南中国海神狐海域天然气水合物上覆地层不排水抗剪强度预测

浅层沉积物不排水抗剪强度（Su）是深水作业的关键参数之一。为了获取南海神狐海域首次海域天然气水合物试采区W18-19框体的基本工程地质特征，试采工程准备阶段开展了原位孔压静力触探测试（CPTU）及大量的室内实验。本文将主要基于CPTU计算不排水抗剪强度的基本模型，采用微型十字板、电动十字板、袖珍贯入仪及不固结不排水三轴实验，确定该区域不排水抗剪强度的基本模式，并提出适用于南海神狐钙质黏土层的不排水抗剪强度纵向分布规律计算模型，对该区域水合物上覆层的不排水抗剪强度进行预测。 结果表明，基于总锥端阻力、有效锥端阻力、超孔隙压力的模型系数分为13.8、4.2、14.4。综合考虑地层压实效应和含气情况，本文提出的分段函数预测模型与室内结果的一致性较好，可用于工程设计阶段进行工区不排水抗剪强度纵向分布规律的预测。另外，基于有效锥端阻力的不排水抗剪强度经验模型适应于浅层极软-较硬压实的钙质粘土层，基于超孔隙压力的不排水抗剪强度模型适用于较硬-坚硬的不含气层，而基于总锥端阻力的不排水抗剪强度计算模型则适用于坚硬含气的钙质黏土层。本文提出的分段函数模型有效的提高了经验模型在南海神狐水合物赋存区的适用性，计算结果可为工程安全评价提供支撑。

Undrained shear strength evaluation for hydrate-bearing sediment overlying strata in the Shenhu area, northern South China Sea

The undrained shear strength of shallow strata is a critical parameter for safety design in deep-water operations. In situ piezocone penetration tests (CPTU) and laboratory experiments are performed at Site W18-19 in the Shenhu area, northern South China Sea, where China''s first marine hydrate exploitation operation is due to be located. The validation of the undrained shear strength prediction model based on CPTU parameters. Different laboratory tests, including pocket penetrometer, torvane, miniature vane and unconsolidated undrained triaxial tests, are employed to solve empirical cone coefficients by statistical and mathematical methods. Finally, an optimized model is proposed to describe the longitudinal distribution of undrained shear strength in calcareous clay strata in the Shenhu area. Research results reveal that average empirical cone coefficients based on total cone resistance, effective resistance, and excess-pore pressure are 13.8, 4.2 and 14.4, respectively. The undrained shear strength prediction model shows a good fit with the laboratory results only within specific intervals based on their compaction degree and gas-bearing conditions. The optimized prediction model in piecewise function format can be used to describe the longitudinal distribution of the undrained shear strength for calcareous clay within all depth intervals from the mud-line to the upper boundary of hydrate-bearing sediments (HBS). The optimized prediction result indicates that the effective cone resistance model is suitable for very soft to firm calcareous clays, the excess-pore pressure model can depict the undrained shear strength for firm to very stiff but gas-free clays, while the total cone resistance model is advantageous for evaluating the undrained shear strength for very stiff and gassy clays. The optimized model in piecewise function format can considerably improve the adaptability of empirical models for calcareous clay in the Shenhu area. These results are significant for safety evaluations of proposed hydrate exploitation projects.