【作者】 宋兵；

【导师】 蔡健；

【作者基本信息】 华南理工大学 ， 结构工程， 2010， 博士

【摘要】 桩基础作为一种重要的深基础形式,具有沉降量小和承载力高的优点,目前得到非常广泛的应用。桩体可视为埋入土中的柱形构件,在受力后会与土体之间产生相互作用,由于土体本身具有分层特点且不同土层的工程地质特性差别很大,造成桩与土之间的相互作用非常复杂。工程实践证明桩的侧摩阻力在不同工况下取值会有很大的差异,为能深入研究桩与土体之间的相互作用,把握侧摩阻力取值的规律,本文进行了以下几方面的研究。(1)通过机理分析指出桩在使用中的侧摩阻力取值并不等于桩土界面处的摩擦力,将桩侧摩阻力的影响因素归纳为五方面:桩土界面强度(包括桩土界面的粘着力及摩擦力)、土体强度、土的受力变形特性(如砂土的剪胀剪缩特性)、桩土共同体的刚度、桩侧及桩端土体强度发挥不同步等。理论分析得出桩侧土压力的增大可提高桩土界面的强度及土体的抗剪强度,但对于不同性质的土层两者提高的幅度是不同的。砂质土层提高的幅度要高于粘性土层。实际桩侧摩阻力随深度呈抛物线分布,主要原因是桩侧土层摩阻力发挥不同步。(2)根据桩侧摩阻力的特性选用有限元法作为分析手段。总结出利用有限元程序进行桩侧摩阻力研究的一般方法:计算初始土应力,建立桩土模型,将界面条件设为粘结进行试算,判断极限侧摩阻力时的破坏位置,若判断破坏发生在界面处,则可以实测侧摩阻力为拟合目标,通过假定界面参数反复迭代试算来确定界面的参数。通过对试验实例的分析验证了程序的准确性,分析了各参数对计算结果的影响。(3)通过预应力管桩的静载试验及内力测试研究了预应力管桩在饱和粘性土及砂质土层中的摩擦粘着特性,发现在饱和粘性土中界面力以粘着力为主,在砂质土层中界面力以摩擦力为主。计算分析了桩侧土压力随土层深度的变化规律,通过对试验桩的有限元拟合分析得出了预应力管桩在淤泥层及细砂层中的计算参数取值。(4)进行了管桩在砂质土层中基坑开挖前后抗拔极限承载力的对比试验,印证了桩侧土压力对桩侧摩阻力的影响。通过有限元分析研究了砂质土层中桩侧土压力的分布规律及桩在加载前后桩侧土压力的变化规律。(5)进行了灌注桩在分层土体中的承载力试验及内力测试,研究了灌注桩在多种土层条件下的摩擦粘着特性。发现对于中等长度的混凝土灌注桩,其侧摩阻力主要由界面强度决定,界面参数与土的抗剪性能相对应。研究了各试验桩的桩侧土压力变化规律。通过对试验桩试验数据的有限元拟合分析得出了灌注桩在粘性土及砂土中的计算参数取值。(6)进行了混凝土短桩的抗拔与抗压极限侧摩阻力试验,研究了混凝土与岩层的摩擦粘着特性,获得桩与岩层在抗压及抗拔情况下侧摩阻力取值及临界位移。通过有限元分析研究了桩岩之间界面力的构成,发现在桩表面粗糙的情况下,桩与岩层之间的机械咬合作用在界面力中占很大的比重。(7)通过模型计算研究了桩端持力层岩土性状及桩受力方向对侧摩阻力的影响,桩端持力层岩土性状与桩受力方向(抗拔与抗压)不同引起侧摩阻力差异的原因在本质上是相同的,可归结为桩侧土压力的变化、由桩长引起的不同土层的侧摩阻力发挥不同步以及土层所受周围土层的限制作用不同三方面的原因。更多还原

【Abstract】 As an important deep foundation form, pile foundation has the virtue of small sedimentation and high load bearing, so it has a broad application. Pile body can be seen as a column buried in soil, when pile bearing load, there will be a interaction between pile and soil. Because the soil is layered and the geologic characteristics of different soil layers are different, the interaction between pile and soil is very complex. It is found in engineering practice that the shaft soil resistance of pile will varied in different situation. To study the interaction between pile and soil profoundly and make the variation rule of shaft soil resistance clear, the following studies were carried out.(1) By mechanism analysis it is concluded that the shaft soil resistance of pile is different from the friction force between pile and soil. The influence factors to shaft soil resistance can be reduced to five aspects: the strength of pile-soil interface (including the adherence force and the friction force), the strength of soil, the load and deformation characteristic of soil (such as shear dilation or shear compression of sandy soil), the rigidity of the combination of pile and soil, the asynchronism of the appearance of ultimate shaft soil resistance or ultimate tip soil resistance of different soil layer. It is found by analysis that the strength of pile-soil interface and the shearing strength of soil will increase with the increase of side soil pressure, but the extend of the increase is different for soil with different character. The increase extend of sandy soil is bigger than that of cohesive soil.(2) Because the shaft soil resistance of pile is influenced by many factors, finite element theory is chosen as the analysis method of it. A common method for the research of shaft soil resistance of pile was summed up. The process includes: calculating the original stress of soil; establishing the calculating model of pile and soil; set the pile-soil interface as“tied”to judge the destroy position. If the destroy position is judged to be in pile-soil interface, the parameters of pile-soil interface can be worked out by fitting the testing result with iterative computation. By comparing the test data and the calculated data obtained from the program, the veracity of the program was verified. The influence of the calculating parameters to the calculating result was analyzed.(3) The friction and adherence characteristics of PHC pipe piles were studied using the result of a still load and internal force test of piles in saturation cohesive soil and in sandy soil. The research showed that for saturation cohesive soil, the interface force is mainly composed of adherence force, while for sandy soil the interface force is mainly composed of friction force. The rule of side soil pressure changed with the depth of soil is analyzed by calculation. The value of interface parameters for PHC pipe pile in silt and in fine sand is estimated by fitting the testing data.(4) A contrastive uplift test of 4 PHC pipe piles before and after excavating were accomplished. This test proves that the side soil pressure have a influence to the shaft soil resistance of pile. Finite element analysis is used to study the rule of side soil pressure changing with the depth of pile and changing with the load on pile.(5) The friction and adherence characteristics of Cast-in-site pile were researched using the result of a still load and internal force test of Cast-in-site piles. Research shows that for middle length pile, shaft soil resistance is mainly decided by interface strength and the interface parameters are related to the shearing resistance of soil. The rule of side soil pressure changing with the depth of soil is studied. The value of interface parameters for Cast-in-site pile in multiple layered soil is estimated by fitting the testing data.(6) A set of uplift and downward load test of short piles were used to research the friction and adherence characteristics of concrete and rock. The ultimate value of shaft soil resistance and its critical displacement under uplift or downward pressure load is obtained from the test. The composing of interface strength is analyzed with finite element calculation. It is found that when the interface of pile is coarse, the general adherence force is the main component part of interface strength.(7) The influence of tip bearing stratum and the load direction of pile to the shaft soil resistance are analyzed. It is found that the reasons why the above two factors can cause a variation of shaft soil resistance are actually same, which can be reduced to three aspects: the variation of side soil pressure, the asynchronism of the appearance of the ultimate shaft soil resistance or tip soil resistance of different soil layers, the different circumjacent restriction to the soil.更多还原