基于变形控制的近海深水桩基承载特性及其设计计算方法研究

For the offshore piles used by cross-sea bridges, straight-pile wharf and offshore wind power tower, not only settlement is caused by the vertical force down from upper structures (such as the deadweight), but obvious lateral loads (i.e., horizontal force, bending moment and torque) as well as flexure deformation will be observed under the influence of the vehicle braking, equipment operation, typhoon and heavy waves. Furthermore, bearing behavior of this kind of piles is more complex than those common piles on land. In addition to the material strength of pile shaft and surrounding subsoil, the bearing capacity of the pile foundations will depend more on the allowable vertical or horizontal displacement values at the pile top by upper structures, which means that the pile bearing capacity should be determined by controlling the deformation. However, the commonly used method in engineering practice is still difficult to achieve this. Therefore, in this study, finite element analysis based on the Probe Test method will be first made to obtain the pile shaft bearing capacity failure envelopes (surfaces) under different combined loads of load, and qualitative discussion will then be carried out to analyze the main influence parameters as well as the rules on the bearing characteristics of offshore pile foundations. Then, indoor load tests on model offshore pile foundations will be designed and finished. By changing the parameters of geometry, material and load, deformation characteristics, load transferring mechanism and failure modes of the pile shaft will be discussed, from which distribution function or models may be established to describe the lateral subsoil resistance, skin friction resistance and end resistance along the pile shaft. Finally, by establishing and solving the deflection differential equation of pile shaft, the power series solutions for the internal forces and displacement of pile shaft will be proposed in detail. After compared and verified with the results from model experiments and numerical analysis, the calculation and design method for offshore piles will be put forward based on deformation control.

对于跨海大桥、全直桩码头及海上风力发电塔等采用的深水区桩基，除因上部结构自重等竖向力作用产生沉降外，还会受车辆制动、设备运转、台风或巨浪影响承受较大侧向荷载(水平力、弯矩甚至扭矩)和变形，导致其承载性状远较一般桩基复杂，除了桩土材料强度影响，桩身承载力更取决于上部结构所容许的桩顶竖向或水平位移值，即应按变形控制设计，而常用方法尚难以实现。为此，本研究拟先通过有限元分析，基于Probe Test法获得不同组合荷载下的桩身承载力破坏包络线(面)，定性探讨深水桩基承载特性的主要影响参数及其规律。然后，设计完成深水桩基室内载荷模型试验，通过改变几何、材料及荷载参数，探讨桩身变形特征、荷载传递机理及破坏模式，由此建立桩侧土抗力、摩阻力及桩端阻力分布函数。最后，通过建立并求解桩身挠曲微分方程，获得桩身内力位移幂级数解答，经与数值分析与模型试验结果对比分析验证后，提出基于变形控制的深水桩基设计计算方法。

近海深水桩基常年受复杂环境条件影响及单个或多个荷载作用，承载性状极为复杂，而常规设计方法多基于简化与叠加分析，难以准确评价其桩身承载力，甚至偏于不安全，本课题为探讨更为合理的计算分析方法展开了较为系统深入的研究。.首先，基于ABAQUS完成了近海深水桩基内力变形数值模拟分析，获得了桩身长径比与土层分布等关键因素对桩身承载力的影响规律以及不同荷载组合与加载顺序下的桩身承载力包络线；通过二次开发编写USDFLD用户子程序，探讨了近海波浪荷载作用下水深、波高、周期、循环次数等参数及桩身几何尺寸对单桩水平累积位移的影响规律。.然后，设计制作了室内加载测试装置，完成了一系列模型桩载荷试验，获得了单一荷载及水平力-扭矩、竖向力-水平力-扭矩等组合荷载作用下的桩顶位移及桩身侧阻、端阻、弯矩、轴力和扭矩分布，并按变形控制确定桩身极限承载力，进而通过数据拟合获得了桩身承载力包络线及其简化计算公式，可供近海深水复杂受荷桩基设计计算参考。同时，开展了水流环境下的模型桩载荷试验，研究了流体作用对桩身承载特性的影响规律，并按桩顶沉降及水平位移控制桩身承载力，获得了桩顶荷载-位移曲线、桩身弯矩及考虑流体影响的桩身承载力包络线。.最后，引入剪切位移法和荷载传递函数，基于模型试验结果建立了桩周土塑性变形随桩顶荷载增加自上而下开展的荷载传递模型，并假定地基土剪切模量沿深度呈幂函数或指数函数分布，采用Bessel函数转化求解出单、双层地基中单桩在纯扭荷载下的弹性、弹塑性和塑性解析解；考虑桩-土接触面上的竖向、切向双向剪应力耦合，导出了单、双层非均质地基中不同竖向力-扭矩加载路径下的桩身内力位移解答。采用改进有限杆单元法，考虑水平简谐荷载-扭矩的动静力耦合作用产生的附加刚度，建立综合刚度矩阵，基于MATLAB编制计算程序，求得了桩身内力位移解答。此外，还结合工程实例和模型试验实测数据将数值分析与理论简化计算结果进行了相互对比分析，验证了研究成果的可靠性。