循环荷载下珊瑚礁钙质砂累积变形宏细观演化规律及本构模型研究

The construction of a series of reclamation islands on coral reef in South China Sea is a strategic project of China. These reclaimed islands on the coral reef are mainly composed of calcareous sand. Because of the special engineering characteristics of calcareous sand, the reclaimed islands will face complex surface settlement problems under the long-term dynamic loading caused by marine hydrodynamic and human activities, however, there are few studies on the settlement deformation of reclaimed island. In this study, firstly, a series of cyclic triaxial drainage shear tests are carried out on calcareous sand and ordinary quartz sand, and the cumulative deformation characteristics of calcareous sand under different initial confining pressures and dynamic stress levels during cyclic loading will be revealed, and the particle breakage characteristics and evolution rule of calcareous sand in the multi-cycle shearing process will be investigated. Then, based on CM constitutive model, the dynamic constitutive model considering the particle breakage effect is proposed by introducing parameters of cumulative particle crushing and revising the slope of the critical state line, and the efficient and stable numerical algorithm is also achieved. The influence of initial particle gradation, static and dynamic load parameters on the cumulative deformation is analyzed combined with finite element method, and the calculation method of settlement deformation of the reef foundation under cyclic loading is proposed. This project results can provide theoretical basis for the prediction of the long-term settlement of reclaimed island foundation, and provide scientific support for the safe construction of reclaimed island in China.

南海岛礁吹填工程是我国的战略性工程，岛礁的吹填陆域主要由珊瑚礁钙质砂组成。由于钙质砂工程性质特殊，在海洋水动力和人类活动等引起的长期动力荷载下，吹填岛礁面临着复杂的地表沉降问题，而目前动荷载下岛礁沉降变形的相关研究较少。首先，本项目进行一系列的钙质砂和普通石英砂的循环三轴排水剪切对比试验研究，揭示循环荷载下钙质砂在不同初始围压和不同动应力水平下的累积变形特性，探明多次循环剪切过程中钙质砂颗粒破碎特点及演化规律；然后，基于CM（cyclic mobility，交变移动）模型，通过引入表示循环累积破碎的相关参数和修正临界状态线的斜率M，建立考虑颗粒破碎效应的动力本构模型，并实现高效稳定的数值算法，与有限元相结合分析初始颗粒级配、静动荷载参数和排水条件对累积变形的影响规律，提出循环荷载下吹填岛礁地基沉降变形计算方法。项目成果可为岛礁地基长期沉降预测提供理论基础，为我国岛礁的安全建设提供科学支撑。

随着国家“一带一路”战略构想贯彻实施，围海造岛工程越来越多，就地取材利用海底疏浚材料作填料进行岛礁工程建设可有效缓解围海造岛填料紧缺矛盾，大大降低工程造价并缩短工期，为吹填造岛建设探索一条新路。珊瑚礁砂作为吹填造岛的主要材料，具有多孔隙、多棱角且易破碎、压实难，压缩性大等特点，它与陆源砂有很大差异，其构成的地基承载力低，沉降大，工程性质复杂。本项目通过对自级配钙质砂样开展高压固结试验，发现了试样的压缩性会受到颗粒形状及颗粒粒组的显著影响，试样总应变量与颗粒破碎率之间呈现良好的幂函数关系；通过循环排水三轴试验，研究了循环荷载下钙质砂的累积变形特性，分析了初始颗粒级配、循环动应力比及固结围压大小对钙质砂土累计变形发展的影响，基于安定理论对其变形发展模式进行了归类，揭示了不同破碎率下钙质砂临界状态线偏移规律，探明了多次循环剪切过程中钙质砂颗粒破碎特点及演化规律，钙质砂颗粒破碎率与循环排水累计塑性功之间呈现良好幂函数关系；通过高效稳定的数值计算方法研究了双点振冲法加固吹填岛礁地基的机理。研究成果可为岛礁地基的长期沉降预测和加固提供理论基础，为我国南海岛礁的安全建设提供科学支撑。