复杂应力条件下南沙群岛饱和珊瑚砂动力特性试验研究

Few studies are carried out on the cyclic characteristics of coral sand of the Nansha Islands in the South China Sea. In order to meet the urgent need for the cyclic characteristic parameters of coral sand in the proper engineering analysis of both military and civilian function facilities located in Nansha reefs and offshore marine areas, it is an important and emergent scientific task to study the cyclic characteristics of coral sand of the Nansha Islands under ocean wave and earthquake systematically. Considering the effects of index properties, constituent mineralogy, particle types and shapes, and crushability of coral sand, a series of undrained cyclic loading comparison tests of saturated coral sands from Mischief and Yongshu reefs in Nansha Islands are performed under complex initial stress conditions and cyclic loading patterns for simulating ocean waves, earthquake and their simultaneous effect. The dynamic modulus, damping ratio and Poisson's ratio characteristics, pore pressure generation characteristics, cyclic stress-strain characteristics, and cyclic strength characteristics of coral sands are analyzed systematically and thoroughly. The desired results will show the influencing rules of particle breakage and anisotropy on the cyclic characteristics, and provide the empirical relationships of small strain dynamic modulus and Poisson's ratio; the unique empirical relationships of the dynamic modulus, damping ratio and Poisson's ratio for a wide strain range are established; the appropriate dynamic constitutive relation describing the influences of different cyclic loading patterns on the deformation characteristics of coral sand is proposed; the physical mechanism and its differences of liquefaction and failure of coral sands under different cyclic loading patterns from ocean waves, earthquake and their simultaneous effect are also revealed, the corresponding failure criteria are proposed; the method and unique empirical relationship characterizing quantificational cyclic resistance of coral sand are obtained. The findings are of special significance to the Nansha Islands in earthquake prone areas, where the port and airport facilities use loose dredged coral sand sediments of this type as filling material. A reassessment of seismic performance of ports, airports and other seafloor structures requires better characterization of the mechanical properties of shallow-water coral sand sediments. The results will provide some important information that is lacking for the proper engineering analysis and design involving these unique materials.

目前对南沙群岛珊瑚砂动力特性的研究极少。为了满足南沙珊瑚岛礁及近岸军事与民事功能设施建设对珊瑚砂动力特性参数的急迫需要，系统而深入地研究海浪和地震作用下南沙珊瑚砂的动力特性已是一项紧迫的科学任务。以永暑礁和美济礁珊瑚砂为研究对象，针对不同物理性能和颗粒形态的珊瑚砂，模拟波浪、地震、波浪与地震联合作用的不同循环加载模式与应力路径，开展复杂初始应力条件下饱和珊瑚砂的系列不排水循环加载对比试验，研究珊瑚砂动模量、阻尼比与泊松比特性、应力-应变关系特性、动强度特性，揭示颗粒破碎和各项异性对珊瑚砂动力特性的影响规律，给出小应变动模量和泊松比的经验关系式，建立动模量、阻尼比、泊松比与应变关系曲线的唯一性经验关系式；提出适宜描述不同循环加载模式及应力路径对变形特征影响的珊瑚砂动本构关系，揭示不同循环加载条件下珊瑚砂液化或失效机理及其差异，提出相应的失效准则，建立珊瑚砂动强度的表征方法与唯一性经验关系。

目前对南海岛礁珊瑚砂动力特性的研究极少。研究地震、波浪作用下饱和珊瑚砂的变形与液化特性，对我国南海珊瑚岛礁的军事和民事功能设施建设具有重要的应用价值。该研究工作具有前瞻性和基础性。以南沙和西沙岛礁的饱和珊瑚砂为研究对象，模拟波浪、地震作用的不同加载模式与应力路径，开展了饱和珊瑚砂的系列不排水试验。发明了负压渗流饱和技术，提高了砂类土的饱和效率。提出了基于极限孔隙状态的砂类土最大动剪切模量预测模型，并采用既有不同类型砂土最大动剪切模量的试验数据，验证了预测模型的可靠性。提出了基于相关函数理论的动模量和阻尼比计算方法，提高了计算精度和可靠性。揭示了饱和珊瑚砂在剪应变水平0.001%-1%的割线动剪切模量和阻尼比的变化规律；提出了基于累积弹性应变能的损伤状态参数，建立了饱和珊瑚砂应力-应变骨架衰退和孔压增长特性的损伤-应变状态耦合模型。揭示了饱和珊瑚砂应力诱发各项异性的基本规律，发现饱和珊瑚砂存在显著的应变硬化现象。揭示了饱和珊瑚砂的液化或失效机理，发现饱和珊瑚砂的液化抗力及传统的循环应力比CSR与液化循环次数(对应广义剪应变幅值γga =2.5%)之间的关系受固结条件和循环加载耦合的显著影响，γga=2.5%是评价饱和珊瑚砂液化或失效的适宜准则。对于相同的固结条件、不同的循环应力路径，发现饱和珊瑚砂的孔隙水压力增长模式为广义剪应变幅值的正切函数；发现单元体循环应力比USR是表征饱和珊瑚砂动强度的适宜指数，且固结条件相同时USR与引起2.5%广义剪应变幅值的循环次数Nf存在事实上的唯一性关系。对于均等固结条件、不同应力路径的循环加载，发现15次循环引起液化所需的USR15与骨架孔隙比存在良好的负幂函数关系, 并用文献中的5种砂类土的试验数据验证了该关系式的适用性。