【作者】 周敏锋；

【导师】 张克绪；

【作者基本信息】 哈尔滨工业大学 ， 岩土工程， 2007， 博士

【摘要】 复合地基是在天然地基土体中设置加固体的一种人工地基。水泥土桩复合地基是复合地基的一种,在土木工程中有着广泛的应用。但是,对复合地基桩-土共同工作的机理和荷载传递机制的认识,仍停留在比较粗浅的阶段,这极大地制约了复合地基的推广和进一步发展。针对这些问题,本课题对水泥土桩复合地基的工作机理和破坏形式进行了深入系统的研究,在此基础上提出了水泥土桩复合地基承载力确定方法的建议。本文首先总结了当前复合地基的研究现状,根据现场和室内试验的结果,提出了一个以分段函数形式表示的水泥土非线性应力-应变关系模型,其参数可以由试验结果确定。模量的表达式也可以由此模型方便地求得。用此模型拟合试验点的吻合程度相当好。通过参数分析发现,水泥土强度的增加主要表现为粘结力的增加;围压增加时,水泥土的模量在对数关系下线性增长,并且围压改善了水泥土的延性;高掺合比的水泥土强度获得的增长比低掺合比水泥土大,但是破坏时其强度的损失也大。根据相关的土力学理论和计算方法,开发了一个可以进行单桩和群桩复合地基受力分析三维有限元程序。将桩体、土体、底板、垫层、桩-土接触面分别划分为不同类型的有限单元;根据对称性取体系的1/4进行计算,以节省计算量;采用分级加荷来模拟加载过程;程序中设置了接触面单元以考虑桩体和土体之间的相互作用;考虑了桩体、土体、桩-土接触面的非线性性能;采用增量割线法以考虑围压的增长对垫层材料模量的影响;采用中点增量法以改进分级加荷计算误差积累的影响。运用本文提出的水泥土的力学模型及模拟分析方法,对一个单桩复合地基进行了三维有限元模拟分析,计算结果和试验数据很接近,验证了该模型的适用性。对两种典型土类的复合地基进行了大量的参数影响分析,研究了水泥土桩复合地基的荷载-沉降曲线、水泥土桩的轴向力、侧摩阻力和桩土应力比,以及母土、水泥掺合比、桩径和桩长等因素的影响。考察了水泥土桩复合地基的荷载传递机制;指出了水泥土桩复合地基荷载-沉降曲线是一条开始为一段直线,破坏后陡降型的曲线;在荷载-沉降曲线上有一个明显的弯曲点,该点相应于水泥土桩顶发生压缩破坏点,因此可按该点来确定水泥土桩复合地基的承载力;基于模拟分析的结果,探讨了随荷载增加桩身轴向力及桩侧摩阻力沿深度分布的曲线,分析了桩土应力比,尤其是破坏时的桩土应力比。研究了水泥土桩端破坏时的桩端变形和桩身压缩。基于水泥土复合地基破坏机制,对确定水泥土桩复合地基承载力提出了建议。更多还原

【Abstract】 Composite foundation is a kind of artificial foundation setting up reinforcement body in natural soil. Cemented soil pile is one type of composite foundation and it has been widely adopted in civil engineering projects in China. However, it is still beyong full understanding of pile-soil co-work mechanism and load transfer behavior and it is at a preliminary stage yet, which greatly hampers the promotion and further development of composite foundation technique. In this paper, the work mechanism and failure mode of cemented soil pile composite foundation are deeply and systematically studied, on which basis proposal of bearing capacity determination method of cemented soil pile composite foundation is put forward.The recent studies on composite foundation are summarized in the first part of the paper. According to the results of in-situ and lab tests, a non-linear stress-strain relationship model of cemented soil expressed in piecewise function is established, whose parameters can be determined from test results. The expressions of modulus can be conveniently obtained from this model as well. The agreement degree is very well adopting this model to fit in-situ and lab test results. Parametric analysis shows that the increase of cemented soil strength is mainly the consequence of adhesion increase in soil; when the surrounding pressure increases, the modulus of cemented soil linearly increases in the mathematical relationship of logarithm correspondingly, and the existence of surrounding pressure improves the ductility of cemented soil; the increase degree of cemented soil strength with high admixture ratio is greater than that of cemented soil with low admixture ratio, but when the former fails, the strength loss is also greater than that of the latter.According to related soil mechanics theories and calculation methods, a three-dimensional FEM computer program is developed in which bearing capacity and deformation analysis of single and group pile composite foundation can be performed. The cemented soil piles, original soil mass, foundation plate, cushion layer and pile-soil interface are discreted into finite calculation elements of different types respectively. Taking advantage of symmetry, one quarter of the whole analysis system is taken to be studied to save calculation capacity, and the load applying process is simulated by step loading method. In the program, the interaction effect between soil mass and pile is considered by setting up interface element, and the non-linear behavior of shaft, soil mass and the interface between pile and soil is considered. The influence of surrounding pressure increase on the modulus of material in cushion layer is considered by using incremental secant method while the influence of error accumulation using step loading method is improved by adopting mid-point incremental method.The three-dimensional FEM simulation analysis of a single pile composite foundation is performed by adopting the cemented soil model and simulation analysis method suggested in this paper. The calculated results well agree with the test data, which testifies the validity of the model. A great amount of parametric influence analysis of cemented soil pile composite foundation in two typical soils is carried out to reveal the influence of factors as load vs. settlement curve, axial forces of shaft, side friction, stress ratio of pile to soil, natural soil, cement admixture ratio and pile length etc. The load transfer mechanism of cemented soil pile composite foundation is estimated as well. The load vs. settlement curve of cemented soil pile composite foundation is a straight line at the beginning and a curve with sharp drop after failure. There is an obvious inflection point on the load vs. settlement curve, which is corresponding to the compressive failure state point of cemented soil pile top. Therefore, the bearing load capacity of cemented soil pile composite foundation can be determined by this point. Based on the simulation analysis results, the distribution curves of shaft axial forces and side friction developing with the increase of load along depth are discussed. Stress ratio of pile to soil is analyzed, especially when the foundation reaches failure state. Pile top deformation and shaft compression at failure state of pile top are studied as well. Determination of bearing capacity of cemented soil pile composite foundation based on failure mechanism is suggested.更多还原