【作者】 张爱军；

【导师】 谢定义；

【作者基本信息】 西安理工大学 ， 岩土工程， 2007， 博士

【摘要】 当前复合地基实践中出现了许多以优化为目的的新的生长点。对这些新的生长点进行总结、提高和理论化研究,促进复合地基新型设计理念的变革和设计理论的形成是一项具有重大现实与理论意义的工作。本论文以推动这些生长点的成熟与应用为主要目标,对复合地基优化的理论与技术措施进行了较为系统的研究。1、论文提出了复合地基优化的总体目标和应该遵循的基本原则,较为全面地分析归纳了复合地基优化的技术措施,使得目前零散、孤立的复合地基优化方法系统化、理论化,从而建立了复合地基优化理论的基本框架体系。该框架体系以复合地基整个系统(包括基础、桩体、土体、褥垫和承台)的共同工作理论和在各阶段的变化规律,以及地基处理的各种有效技术为基础,以实际工程对象对地基承载力和地基沉降量的要求彼此协调为目标,面向“全系统”、“主过程”和“多方法”整体配合互补寻优,抓住了复合地基优化的实质因素。2、论文采取结合适当原型观测的数值试验方法对复合地基优化问题进行研究,建立了基于Adina软件平台,以Biot固结理论为基础的复合地基弹塑性大位移分析模型,并用原型观测结果对建立的数值模型的正确性进行验证。模型中用剑桥模型反映地基土的本构,用接触力学模型反映桩土之间的接触,用“单元生死”技术准确模拟了复合地基的施工过程,用“重启动”功能和开挖“Elment death Decay”功能模拟桩体开挖成孔和填石过程,用“Rebar”单元模拟加筋网,从而可以灵活地模拟桩式、层式和桩层组合式复合地基在各种施工过程下的承载特性,在计算技术上具有先进性。3、论文分别以复合地基型式组合优化、施工过程优化以及工后加固优化等技术措施的典型条件为例,利用本文建立的数值分析模型,分析了各典型条件下复合地基的工作机理和特性。(1)对型式组合优化的条件,揭示了层桩组合式复合地基与层式、桩式复合地基的承载特性差别;完善了层桩组合式复合地基“工后沉降”的定义,以及其在数值分析中的确定方法;分析了桩体和网体刚度、桩体的渗透性、地基的渗透性以及运行荷载对复合地基“工后沉降”的影响,为复合地基型式组合优化方法的应用提供了理论支持;(2)对无砂混凝土小桩复合地基技术不同施工过程优化条件的研究得出:不同的施工过程所形成的复合地基具有不同的承载能力,良好的施工过程可以充分发挥上部建筑物荷载对地基的压密和加速固结作用,提高地基土本身的承载力,从而提高复合地基的承载力,并可以使沉降量绝大部分发生在施工期间,大大减少建筑物的工后沉降;(3)对工后加固的优化条件,根据复合地基中桩间土本身的承载能力决定于浅层部位桩间土的力学特性,而桩体的承载能力关键在于其深部的特点,提出复合地基浅层加固技术和利用浅层加固和深层后灌浆技术进行复合地基工后优化的方法。以浅层换填的工后加固技术为例,采用数值试验的方法,证实了工后浅层加固技术可以较大地提高复合地基承载力的事实,并分析了不同加固深度对复合地基变形、承载力和桩体、桩间土塑性区的开展等各个方面的影响。4、论文还探讨提出了复合地基数值计算中桩土应力比和极限承载力、承载力特征值的确定方法。前者鉴于桩顶应力状态复杂、群桩中桩之间相互影响使得与桩土应力比定义不符以及常规Goodman等界面单元计算的法向应力不准等因素,而使数值计算中不易准确确定桩土应力比的实际,提出将桩土接触面按接触力学问题处理,以桩体顶部中段平均法向接触应力为桩所受应力值,以等应变假定为基础,按照桩与土的力平衡条件计算桩间土应力,以两者之比得到桩土应力比的方法;后者鉴于数值计算中复合地基的极限承载力和承载力特征值不易确定的实际,提出用数值计算方法模拟载荷试验,按照非线性迭代不收敛时载荷板上作用的外荷载为复合地基极限承载力,而用计算出的p～s曲线,按照规范规定的方法确定复合地基承载力特征值的方法。这些方法的提出是复合地基数值计算技术的一个进步。5、论文鉴于浅层加固时主体桩已经施工完毕,可以进行载荷试验的有利条件,提出了以加固前复合地基的载荷试验结果为依据,从加固前后地基综合变形模量的变化出发,估算加固后复合地基承载力的非线性弹性方法,并编制了相应的计算程序。该方法避免了室内试验结果不符合实际的缺陷且计算简单,使用方便,便于推广应用。在这个基础上,进一步提出了复合地基“承载力可调节型设计”的思路和方法,通过用数值计算和实际工程原型观测结果的验证,证明了它的正确性。论文以上的系列研究将为复合地基由基础理论向优化理论的发展和深化起到重要的促进作用。更多还原

【Abstract】 A series of new growth point appear recently in the engineering practice of composite foundation deign and construction from the angle of its optimization. To make them summarizing and improving in a higher level so that to form a new transformation and a new design theory of composite foundation is now an important work both in practical and in theoretical sense. This paper is just aimed at the direction pushing these growth points a further mature in application and establishing the theory and measure of composite foundation optimization.1. The general target, the fundamental principal and the technical measures of composite foundation optimization are given and summarized, which makes the scattered and isolated methods for optimizing the composite foundation systematical and theoretical and then establishes the fundamental frame system of composite foundation optimization. This frame system is based on the interaction theory of the whole system of composite foundation, including the foundation, pile, soil basement, cushion and also pile platform, on their regulations of variation in different stages of composite foundation construction and execution, and also on the series of effective techniques in soil base treatment, hitting the target at harmonizing the requirements both in basement bearing capacity and basement settlement of practical engineering object. It is faced to“the whole system of composite foundation”, to“the principal sequences of composite foundation”and to“the manifold measures of soil base treatment”for finding the optimized scheme in view as a whole, which grasps the essential factors reflecting the composite foundation optimization.2. The method coupling the appropriate results of prototype measurement with that of numerical calculation is adopted as an effective method in researching the problems of composite foundation optimization. An elastic-plastic analyzing model in large displacements is developed based on the Biot’s consolidation theory, taking the ADINA software as its calculating platform. This model is characterized by applying the Cambridge model to describe the base soil, the contact mechanics model to describe the contact property between soil and piles, the“cell life death decay”technique and the“Reset and dig element death decay”to simulate the excavation of pile holes and stone filling in them, the“Rebar element”to simulate the vain net strengthening the soil base. These give a possibility to model flexibly the bearing characteristics of composite foundation in types of pile, of stratum and of pile-stratum during different period and order of their construction. This appears an obvious advance in calculating technique of composite foundation.3. The action mechanism and characteristics of composite foundation under some typical conditions selected as examples of optimizing in types of base soil improvement, in construction order as well as in post-stabilization of base soil are analyzed by using the developed software and calculating model. The result of analysis under the condition optimized in type of basement improvement has revealed the differences of bearing characteristics between the composite foundations in type of pile and in type of stratum, has improved both the definition of“post-construction settlement”of composite foundation and the method of its determination, and has analyzed also the effect of stiffness of pile and net, the effect of permeability of pile and base soils, and the effect of loading action on the magnitude of“post-construction settlement”during the execution of composite foundation. The result of analysis under the condition optimized in construction order of a composite foundation built with small piles of concrete without sand shows that the composite foundation of different construction order has different bearing capacity, the better planed in construction order composite foundation can bring the supper-construction weight into play and speed up the process of soil compression and soil consolidation, that leads to improving the bearing capacity of soil base or composite foundation. Meanwhile, a large part of settlement occurs during the period of construction, which largely decreases the“post-construction settlement”of building. As for the composite foundation treated by the method of post-construction stabilization, such as strengthened by different method in shallow part of soil base (for example, the soil replacement) and strengthened by grouting the deeper part of soil around piles, the result of their analysis has proved the fact that even the shallow improvement technique can obviously increase the bearing capacity of composite foundation. The influence of strengthened depth on basement deformation, on bearing capacity and on plastic domain in soil between piles are also analyzed.4. The methods of determining the“pile-soil stress ratio”and determining the“ultimate bearing capacity”of composite foundation under the condition of numerical calculation are proposed, which makes their values calculated close to that in practical condition. It makes also a progress in the field of numerical calculation of composite foundation.5. The shallow strengthening technique is extended to form a new thinking about“adjustable-bearing capacity design”of composite foundation, which can be used not only to avoid the shortcomings of the test result in laboratory, but also to apply simply, conveniently and easily. This possibility comes from that the piles as a main part of composite foundation are usually established at first, which gives a convenient condition to conduct the field loading test and to obtain a loading-settlement curve of soil base. Based on the change of general modulus of deformation before and after soil improvement this curve can be used to put forward a non-elastic method for estimating the bearing capacity of composite foundation. Such a method has been recommended and compiled it into a calculating program in this paper. The validity of the thinking and method of“adjustable-bearing capacity design”of composite foundation has been proved by comparing the results from numerical calculation and prototype measurement.The abovementioned series of research work may be taken as an important step leading the theory of composite foundation from its fundamental to its optimized and promoting the later theory into practical application.更多还原