竖向受荷螺杆桩与土相互作用的宏细观机理研究

Half-screw pile foundation has been popularized and applied in some projects, but the theory development still fell behind the project practice. In order to further promote the research work of complex pile-soil interaction mechanism and bearing capacity calculation problem, the model tests based on digital image correlation technique are used to study the pile-soil interface friction transmission and soil progressive deformation regularity around pile side and tip. The three-dimensional particle flow simulation method is used to investigate the variety characteristics of soil stress, porosity and average contact number, the connection between mesoscopic structure changes and macroscopic mechanical responses such as pile-soil interface friction transmission characteristics, pile tip resistance distribution characteristics and soil deformation around pile is also be revealed. The thread section bearing capacity composition of half-screw pile is presented according to the results got from model tests and numerical simulation, and the ultimate bearing capacity calculation method is proposed based on the load transmission and failure model. Development of this research work will deepen the understanding on pile-soil interaction inherent essence and rule of half-screw pile from the macro-and meso-levels, and improve the half-screw pile ultimate bearing capacity calculation method. This research work has important value for raising the level of theoretical research and promoting the engineering application of half-screw pile technique.

螺杆桩基础在工程中已经得到了推广和应用，然而理论的发展仍然落后于工程实践。为进一步推动螺杆桩与土体之间复杂的相互作用机理以及承载力计算问题的深入研究，课题基于数字图像相关技术，开展不同设计参数条件下螺杆桩物理模型试验，揭示竖向受压螺杆桩桩土界面摩擦力传递以及桩侧和桩端土体变形渐进发展规律，利用三维颗粒流数值模拟方法，探讨桩周土体应力、孔隙率及平均接触数的变化特征，揭示土体细观结构参数变化与螺杆桩桩土界面摩擦力传递、桩端阻力发挥及桩周土体变形等宏观力学反应之间的内在关联，在此基础上明确螺杆桩螺纹段承载力构成，并根据构建的荷载传递及破坏模型，提出螺杆桩极限承载力计算方法。本课题研究工作的开展将从宏观和细观层面深化对竖向受压螺杆桩与土相互作用内在本质和规律的认识，完善螺杆桩极限承载力计算方法。课题研究工作对螺杆桩理论研究水平的提高及工程应用推广具有重要意义。

螺杆桩是一种现场浇注的变截面异型桩，已在多项工程中得到了推广应用。然而，目前对于螺杆桩与土体之间复杂的相互作用机理的认识还不够深入，其设计计算理论的发展亦落后于工程实践，需要进一步开展研究工作。项目综合运用物理模型试验、数值模拟和理论分析方法，从宏观和细观层面对竖向受荷螺杆桩与土相互作用的内在本质和规律以及承载力计算等问题进行系统研究，拟定的主要研究内容为：（1）竖向受荷螺杆桩桩土界面传力与变形特性室内试验；（2）竖向受荷螺杆桩与土相互作用细观特征的颗粒流数值模拟；（3）竖向受荷螺杆桩桩土界面荷载传递及破坏模型。项目重要研究结果及关键数据：（1）螺杆桩H/D在1.0-1.33之间时，荷载-沉降曲线呈缓降型，单桩极限承载力最高，较常规单桩提高17.6%；（2）螺杆桩螺纹段荷载分担比最高，底层叶片控制螺纹段承载力发挥，桩身轴力沿着深度方向分三段衰减且衰减速度逐渐加快，建议螺纹段长度一般不超过2/3L；（3）不同螺距螺杆桩直杆段及叶片间直杆周围土体破坏模式呈柱状，H/D≤1.33螺杆桩底层叶片及桩端周围土体呈整体剪切破坏，HSP8螺杆桩螺纹段上部叶片周围土体形成局部剪切破坏区，H/D=1.33是在此工况下较为合理距径比；（4）在室内试验和数值模拟基础上，建立了螺杆桩荷载传递模型，给出了考虑距径比影响的螺杆桩极限承载力计算方法，并初步验证了计算方法的合理性。项目研究工作的开展，深化了对螺杆桩与土相互作用机理的全面认识，丰富和发展了螺杆桩设计计算理论。