【作者】 周志军；

【导师】 陈昌富；

【作者基本信息】 湖南大学 ， 岩土工程， 2010， 博士

【摘要】 公路工程中大面积不均匀软弱地基处理是一个比较棘手的问题,而复合地基技术被证明是一种有效方法,随着复合地基技术的发展,工程中应用的复合地基类型越来越新颖,但其设计计算理论远滞后于工程应用,特别是在承载机理上,故对路堤下复合地基进行研究,具有重要的理论和实际意义。为此,本文通过理论和数值模拟,对复合地基的承载力、沉降、土拱效应、褥垫层设计等进行了研究,主要研究工作和成果如下：(1)由于路堤为柔性基础,不能简单套用由刚性基础建立起来的地基承载力公式,本文首先把滑移线法与极限分析上限法相结合,推导出一种新的倾斜荷载下地基极限承载力计算公式。针对路堤下水平加筋复合地基,路堤侧向土压力、重力以及水平加筋体的拉力对地基成倾斜荷载,本文通过确定可能滑动宽度范围,用优化方法搜索出地基最危险滑动面,应用推导的承载力公式计算出路堤的极限高度值。通过工程实例分析,验证了本文推导公式的可靠性,从而为路堤下水平加筋复合地基极限承载力提供了一个新的计算方法。(2)针对路堤下水平加筋与散体材料桩复合地基的极限承载力计算问题,本文首先分析了水平加筋体、路堤和地基之间的相互作用,得到了三者的作用力关系以及速度关系；然后研究了散体材料桩对地基承载力的影响,得到了考虑桩体置换率、地基土固结度影响的复合地基土体强度参数的计算公式；推导出水平加筋与散体材料桩复合地基极限承载力公式；并根据承载力公式计算路堤极限填土高度。工程实例分析表明,本文计算结果符合实测情况,结果较为理想。同时,基于工程实例详细分析了筋材发挥系数、固结度等因素对地基极限承载力的影响,进一步认识了路堤下水平加筋与散体材料桩复合地基的承载机理。(3)首先讨论路堤下水平加筋与刚性桩复合地基的破坏模式,然后基于极限分析理论和被动桩遮拦理论,分别推导出考虑桩体遮拦效应的桩间土极限承载力公式,以及考虑地基土滑移影响的刚性桩极限承载力公式,最后根据复合地基桩土共同受力得到复合地基的总极限承载力。算例分析说明,本文方法比规范法更符合刚性桩复合地基的实际承载机理。(4)针对带褥垫层桩体复合地基在荷载作用下桩会向褥垫层刺入,并且在桩侧上部产生负摩阻力的特点,考虑复合地基中桩、土间相对滑移和相互作用,引入Boussinesq解和Mindlin解改进基桩弹塑性分析法,提出了桩体复合地基沉降计算弹塑性分析法；并把桩体复合地基沉降计算推广到群桩情况,得到了群桩均匀受力与不均匀受力两种情况下沉降计算公式。通过试验对比分析,证明了本文方法的可靠性,且分析得到了桩体复合地基桩体、桩间土受力和变形的相关规律。(5)根据极限分析下限法对褥垫层的极限承载力进行了研究,分别推导出了二维厚垫层、二维薄垫层以及三维圆形基础下薄垫层的极限承载力计算公式。并根据几何关系推导出厚垫层的有效最大厚度计算公式。然后,分析了垫层内摩擦角、垫层有效最大厚度、垫层桩土应力比的之间的关系,得到了垫层承载机理的内在规律。最后,本文基于承载力与沉降两方面考虑,提出了新的垫层设计方法,此方法比原先仅从破坏模式角度,或从垫层弹性变形角度考虑更加全面。(6)考虑路堤下双向增强体复合地基中,由于桩体与桩间土变形不协调,导致路堤填土内部垂直变形有差异,并产生土拱效应的特点。采用马斯顿土压力理论分析模型,引入剪切位移法分析填土、土工合成材料的应力和变形规律,分别推导出了土工格室+桩体复合地基和多层土工格栅(或土工布)+桩体复合地基的桩土应力比、桩土沉降差计算公式。最后通过算例分析验证了计算公式的可靠性,同时系统讨论了路堤填料力学性质(c,φ)、桩身半径、桩间距、土工合成材料的抗拉强度和抗弯刚度等因素对桩土应力比的影响,得到了一些有益的结论。(7)根据某路堤下双向增强体复合地基现场试验,建立ADINA有限元分析模型。通过对比分析复合地基沉降的有限元分析结果、理论计算值与实测结果,验证了有限元分析结果的可靠性。并根据有限元计算结果深入分析研究了水平加筋体的拉力、摩阻力,桩体的轴力、弯矩、摩阻力,以及桩间土的侧向位移等受力变形特性,对路堤下双向增强体复合地基的承载特性有了很深入的认识。更多还原

【Abstract】 It is quite difficult to solve the problem of the uneven soft ground in large areas of highway and railway engineering, to which the technology of composite foundation proves to be an effective method. With the development of the technology of composite foundation, various innovative types have applied to the engineering field. But its designed computation theory is left far behind, especially in the aspect of bearing mechanism. Therefore, the research into composite foundation under embankment has an important theoretical and practical significance. The dissertation studied on the bearing capacity, settlement, soil arching effect, cushion design, etc. through theoretical analysis and numerical simulation analysis. The main researches and achievements are as follows:(1)Since the embankment is a flexible foundation, the formulas for the bearing capacity of ground developed from rigid foundation could not be applied mechanically. The combination of the Slip Line Method and the Limit Analysis Upper Bound Method was set forth to deduce new ultimate bearing capacity calculating formulas of ground under inclined loads. As regards to the composite foundation under embankment where the lateral earth pressure of the embankment, the gravity, and the tensile force of the horizontal reinforced body would bring inclined loads to the ground, the possible range of sliding width was first determined, then the most dangerous sliding surface of the ground was searched out with the optimization method, and finally the limit height of embankment adopting the deduced capacity formulas were calculated. The deduced formulas were proved reliable by sample engineering analyses. A new calculating method was provided for the ultimate bearing capacity of composite foundation under embankment.(2)To solve the problem of the ultimate bearing capacity calculations of horizontal reinforcement and gravel pile composite foundation under embankment, the interaction among horizontal reinforced body, the embankment, and the ground were first analyzed, in order to find out their relationships in terms of the acting force and speed. Then the calculating formulas, which considered the influences of replacement rate of pile and consolidation degree of ground soil, for the strength parameters of composite foundation soil were obtained after analyzing the effects which the gravel pile had on the bearing capacity of horizontal reinforcement and gravel pile composite foundation were deduced. And according to those formulas, the limit filling height of embankment were calculated. As the engineering case studies showed, the calculating results in the dissertation agreed with the practical measurements and turned out to be ideal. Meanwhile, based on the engineering cases, the dissertation made a detailed analysis of the effects which factors like the efficient coefficient of reinforcements and the consolidation degree, had on the ultimate bearing capacity of the ground. And the bearing mechanisms of horizontal reinforcement and gravel pile composite foundation were further studied.(3)First, the failure mode of horizontal reinforcement and rigid piles composite foundation under embankment was discussed. Then based on the Limit Analysis Theory and masking theory for passive piles, the dissertation relatively deduced a formula for ultimate bearing capacity of soil between piles, which took the masking effect of pile into account, and one for ultimate bearing capacity of rigid piles, which considered the slipping impacts of ground soil. Finally the total ultimate bearing capacity was received based on the joint force of composite foundation pile-soil. The sample analyses showed that the method used in this dissertation, rather than the normative one, was more in line with the actual bearing mechanism of rigid pile composite foundation.(4)When the composite foundation with a cushion pile was loaded, its pile would pierce into the cushion and a negative friction resistance would be elicited along at the upper side of the pile. With regard to the features of the relative slipping from the pile and soil of composite foundation and the mutual reaction between them, the elasto-plastic analysis method was improved by absorbing the Boussinesq solution and the Mindlin solution, and the method was introduced in the calculation of the settlement of pile composite foundation. Also, the method was extended in the case of groups of piles, so formulas for the piles with uniform forces and non-uniform forces were relatively received. Through model tests and comparison, it proved that the method in the dissertation was quite reliable. And a new method of calculating the settlement of composite foundation was found.(5)The ultimate bearing capacity of cushion was studied according to the Limit Analysis Lower Bound Method. And the formulas for calculating the ultimate bearing capacity of cushion with the bases of two-dimensional thick cushion, two-dimensional thin cushion and three-dimensional thin cushion under a circular base were relatively deduced. Also the formulas for calculating the maximum effective width of the thick cushion were deduced according to the geometric relationship. Then the relation among the internal friction of the cushion, the maximum width of the cushion, and the stress ratio of the cushion’s pile and soil was studied, and also some inherent laws on the bearing mechanism of cushion were obtained. In the end, considering both the aspects of bearing capacity and settlement, a new design method of cushion was proposed, which was more comprehensive than the previous ones either studying from the perspective of the failure mode or the elastic deformation of cushion.(6)In double direction reinforced composite foundation under embankment, the deformation of the pile and soil between piles would bring a mismatch. Consequently it caused differences in the vertical deformation within the filling of embankment, leading to soil arching. Based on the analysis model of Marston’s soil pressure theory and using Shear Displacement Method to analyze the stress and deformation rules of filling and geosynthetics, the dissertation deduced formulas for the stress ratio of pile-soil and settlement difference of pile-soil, of the composite foundation with geocell plus pile and composite foundation with multi-layer Geogrid(or geotextile) plus pile. At last, the formulas proved to be reliable through sample analyses. Meanwhile the way how the mechanical properties of embankment fillers(c,φ), pile radius, pile space, and tensile strength and bending rigidity of geosynthetics influenced the stress ratio of pile-soil were systematically discussed and some useful conclusions were made.(7)Based on field experiments on double direction reinforced composite foundation under embankment, an ADINA finite element analysis model was established. And by analyzing and comparing the finite element results, the theoretical results, and the measured results of the settlement of composite foundation, the finite element results proved to be reliable. Based on the finite element results, deep analyses were made into the deformation characteristics, such as the tensile force and friction of horizontal reinforced body, the axial force, bending moment and friction of the pile, and the lateral displacement of the soil between piles, in order to know further about the bearing characteristics of double direction reinforced composite foundation under embankment.更多还原