论价值,同时还有着广阔的工程应用前景,将具有显著的经济效益和社会效益。
在对柔性基础下桩体复合地基的试验研究、解析法研究和数值分析等方面现状作了较详细分析基础上,本文主要作了以下工作:
1)引进“典型单元体”与“虚土桩”模型,将柔性基础-垫层-复合地基-下卧层土体视为上下部共同作用的系统;引进侧阻分布函数和相对位移函数,统一了Alamgir型假设位移模式,从而可以考虑桩土界面之间存在相对滑移且同一水平面上地基土沉降不同;结合系统的荷载传递规律和桩土界面的理想弹塑性本构关系,得到摩阻力发挥水平沿深度的分布,实现非直线型桩侧摩阻力分布模式;基于以上假设,得到了柔性基础下桩体复合地基“虚土桩”单元体物理模型的弹性力学微分解答,并通过工程实例验证了该解析解的合理性.
2)基于本文解析解和正交试验方法,探讨了填土高度与模量、垫层厚度与模量、桩体置换率、桩径、桩长、桩材模量、桩间土压缩模量、下卧层压缩模量等因素对等沉面和中性面位置、桩土应力比或荷载分担比等的影响,研究了典型单元体柔性基础下桩体复合地基的工作性状.
3)基于三维有限元和正交试验方法,分析了路堤宽度效应、群桩效应、垫层中的格栅加筋效应以及填土的分层加载,通过基本算例,分析了系统总沉降、加固区与下卧层压缩量、桩顶与桩底刺入量、桩身最大轴向应力和桩侧摩阻力、格栅受力等性状;结合工程实际情况,着重探讨了填土高度、填土宽度、格栅层数、桩材模量、桩体置换率、下卧层压缩模量等6个因素对桩土应力比或荷载分担比、等沉面和中性面位置的影响,研究了分层堆填时柔性基础下群桩复合地基的工作性状,为柔性基础下桩体复合地基的设计和实践提供有价值的参考。
4)提出柔性基础下桩体复合地基的设计思路,给出与荷载传递相关的土拱等沉面高度、中性面位置和桩土应力比的确定方法,提出考虑上下部共同作用的柔性基础下桩体复合地基承载力、稳定性的验算方法和沉降计算方法;
5)将柔性基础下桩体复合地基的荷载传递规律和设计方法运用于某原料堆场工程事故的原因分析,并按刚性、柔性基础分别复核了原设计的复合地基承载力;以此为基础,提出了“半刚性垫层”、“长短桩复合地基”、“复合挡墙”的二次加固综合措施,全过程的监测验证表明其效果是明显的,二次加固处理措施获得成功。
People find there lies different working behaviors between the pile composite ground under the rigid foundation and the flexible foundation,such as embankments of highway and railway, dams,stack yards,oil tanks etc..Applying the design method,which is applicable to the rigid foundation,to the flexible foundation,accordingly a notebale gap between the calculated value and the measured will be induced,and it is apt to insecurity.Therefore,it is of great theoretical significance to study on the working behavior of the pile composite ground under the flexible foundation.The results of study on the design method,which can be used to the engineering practice,have great values of social and economical benefits.
Based on an overview of experimental research,analytical method,numerical analysis and design methods,etc.,the principal contents and original work are as follows:
1) The flexible foundation-the cushion-the pile composite ground-the underlying strata is considered as an interactive system by bringing in a typical unit model and virtual soil pile model.The Alamgir-type assumption of the soil displacement is unified by bringing in a pile's skin friction distribution function and a relative movement function,which can consider the relative movement between the pile and soils around the pile,and can assume a different settlement at a same original height of soil around the pile.Combining the load transfer mechanism with the ideal elastic-plastic constitutive of the pile-soil interface,the distribution along depth of the exerting degree of nonlinear skin friction is obtained.Based on the foregoing assumptions,the differential quations of the system is presented and solved.The rationality of the presented analytic method is verified by an engineering case.
2) Based on the presented analytic method and orthogonal test method,the impact of height and modulus of fill,thickness and modulus of cushion,replacement ratio of pile,pile diameter,pile length,pile modulus,modulus of soil between piles,and the modulus of the underlying strata on the characteristic of the system,such asthe equal settlement plane,neutral point plane,pile-soil stress ratio or pile-soil load share ratio,are studied.
3) Through 3D FEM and orthogonal test method,the effects of embankment width,pile group,grid reinforeced cushion,and layered filling are researched.Based on the basic case,the total settlement,the amount of compression of the pile improved area and the underlying area, the amount of penetration at the top and bottom of the pile,the maximum axial stress of the pile, the skin friction of the pile,and the stress of the grid are analyzed.Considering the situations of the engineering practice,the impact of the fill height and width,the layer number of grid,the replacement ratio and modulus of pile,the modulus of the underlying strata on the equal settlement plane,neutral point plane,pile-soil stress ratio or pile-soil load share ratio,are studied. The results could provide some references to the engineering practice.
4) The design idea of the pile composite ground under the flexible foundation is presented. And the method to determine the position of the equal settlement plane,the neutral point plane, pile-soil stress ratio,which are relevant to the load transfer mechanism,are provided.Then the method to calculate the bearing capadty,the settlement arid the global stability of the pile composite under the flexible foundation baesd on super-sub structure interaction is put forward.
5) In the end,the load transfer mechanism and the method of the pile composite ground under the flexible foundation are applied to retreat a foundation acddent of a stack yard. Comparative bearing capacity of the original composite foundation is calculated based on a rigid foundation and flexible foundation respectively.And according to the load transfer mechanism and comparative calculation,the causes of the accident are analyzed.Accordingly,three comprehensive measures of the semi-rigid cushion,binary composite ground,compound retaining wall were adopted in the retreatment design.The whole process of monitoring verified that the measures worked soundly,for the yard reached its scheduled height smoothly.
在对柔性基础下桩体复合地基的试验研究、解析法研究和数值分析等方面现状作了较详细分析基础上,本文主要作了以下工作:
1)引进“典型单元体”与“虚土桩”模型,将柔性基础-垫层-复合地基-下卧层土体视为上下部共同作用的系统;引进侧阻分布函数和相对位移函数,统一了Alamgir型假设位移模式,从而可以考虑桩土界面之间存在相对滑移且同一水平面上地基土沉降不同;结合系统的荷载传递规律和桩土界面的理想弹塑性本构关系,得到摩阻力发挥水平沿深度的分布,实现非直线型桩侧摩阻力分布模式;基于以上假设,得到了柔性基础下桩体复合地基“虚土桩”单元体物理模型的弹性力学微分解答,并通过工程实例验证了该解析解的合理性.
2)基于本文解析解和正交试验方法,探讨了填土高度与模量、垫层厚度与模量、桩体置换率、桩径、桩长、桩材模量、桩间土压缩模量、下卧层压缩模量等因素对等沉面和中性面位置、桩土应力比或荷载分担比等的影响,研究了典型单元体柔性基础下桩体复合地基的工作性状.
3)基于三维有限元和正交试验方法,分析了路堤宽度效应、群桩效应、垫层中的格栅加筋效应以及填土的分层加载,通过基本算例,分析了系统总沉降、加固区与下卧层压缩量、桩顶与桩底刺入量、桩身最大轴向应力和桩侧摩阻力、格栅受力等性状;结合工程实际情况,着重探讨了填土高度、填土宽度、格栅层数、桩材模量、桩体置换率、下卧层压缩模量等6个因素对桩土应力比或荷载分担比、等沉面和中性面位置的影响,研究了分层堆填时柔性基础下群桩复合地基的工作性状,为柔性基础下桩体复合地基的设计和实践提供有价值的参考。
4)提出柔性基础下桩体复合地基的设计思路,给出与荷载传递相关的土拱等沉面高度、中性面位置和桩土应力比的确定方法,提出考虑上下部共同作用的柔性基础下桩体复合地基承载力、稳定性的验算方法和沉降计算方法;
5)将柔性基础下桩体复合地基的荷载传递规律和设计方法运用于某原料堆场工程事故的原因分析,并按刚性、柔性基础分别复核了原设计的复合地基承载力;以此为基础,提出了“半刚性垫层”、“长短桩复合地基”、“复合挡墙”的二次加固综合措施,全过程的监测验证表明其效果是明显的,二次加固处理措施获得成功。
People find there lies different working behaviors between the pile composite ground under the rigid foundation and the flexible foundation,such as embankments of highway and railway, dams,stack yards,oil tanks etc..Applying the design method,which is applicable to the rigid foundation,to the flexible foundation,accordingly a notebale gap between the calculated value and the measured will be induced,and it is apt to insecurity.Therefore,it is of great theoretical significance to study on the working behavior of the pile composite ground under the flexible foundation.The results of study on the design method,which can be used to the engineering practice,have great values of social and economical benefits.
Based on an overview of experimental research,analytical method,numerical analysis and design methods,etc.,the principal contents and original work are as follows:
1) The flexible foundation-the cushion-the pile composite ground-the underlying strata is considered as an interactive system by bringing in a typical unit model and virtual soil pile model.The Alamgir-type assumption of the soil displacement is unified by bringing in a pile's skin friction distribution function and a relative movement function,which can consider the relative movement between the pile and soils around the pile,and can assume a different settlement at a same original height of soil around the pile.Combining the load transfer mechanism with the ideal elastic-plastic constitutive of the pile-soil interface,the distribution along depth of the exerting degree of nonlinear skin friction is obtained.Based on the foregoing assumptions,the differential quations of the system is presented and solved.The rationality of the presented analytic method is verified by an engineering case.
2) Based on the presented analytic method and orthogonal test method,the impact of height and modulus of fill,thickness and modulus of cushion,replacement ratio of pile,pile diameter,pile length,pile modulus,modulus of soil between piles,and the modulus of the underlying strata on the characteristic of the system,such asthe equal settlement plane,neutral point plane,pile-soil stress ratio or pile-soil load share ratio,are studied.
3) Through 3D FEM and orthogonal test method,the effects of embankment width,pile group,grid reinforeced cushion,and layered filling are researched.Based on the basic case,the total settlement,the amount of compression of the pile improved area and the underlying area, the amount of penetration at the top and bottom of the pile,the maximum axial stress of the pile, the skin friction of the pile,and the stress of the grid are analyzed.Considering the situations of the engineering practice,the impact of the fill height and width,the layer number of grid,the replacement ratio and modulus of pile,the modulus of the underlying strata on the equal settlement plane,neutral point plane,pile-soil stress ratio or pile-soil load share ratio,are studied. The results could provide some references to the engineering practice.
4) The design idea of the pile composite ground under the flexible foundation is presented. And the method to determine the position of the equal settlement plane,the neutral point plane, pile-soil stress ratio,which are relevant to the load transfer mechanism,are provided.Then the method to calculate the bearing capadty,the settlement arid the global stability of the pile composite under the flexible foundation baesd on super-sub structure interaction is put forward.
5) In the end,the load transfer mechanism and the method of the pile composite ground under the flexible foundation are applied to retreat a foundation acddent of a stack yard. Comparative bearing capacity of the original composite foundation is calculated based on a rigid foundation and flexible foundation respectively.And according to the load transfer mechanism and comparative calculation,the causes of the accident are analyzed.Accordingly,three comprehensive measures of the semi-rigid cushion,binary composite ground,compound retaining wall were adopted in the retreatment design.The whole process of monitoring verified that the measures worked soundly,for the yard reached its scheduled height smoothly.
引文
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