【作者】 孔纲强；

【导师】 杨庆；

【作者基本信息】 大连理工大学 ， 岩土工程， 2009， 博士

【摘要】 桩基负摩阻力问题是工程界广泛关注和研究的重点和难点课题之一。由于问题的复杂性,目前仍有很多问题没有得到很好地解决。本文采用模型试验、数值模拟和理论计算相结合的方法,对竖直/倾斜群桩中各位置桩基的桩身下拽力、中性点位置、群桩效应以及地基土固结时间效应、桩顶荷载与地面堆载的施荷顺序等影响因素进行了系统研究;基于极限平衡原理建立了负摩阻力群桩效应系数计算公式和群桩竖向抗拔承载力计算公式:开发了减少负摩阻力措施的新方法——扩底楔形桩及其施工方法,并对其力学特性进行了初步探讨,以求得到一些能为工程设计提供有益参考的结论。具体地,论文主要研究内容及所取得的主要研究成果包括以下几个方面:（1）设计了考虑桩周土体含水率、桩端持力层刚度特性等因素对桩身下拽力和中性点位置影响的模型试验,结合数值模拟方法分析了桩顶荷载与地面堆载的施荷顺序、桩基长径比以及桩-土摩擦系数等因素对桩基负摩阻力特性的影响。研究结果表明,先进行地面堆载后进行桩顶加载情况比先进行桩顶加载后进行地面堆载情况的桩身轴力和桩顶下拽位移要小,同时加载情况所得的相应值介于两者之间;相同地面堆载等级下,摩擦端承型桩负摩阻力作用引起的桩身下拽力比端承型桩情况小;摩擦端承型桩,当桩周土体含水率在最优含水率附近时,土层的下陷量较大致使桩体沉降量也相对增大,中性点位置反而有所上升。（2）设计了桩顶荷载与地面堆载共同作用下竖直单、群桩的模型试验,结合数值模拟方法对竖直群桩的负摩阻力特性进行了系统研究。分析了地基土固结时间效应、加载速率与加载方式、桩间距、桩基位置（中心桩、边桩和角桩）、桩基数目以及地面堆载等级等因素对桩身下拽力、桩顶下拽位移和中性点位置的影响规律。研究结果表明,随着地基土固结沉降的发展,桩侧负摩阻力值也得到发展并逐渐趋于稳定,表现出明显的时间效应;当桩-土相对位移达到一个较小值（几毫米）时,桩侧负摩阻力就能得到充分发展;桩侧负摩阻力的发展随着加载速率的减慢而减小并最终趋于一个稳定值;分级加载比直接加载产生的桩身下拽力值略小,随着荷载等级的增大,两者的差异减少并最终趋于一致;由于受群桩效应的影响,群桩中角桩的桩身下拽力值和桩顶下拽位移值大于边桩和中心桩的相应值。（3）设计了桩顶荷载与地面堆载共同作用下倾斜单、群桩的模型试验,结合数值模拟方法对倾斜群桩的负摩阻力特性进行了研究,并与相应的竖直单、群桩进行了对比分析。探讨了桩基倾斜角、桩间距等因素对桩身下拽力、桩项下拽位移和侧向位移的影响规律。研究结果表明,由于受桩基倾斜角的影响,群桩效应相对不明显;在小倾斜角范围内,桩身下拽力沿桩深方向的分布规律与竖直桩基类似,且桩身下拽力随着桩基倾斜角的增大而略有所增大,桩顶下拽位移随着桩基倾斜角的增大而有明显增大;地面堆载作用下,斜群桩中桩顶产生明显的侧向位移,并且随着地面堆载等级、桩基倾斜角的增大而增大。（4）基于极限平衡原理,假定土体受力时对桩体作用的有效影响面积,建立了负摩阻力作用下群桩效应系数的理论计算公式。该计算公式综合考虑了群桩布置形式、桩间距、桩基长径比以及桩周土体性质等因素对负摩阻力群桩效应系数的影响;该公式计算简便且公式中的参数容易得到,尤其是对有效影响面积进行简化后的计算模型。通过运用该公式对本文模型试验及工程实例进行计算,并与实测结果进行比较分析,计算结果与实测结果吻合良好,从而验证该计算方法的准确性。（5）提出了一种既可以提高桩基承载力又能有效降低负摩阻力对桩基影响的新桩型——扩底楔形桩,并申请了国家发明专利（专利号:200810011854.4）。简要介绍了该新桩型的施工工艺和力学机理;基于数值模拟分析方法,初步探讨了其竖向承载力、竖向抗拔力以及地面堆载引起的桩侧负摩阻力等力学特性,并与等体积混凝土的常规扩底桩、楔形桩和等截面桩进行对比分析;研究了桩端土体与桩周土体模量比、桩-土摩擦系数、楔形角、扩大头直径以及桩体模量等因素对扩底楔形桩力学性状的影响规律。研究结果表明,扩底楔形桩桩身轴力沿桩深方向的分布规律兼有楔形桩和扩底桩的特点;当桩端土体与桩周土体模量比适中时,选用扩底楔形桩比较适合,可以同时发挥桩侧摩阻力和桩端阻力值的优势;相同地面堆载等级作用下,扩底楔形桩中桩侧负摩阻力引起的桩顶下拽位移最小、桩身下拽力值介于常规楔形桩和扩底桩两者之间。由此说明,扩底楔形桩是降低负摩阻力对桩基影响的有效方法之一。（6）基于极限平衡原理,假定桩基上拔破坏时的破坏面是由倒置截顶圆锥面和圆柱面组成的复合破坏面,建立了计算单、群桩极限抗拔力的简单理论计算公式。利用“最大最小值原理”确定复合破坏面的临界桩长L_cr和破坏曲线中的待定系数N,从而确定破坏面形状和极限抗拔力值P_u。该计算公式考虑了桩基长径比、土体容重、土体凝聚力、内摩擦角以及桩-土摩擦系数等因素对破坏面形状和极限抗拔力值的影响。通过运用简化计算公式和数值模拟方法对工程实例进行计算,并与实测结果进行对比分析,理论计算结果与数值模拟、实测结果吻合良好,由此说明采用本文方法计算桩基的极限抗拔力的可靠性。更多还原

【Abstract】 Negative skin friction is one of most important and difficult projects on pile foundation design field,which is widely attentioned by engineers.However,the problem of negative skin friction is very complex and there are some issues that have not been well solved in engineering practice.Study on dragload,neutral point,group effect and considering consolidation time of soil,loading sequence effect for vertical or inclined pile groups are taken by using model test,numerical simulation and theoretical calculation methods.Based on limit equilibrium principle,the theoretical calculation formulas for pile group effect coefficient under negative skin friction and uplift capacity of pile groups are built.A new method:pedestal wedge pile which can reducing the influence of negative skin friction is developed,and its mechanical properties are preliminary studied.These researches may be providing helpful reference conclusions for engineering design.Concretely,the main research contents and results of this paper includes:（1） Model test on dragload and neutral point which influenced by the water content of pile surrounding soil and the stiffness of pile end soil are designed.Combined with numerical simulation method,the characteristics of negative skin friction which influenced by loading sequence of pile head load and surface load,friction coefficient of pile-soil interface and length-diameter ratio of pile are discussed.The results show that,in surface loading after pile head load conditions,the dragload and downdrag which caused by negative skin friction are maximum.The dragload of end bearing pile are larger than that of friction end bearing pile under the same surface load.When the water content of pile surrounding soil is close to optimum water content,the position of neutral point is higher than other conditions.（2） Model test and numerical simulation on negative skin friction for vertical single pile and pile groups under pile head load and surface load are designed.Dragload,downdrag and neutral point which are influenced by soil consolidation progresses,loading rate or mode,pile spacing,the positions of pile（central pile,edge pile and corner pile）,the numbers of piles and surface load grade are discussed.The results show that,negative skin friction increases rapidly at beginning and then slowly down,finally stabilizes at a constant as soil consolidation progresses.Negative skin friction can be fully developed when the relative displacement of pile-soil reaches to a small value（several mm）.Negative skin friction decreases rapidly at beginning and then slowly down,finally stabilizes at a constant as loading rate decrease.Dragload caused by step loading mode is a little smaller than that caused by direct loading modes,and the difference become smaller when surface load increasing.Dragload and downdrag of comer pile are larger than that of edge pile and central pile because of pile group effect.（3） Model test and numerical simulation on negative skin friction for inclined single pile and pile groups under pile head load and surface load are designed,and compares with the results of relevant vertical single pile and pile groups.Dragload,downdrag and neutral point which are influenced by tilt angles and pile spacing are discussed.The results show that,pile group effect is relative not significantly because of the existing of tilt angles.In small tilt angles range,the dragload distributions along pile depth of inclined pile is similar to that of vertical pile,and its values increased slightly with the tilt angles increasing.Downdrag increased significantly with the tilt angles increasing.The lateral displacement is developed significantly under surface loading,and its values increased with the surface load or tilt angles increasing.（4） Based on limit equilibrium principle,assuming an effective influence area around various locations of piles,the theoretical calculation formula for pile group effect coefficient under negative skin friction is built.Various pile parameters such as the arrangement of group piles,pile spacing,length-diameter ratio of piles and soil properties such as density,angle of internal friction and pile-soil interface friction coefficient are considered in the formula.This formula can be calculated easily,and the parameters in the formula can also be obtained easily.Especially,when the effective influence area calculation method is simplified,the formula is more easy to taken.The calculatied values are then compared with model test results carried out by this paper and also with the engineering example results reported on literature.The calculation results are found to be in good agreement with the measured values, which validating the accuracy of the developed calculation formula.（5） A new method:pedestal wedge pile which can improving bearing capacity and reducing the influence of negative skin friction is developed,and national patent（patent number:200810011854.4） is application.The construvtion technology and mechanical mechanism of new pile type are brief introduced.Its mechanical properties of bearing capacity,uplift capacity and negative skin friction are preliminary studied,and the conventional pedestal pile,wedge pile and equal section pile which are in the same concrete volume are comparative analysied.The mechanical properties of pedestal wedge pile which are influenced by modulus ratio of pile end soil and pile surrounding soil,friction coefficient of pile-soil interface,wedge angles,diameter of enlarged pile head and pile modulus are discussed.The results show that,the mechanical properties of pedestal wedge pile have the characteristics of both conventional pedestal pile and wedge pile.When modulus ratio of pile end soil and pile surrounding soil is in middle,it is suitable to chose pedestal wedge pile.It can develop pile shaft and pile top resistance adequately in the same time.In the same surface load,downdrag of pedestal wedge pile is minimum,and dragload are between conventional wedge pile and pedestal pile.It is showed that,pedestal wedge pile is one of effective methods which can reducing the influence of negative skin friction.（6） Based on limit equilibrium principle,assuming a composite failure surface（inverted truncated circular pyramidal and cylindrical surface）,the theoretical calculation formula for uplift capacity of single pile and pile groups is built."Maximum minimum principle" is used to determining the coefficients of critical length of pile(L_cr) and coefficient of failure curves （N）,which are assumed for the failure surface shape.Then the failure surface and uplift capacity（P_u） are determined.Various pile parameters such as the arrangement of pile groups, pile spacing,pile length,pile diameter and soil properties such as unit weight,angle of internal friction and pile-soil interface friction angle,which influencing the uplift capacity of pile groups directly are considered in the formula.The calculatied values of uplift capacity and failure surface of pile groups are then compared with numerical simulation results and experimental results.The calculation results are found to be in good agreement with numerical simulation and measured values,which validating the accuracy of the developed calculation method.更多还原