钙质砂中螺旋锚贯入诱发的颗粒破碎机制及上拔承载特性研究

Helical anchor, as a new type of offshore foundation system, has been widely used in many aspects of offshore geotechnical engineering, such as foundations for pipelines, ocean transmission towers and offshore wind turbines. To date, substantial research has been carried out on the uplift behavior of helical anchor in clay and silica sand, but no attempts have been made to study its behavior in calcareous sand. In calcareous sand deposit, particle breakage is a dominant factor affecting the uplift capacity of helical anchor. For this purpose, the main objectives of this research are to explore the particle breakage mechanism of crushable sand due to anchor installation and to disclose the uplift behavior of helical anchor in calcareous sand involving particle breakage. Firstly, a series of single particle crushing tests will be carried out on selected calcareous sand particles to study the crushing behavior of particles with different particle sizes, and to build a particle breakage criterion for calcareous sand. The fracture modes of particles under compression can be captured using a high-speed microscopic camera. Secondly, discrete element models of crushable particle elements will be established on the basis of the experimental results of the breakage criterion and fracture modes of sand particles. Using the above established particle crushing models, three dimensional discrete element simulations will be conducted on hollow cylindrical samples in the element scale and anchor penetrating into large scale samples. Thirdly, calibration chamber tests will be performed on half-cut helical anchor models in laboratory. Digital image correlation technique will be adopted to capture the rupture modes of calcareous sand during anchor uplifting. Effects of the embedment depth of anchors and the spacing of helixes on the shape of rupture surface in calcareous sand will be discussed. Finally, centrifuge tests on full-size helical anchor models will be carried out. The outcome of the present research will be a practical method for predicting the ultimate uplift capacity of helical anchor in calcareous sand, considering the effect of particle breakage.

螺旋锚是一种新型的海洋工程基础，可应用于海底管道铺设、海上输电线塔、海上风机等海洋工程领域。然而，目前关于螺旋锚上拔特性的研究成果仅针对海洋黏土或石英砂地基，尚未涉及极易发生颗粒破碎的钙质砂地基。本项目首先基于钙质砂单颗粒压碎试验研究，在分析单颗粒破碎宏观强度分布规律的同时，采用高速细观摄像技术捕捉钙质砂颗粒破裂模式；接着，在试验得到的颗粒破碎准则和破裂模式基础上，进行颗粒破碎离散元细观建模，并用于单元尺度试样空心圆柱模拟和螺旋锚贯入的三维大尺度仿真模拟，揭示螺旋锚贯入过程压－剪应力耦合作用下的颗粒破碎宏细观力学机制；随后，将数字图像相关技术融入室内1g半模模型试验，研究钙质砂中螺旋锚上拔土体破裂面模式，分析锚体埋深、锚片间距等因素对破裂面模式的影响；最后，在考虑钙质砂颗粒破碎影响的基础上，基于螺旋锚上拔离心模型试验研究，分析钙质砂中螺旋锚上拔承载性能，提出上拔极限承载力的实用计算方法。

作为一种新型的海洋工程基础，螺旋锚具有安装快捷、可循环利用且上拔承载力高等优点，非常适用于我国岛礁工程建设。针对这一工程需求，本项目以钙质砂场地中的工程应用为背景，以螺旋锚为研究对象，综合运用理论分析、室内试验、数值仿真等手段，研究了钙质砂中螺旋锚的贯入机理及上拔承载特性。首先，基于室内模型试验手段，分析了砂土中螺旋桩贯入成孔特性及孔周土体的细观组构演化规律，重点探讨了转速比和相对密度的影响；其次，针对螺旋桩贯入诱发的土体压―剪复合应力条件，通过在离散元平台中构建空心圆柱型数值试样，分析了复杂应力状态下颗粒材料的剪切力学性状，揭示了试样剪切带的触发与扩展规律；接着，在单元试验数值模拟基础上，借助PFC3D平台，开发了三维离散元变粒径模型，并将其用于螺旋桩贯入的大尺度模拟；随后，利用开发的三维变粒径模型，将颗粒破碎模型嵌入至土体建模中，进行了可破碎砂层中螺旋桩贯入的细观模拟，揭示了颗粒破碎对螺旋桩贯入特性的影响及其机理；最后，基于数字图像相关技术DIC，对砂土中螺旋锚的上拔承载特性进行了宏细观模型试验研究，得到了松、密砂中螺旋锚上拔承载力变化规律及锚周土体的变形特性。本项目研究成果可为钙质砂场地中螺旋锚等新型海洋工程基础的安装及上拔承载设计提供参考。