【作者】 戚炜；

【导师】 赵法锁；

【作者基本信息】 长安大学 ， 地质工程， 2004， 硕士

【摘要】 桩基础的设计、计算理论体系中，单桩在竖向荷载下的沉降和承载力问题是很关键的。目前应用在竖向荷载下单桩沉降方面的理论分析方法主要有荷载传递法（Load Transfer Approach）、弹性理论法（Elastic Theory Method）、剪切变形传递法（Sheafing Deformation Transfer Approach）、有限元法（Finite Element Method）与边界元法（Boundary Element Method）等数值分析方法、其它经验简化法等五个方面。荷载传递法利用标准化的地区性桩荷载试验资料和现有的计算机专用程序，分析工作简单。此法一旦确定荷载传递函数后，只要进行一些简单的数据运算便可得到桩的荷载—沉降曲线，并由此分析群桩及单桩的工程力学性能。荷载传递法还可以有效地解决土的分层性和非线性等问题。理论分析方法虽然不如现场原型试验直观和准确，但是具备经济、快捷、预测、定量对比的优势。 本文从西安东郊某典型静荷载实例出发，在深入分析竖向荷载作用下单桩的荷载传递过程、变化规律和荷载—沉降特性之后，分别对上部非饱和黄土层和下部饱和黄土层对应的桩侧阻力的发挥特征作进一步分析，依据不同土层呈现出不同的应力应变关系，提出桩侧摩阻力和沉降位移的改进荷载传递模型——随土体变形变化的桩侧摩阻力三折线模型。该模型利用桩身侧摩阻的刚度系数K_s2i在桩周土体弹塑性阶段(S_li～S_2i)可正、可负或为零的概念，能将线弹性全塑性模型、加工软化型模型和加工硬化型模型都统一起来。在打入桩和静压桩的设计中需要考虑桩侧摩阻力的深度效应问题，因为如果将桩侧某一土层的侧摩阻取为一定值会引起承载力计算结果与工程实际之间存在较大差别。为解决这个问题，本文引入深度效应系数ζ_i。 对于Q～s曲线一般呈缓变型，无明显破坏特征点的桩基以及那些对沉降极为敏感的上部结构，应根据桩顶的允许沉降量加以控制。本文提出的按桩项沉降量计算桩基竖向承载力的分析方法，利用“荷载传递函数模型的深化研究”这一章中提出的荷载传递函数——桩侧摩阻力改进三折线模型，借助埃特金迭代法，计算桩顶沉降分别处于弹性、弹塑性或塑性状态等不同水平状态时所对应的桩顶荷载值，推导出桩顶沉降与承载力之间的一系列公式，用以较全面地模拟桩—土相互作用机理，从而实现以桩项沉降量来控制桩的承载力的设想。 最后。对改进荷载传递模型——三折线模型进行实例验证。在黄土地区西安南郊某教学实验楼现场静荷载试验和软土地区某大桥主墩灌注桩的静载荷试验资料分析当中，先将弹性直线段末端对应的桩身变形量作为弹性变形临界值S_li再把走势较平稳的塑性段的各点拟合成一条水平直线，该直线与实测q_s～s关系曲线的交点作为塑性变形临界值s_2i。桩侧摩阻的刚度系数K_sli与K_s2i的大小分别取值于弹性段与弹塑性段的斜率。两桩对应的各主要土层q_s～s关系曲线的发展趋势与本文提出的桩侧摩阻力的改进三折线模型比较吻合。 将本文提出的按照桩项沉降量控制承载力的迭代计算方法用西安南郊某教学实验楼实例验证，计算的桩顶荷载值与实际观测值进行比较，结果Q～s曲线较为吻合。而且收敛速度较快。说明本文方法可用于指导工程实践，具有较好的实践价值。用软土地区静荷载实测资料验证结果也较为吻合，说明本文方法在软土地区也可以参考应用。更多还原

【Abstract】 In the pile foundation calculation theory and designing, the settlement and bearing capacity of single pile under axial loads is very important. Domestic and overseas scholars have achieved a series of valuable conclusions in this field through long-term exploration. Subsequent research has been done on the bearing mechanism and settlement analysis of piles under axial loads. But the pile-soil system is a very complex mechanical system and its engineering design is still in a semi-theory and semi-experience state till now. Lots of researches are still needed due to the complicated inter-action between piles and the surrounding soil. There are five main theory methods used in analyzing pile settlement: LTA-load transfer approach, ETM-elastic theory method, SDTA-shearing deformation transfer approach, NCM-numerical computation method(including the FEM-finite element method, BEM-boundary element method etc.), and other predigest or experiential methods. Using normative information in certain regions and ready-made, appropriative computer programs, the LTA is very simple and effective. Once the load transfer function is confirmed, the load-settlement curve can be obtained by simple data calculation .The LTA is effective in analyzing the mechanic character of single pile and pile groups and in solving the vertical stratification and nonlinear response. The test in-situ is simple and accurate, but the theory analysis method can predict and contrast quickly and accurately, and is economical to use.There are 9 load-transferring functions put forward by scholars at the present. A detailed analysis of the load transfer procedure, the load changing regularity and the load-settlement mechanism of vertically loaded plies have been tested in east of Xi’an city. In this paper I will firstly analyze how the friction resistance force work between the pile shaft and the loess. As we know, the upper layers of loess are unsaturated and lower layers are saturated. According to the different relationships between stress and strain of different soil layers ,then propose an improved load transfer model :the trilinear model for lateral friction resistance and the bilinear model for end resistance. The detailed formulas are followed as:Three conditions are discussed. Firstly the rigidity coefficient-kS2i is equal to zero, then ks2i is bigger than zero, at last ks2i is smaller than zero. When the rigidity coefficient kS2i is equal to zero the formula is same as the elastic-plastic function, when kS2i is bigger than zero the formula is same as the hardening function, when ks2i is smaller than zero the formula is same as the softening function. Then I will unite the elastic-plastic function, hardening function and softening function into a singleformula. There is a need to calculate the depth effect when designing the driven pile and the pressed pile. When fixed lateral friction resistance is taken there is a big error between the computation and practical measurement for driven pile and pressed pile. The depth effect is taken into account by means of the depth effect coefficient i of the lateral friction resistance. The trilinear model changes according to the soil deformation. The improved load transferring function can comprehensively simulate the actuality of the pile-soil interaction.For the superstructure which is sensitive to settlement or whose Q-s curve is gentle without obvious failure point, the top settlement of pile is the dominating parameter in design and calculation. Based on the improved trilinear model and Eitiking Iterated Algorithm, mainly considering the important factor of the top settlement in elastic, elastic-plastic or plastic status, a series of formulas are deduced for controlling the vertical bearing capacity of pile by means of the top settlement values. The formulas can simulate the pile-soil interaction accurately and simply as well as considering the sandwich construction of soil, nonlinear prosperity and so on. Compared with the finite element method, these formulas are suitable for field practice becau更多还原