承台对群桩水平承载力影响的数值分析

发布时间：2018-11-27 19:23

【摘要】：桩基础是一种重要的基础形式,因其造价低、承载能力高、承载力发挥稳定等优点在工程中得到了广泛的应用。对于桩基础在水平荷载作用下的受力机理及变形特性研究一直是研究的热点和难点。吸收塔基础中常常将大块式基础作为承台,在其下设置群桩基础。现有的设计中,群桩的水平承载力计算只是单桩水平承载力的简单叠加,没有考虑群桩效应,也没有考虑大体积承台受到的水平抗力以及对群桩水平承载力的提高作用,因此现有设计方法不能反应吸收塔基础真实的水平承载力,低估了抵抗水平荷载的能力,造成了浪费。本文利用PLAXIS 3D有限元软件对大体积承台群桩基础在水平荷载作用下的受力和变形特性进行了模拟计算,结合理论和设计实践,以及室内模型试验,深入探究承台对群桩水平承载力的影响。主要工作和成果如下:(1)通过对某实际工程现场桩基水平载荷试验进行数值模拟,将数值计算结果与原始数据进行对比,验证PLAXIS 3D对桩进行水平荷载计算的可靠性;(2)建立单桩模型,分析承台侧面8体的抗力以及侧面和底面的摩擦力对于单桩水平承载力的影响,承台边长为2 m、3 m、4 m、5 m时单桩水平承载力分别提高了 46.9%、105.9%、170.4%、207.1%。(3)建立群桩模型,对比分析了承台高度、桩间距、承台厚度对群桩水平承载力的影响,经过计算发现上述情况下,群桩效应综合系数均大于1。具体来说:1)桩间距为唯一变量时,群桩效应综合系数η随桩间距的增加而增大,以9桩低承台为例,桩间距由3D～6D时,η值1.65-3.49。2)承台厚度为唯一变量时,随着承台厚度的增加,群桩效应综合系数η呈线性增加,以9桩低承台群桩为例,承台厚度0.5 m～2.0 m时,对应的η值分别为1.31-1.67。(4)选取9桩低承台群桩基础模型进行受力分析,根据承台前端土体的水平抗力和水平位移可以得到得到地基土水平抗力系数K(x)的分布形式,K(x)=K0+mx_,地表处水平抗力系数K0不为零,与m法相比该公式对K(x)值的计算与实际情况更为接近。(5)将群桩效应综合系数η的数值计算结果与规范中的计算值以及模型试验中的值进行比较,由于没有考虑竖向荷载,数值计算值比理论值小;而模型试验由于受模型边界条件、加载方式等影响,η值比数值计算值小。
[Abstract]:Pile foundation is an important foundation form, which has been widely used in engineering because of its low cost, high bearing capacity and stable bearing capacity. It is a hot and difficult point to study the stress mechanism and deformation characteristics of pile foundation under horizontal load. In the absorption tower foundation, the large pile foundation is often used as the cap, and the pile group foundation is set under it. In the present design, the calculation of the horizontal bearing capacity of pile group is only a simple superposition of the horizontal bearing capacity of a single pile, without considering the effect of pile group, the horizontal resistance of large pile cap and the effect of improving the horizontal bearing capacity of pile group. Therefore, the existing design methods can not reflect the true horizontal bearing capacity of the absorption tower foundation, underestimate the ability of resisting horizontal load, and cause waste. In this paper, PLAXIS 3D finite element software is used to simulate and calculate the stress and deformation characteristics of pile group foundation under horizontal load, combined with theory and design practice, as well as indoor model test. The influence of pile cap on the horizontal bearing capacity of pile group is studied. The main work and results are as follows: (1) through the numerical simulation of the pile foundation horizontal load test in a practical project, the numerical results are compared with the original data to verify the reliability of the PLAXIS 3D horizontal load calculation of the pile; (2) the model of single pile is established, and the resistance of 8 bodies on the side of the cap and the effect of friction between the side and the bottom of the pile on the horizontal bearing capacity of the pile are analyzed. The side length of the cap is 2 m ~ 3 m ~ 4 m. At 5 m, the horizontal bearing capacity of single pile is increased by 46.9% and 105.9%, respectively. (3) the pile group model is established, and the influence of pile cap height, pile spacing and pile cap thickness on the horizontal bearing capacity of pile group is compared and analyzed. It is found by calculation that the comprehensive coefficients of pile group effect are all greater than 1. Specifically, 1) when the pile spacing is the only variable, the comprehensive coefficient 畏 of pile group effect increases with the increase of pile spacing. Taking 9 piles with low pile cap as an example, when the pile spacing is 3D~6D, 畏 value 1.65-3.49.2) when the cap thickness is the only variable, the comprehensive coefficient 畏 of pile group effect increases linearly with the increase of cap thickness. Taking 9 piles with low cap as an example, the cap thickness is 0.5 m or 2.0 m. The corresponding 畏 values are 1.31-1.67 respectively. (4) 9 pile cap pile group foundation models are selected for force analysis. According to the horizontal resistance and horizontal displacement of the soil at the front end of the cap, the distribution form of the horizontal resistance coefficient K (x) of the foundation soil can be obtained, K (x) = K0 mx_, the horizontal resistance coefficient K0 is not zero. Compared with m method, the calculation of K (x) value by this formula is closer to the actual situation. (5) the numerical calculation results of the comprehensive coefficient 畏 of pile group effect are compared with the calculated values in the code and the values in the model test. Because the vertical load is not considered, the numerical value is smaller than the theoretical value. However, the 畏 value of the model test is smaller than that of the numerical calculation because of the influence of the boundary condition and loading mode of the model.
【学位授予单位】：北京交通大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TU473.1

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