【作者】 文华；

【导师】 程谦恭；

【作者基本信息】 西南交通大学 ， 防灾减灾工程及防护工程， 2008， 博士

【摘要】 矩形闭合型地下连续墙（以下简称闭合墙）是近三十年来发展起来的一种新型的桥梁基础。它以承载力高、刚度大、工程量小、造价低等优点,受到了桥梁工程界的广泛关注和青睐,具有很高的推广价值和广阔的工程应用前景。在我国西部辽阔的黄土地区,巨厚的黄土地层中没有良好的桩端持力层,如果采用闭合墙作为桥梁基础,充分利用其良好的工程特性,将会对桥梁的安全、经济和施工等带来明显的效益。然而,在黄土地区采用闭合墙作为桥梁基础,一个不容忽视的问题便是湿陷性黄土地基引发的负摩阻力问题。本文采用了室内模型试验、数值模拟和理论分析三种方法对闭合墙基础的竖向承载特性、负摩阻力的作用机理、负摩阻力作用下闭合墙基础的承载性状以及负摩阻力的计算方法等进行了系统的研究,研究成果为黄土地基中闭合墙基础的设计提供了理论依据和参考。主要研究内容和成果如下:（1）根据弹性力学原理,提出了模型闭合墙的简化力学模型,并采用半逆解法推导了闭合墙基础内、外侧表面处的轴向应变与单位侧摩阻力关系的解析解。该解析解的求出,为竖向载荷模型试验中闭合墙以及筒桩等同类型基础所量测的内、外侧应变进行数据处理提供了理论依据,进而为基础一土相互作用研究提供了真实、准确的试验结果。（2）采用自行研发的室内模型试验装置,进行了闭合墙竖向载荷模型试验和室内浸水模型试验。其中,竖向载荷模型试验对闭合墙基础的竖向承载特性、墙—土—承台相互作用和群墙效应作了系统的研究。研究发现:单片墙基础的竖向承载特性与灌注桩十分类似,而闭合型地连墙基础的承载机理较之单片墙基础要复杂得多。闭合墙基础内、外侧摩阻力并非同步发挥,内摩阻力沿埋深呈幂函数曲线分布,内摩阻力的最大值出现在墙端附近。（3）通过单片墙与闭合墙的室内浸水模型试验,研究了矩形闭合墙基础负摩阻力的作用机理以及浸水后闭合墙基础的竖向承载性状,试验结果表明:负摩阻力分布曲线大致呈抛物线型。闭合墙基础的中性点位置比单片墙的低,在相同的湿陷性黄土地层且浸水条件相同的情况下,闭合墙浸水后的附加沉降小于单片墙,表现出良好的抗沉降特性。（4）提出了一种模拟黄土湿陷变形的简便合理的新方法——模量折减法。模量折减法通过对发生湿陷区域土体变形模量的折减和容重的增加来进行自重湿陷变形的模拟,并在底部非湿陷性土层中采用减小容重法对底部土层的压缩变形进行控制。从蒲城现场浸水试验和阿塞拜疆现场浸水试验两个数值模拟算例的计算结果来看,模量折减法概念清楚、操作简便,计算结果合理,较好地实现了对黄土自重湿陷变形的数值模拟,其计算结果的准确性与合理性明显优于以往的模拟方法。（5）基于模量折减法,采用数值分析软件FLAC^3D,分别建立多层地基中不同截面尺寸的方形闭合墙和普通矩形闭合墙数值模型,研究了多层地基中墙周土层逐步发生湿陷的情况下,不同截面尺寸的方形闭合墙和普通矩形闭合墙的竖向承载性状和负摩阻力的发挥发展规律,探讨了方形闭合墙与普通矩形闭合墙竖向承载力发挥的异同。（6）提出了闭合墙负摩阻力与下拉荷载的计算方法（抛物线法）以及考虑负摩阻力的闭合墙基础容许承载力计算公式。（7）基于荷载传递法和剪切位移法,提出了负摩阻力作用下闭合墙基础沉降计算的一种迭代方法。此方法充分考虑了闭合墙基础的截面形状特点与荷载传递特性,并建立了相应的数学模型,推导了闭合墙基础的剪切刚度系数,提出了计算内侧摩阻力的幂函数法。采用迭代方法对某一级公路上一座跨线桥的闭合墙基础进行了计算,计算结果所反映的负摩阻力作用规律与桩基浸水试验实测结果相似,初步验证了迭代方法的合理性。更多还原

【Abstract】 Rectangular closed diaphragm wall foundation is a new type of bridge foundation which is developed in the past 30 years. The new type foundation has aroused extensive concern for its many advantages such as high bearing capacity, large stiffness, smaller work amount and lower cost. So, rectangular closed diaphragm wall foundation has a good prospect of application and extension.When pile foundations are applied in the loess area in western China, there is no rigid stratum for pile foundations in the loess layers with huge thickness. If rectangular closed diaphragm wall instead of pile foundation is applied, better benefit will be obtained for bridge engineering. But negative skin friction （NSF） will be an important problem for rectangular closed diaphragm wall which is used in collapsible subgrade.Model tests, numerical simulation and theoretical analysis were utilized in the research to study the bearing characteristics of rectangular closed diaphragm wall without NSF, the action mechanism of NSF, the bearing behaviors, of rectangular closed diaphragm wall subjected to NSF, and the calculation methods for NSF. Primary coverage and achievements as follows:1. Simplified mechanical model of tubular pile or closed diaphragm wall was proposed base on elasticity mechanics. An analytical solution of unit shaft resistance-axial strain relationship by semi-inverse method was obtained. This solution will provide theoretical basis for data processing of outer and inner axial strain which were measured in the model test of axially loaded cast-in-situ tubular piles or closed diaphragm wall.2. Two groups of model tests were done by the new developed testers, the first group was load tests, and the second is immersion tests. In the load tests, bearing performance, wall-soil-cap interaction, wall group effect of rectangular closed diaphragm wall were studied. It was observed that the bearing performance of single diaphragm wall is similar to cast-in-situ pile. The load transfer mechanism of rectangular closed diaphragm wall is much more complex than single diaphragm wall. Outer shaft resistance’ and inner shaft resistance were not developed at the same time. And distribution curves of inner shaft resistance are similar to power function curves. The maximum inner shaft resistance takes place at the toe of wall.3. Action mechanism of negative skin friction and the bearing behaviors of rectangular closed diaphragm walls after immersion were studied through model tests. It was observed that distribution curves of negative skin friction are similar to parabola, the neutral plane depth of rectangular closed diaphragm walls is lower than that of single diaphragm wall, and additional settlement of rectangular closed diaphragm walls is smaller than that of single diaphragm wall in the same loess stratum under the same immersion condition.4. Modulus reduction method was put forward to simulate collapsible deformation of loess. According to the method, after reducing modulus and increasing unit weight of the loess, collapsible deformation will occur. In order to decrease the compress deformation of bottom soil layer, unit weight reduction method was applied. The results of FDM analysis show that modulus reduction method is theoretically reliable, simple in application and useful in practice.5. Behavior of negative skin friction on rectangular closed diaphragm wall was modeled by FLAC^3D on the basis of modulus reduction method. The influences of sectional size, sectional shape on behavior of negative skin friction were studied in numerical simulation analysis.6. Calculated method of negative skin friction and dragload, calculation formula of allowable bearing capacity were proposed for rectangular closed diaphragm wall in collapsible loess subgrade.7. An iterative approach was proposed for evaluating the settlement of rectangular closed diaphragm wall subjected to negative skin friction. In the iterative approach, shear stiffness index was deduced, and power function method was put forward to calculate the inner shaft resistance. Calculated results indicate the iterative approach is reasonable and feasible.更多还原