土表不对称边载作用下群桩负摩阻力特性及计算理论研究

Negative skin friction can induce additional vertical loading and settlement on a pile, and can even lead to ultimate failure of pile or the structure supported by piles. Most of the finished researches focus on the negative skin friction caused by large-scale surcharge load. However, the asymmetric side load on soil surface often appears in practical engineering. Due to the interaction of soil settlement and lateral soil displacement, the method of negative skin friction caused by large-scale surcharge load cannot evaluation that caused by the asymmetric side load. The research on characteristics and evaluation of negative skin friction considering the effect of lateral pressure of side load is one of key points for this problem. In this project, based on the indoor model test and numerical simulation, the basic characteristics of negative skin friction on pile groups, settlement of pile groups and settlement of soil between piles will be investigated in details, and the influences of several factors will also be discussed. The transfer law of lateral pressure of side load in the soil between piles and the distribution law of lateral pressure of side load along pile shaft will be summarized, and the shadowing effect of lateral pressure due to piles will be revealed. Furthermore, the influence of the interaction of soil settlement and lateral soil displacement on pile, and the group effect of negative skin friction will be researched. The evaluation method of negative skin friction on pile groups caused by the asymmetric side load will be presented, and that of pile group settlement which considering the coordination of the pile-soil settlement with the neutral point depth will be proposed. The results may be helpful to comprehend the mechanism of negative skin friction on pile groups caused by the asymmetric side load, and it can also provide some references for the design of related engineering.

桩基础在负摩阻力作用下将导致桩身轴力、桩基沉降增加，甚至造成桩身以及上部结构破坏等严重后果。已有桩基负摩阻力研究多针对均布堆载的情况，而实际工程中土表堆载常以不对称边载形式存在。由于承受土体下沉和侧移的共同作用，均布堆载的群桩负摩阻力理论难以准确计算边载负摩阻力作用，亟需开展考虑边载侧压力影响的群桩负摩阻力特性及计算理论研究。本项目通过室内模型试验以及数值模拟分析，研究不对称边载作用下群桩负摩阻力、群桩沉降和桩间土沉降的基本特性并分析相关因素影响，总结边载侧压力在桩间土中传递规律以及在桩侧分布规律，揭示基桩对边载侧压力的遮蔽效应。在此基础上，阐明土体竖向、侧向变位对桩基的共同作用机理以及负摩阻力群桩效应，提出边载作用下基桩负摩阻力计算方法，建立考虑桩土沉降与中性点位置协调的群桩沉降计算理论。研究成果有助于明确边载负摩阻力对群桩的作用机理，同时可为该类工程设计提供参考。

桩基负摩阻力作用会造成桩身以及上部结构破坏等严重后果。已有研究多针对均布堆载的情况，而实际工程中常出现土表不对称堆载作用形式。由于承受土体下沉和侧移的共同作用，均布堆载的群桩负摩阻力理论难以准确计算边载负摩阻力作用，故需本项目开展考虑边载侧压力影响的群桩负摩阻力研究。本项目研究主要从计算理论以及模型试验两方面对复杂堆载作用下的桩基负摩阻力作用问题开展研究。主要研究内容：分析了边载以及局部堆载作用下的桩基负摩阻力作用特点，提出了基桩下拉荷载以及沉降计算方法；对比分析了均布堆载以及边载等荷载分布形式对桩基负摩阻力作用的影响，并且探讨了土表堆载和桩顶轴向荷载施加顺序的影响；基于黏土中群桩负摩阻力模型试验，系统研究了荷载距离、群桩布置以及桩基刚度等因素对基桩受力的影响，揭示群桩基桩对边载负摩阻力的遮蔽效应。基于弹性理论，计算了边载以及局部堆载作用下土体沉降分布，明确了附加土压力随桩长分布特点并对桩侧摩阻力开展线性化拟合，形成群桩基桩中性点位置以及基桩沉降等关键指标计算方法。土载分布形式对桩基附加沉降具有较明显的影响，砂土中桩基边载时沉降为均布堆载时的0.77倍且中性点位置低于均布堆载时的0.03～0.08L，边载时黏土中桩基最终附加沉降为均布堆载时的0.74倍，且边载时桩基中性点位置约低于均布堆载时0.02L。桩顶荷载与土表荷载的施加顺序对作用在模型桩上的负摩阻力有较大的影响。先施加土载的工况，桩身中性点位置靠近桩底，而先施加桩顶荷载的工况，中性点最终位于0.4倍桩长处。边载作用产生了额外的桩身弯矩和水平向偏移，中心桩的轴力、弯矩、桩顶沉降和桩顶水平位移的群桩效应系数分别为0.89、0.90、0.90和0.92，桩身设置翼板可以提高对边载负摩阻力的遮蔽效应，可减少后方基桩13.06%的轴力、18.33%的弯矩，14.07%的沉降和19.44%的水平位移。本项目研究有助于明确边载负摩阻力对群桩基础的作用机理，同时可为相关工程设计提供参考，不仅具有科学意义，而且有重要的实用价值。