南海北部陆坡神狐海域峡谷区黏性土海底滑坡过程及其动力学机制研究

The submarine landslides process has been considered to be the key content for the mass transport deposits research and pipeline safety risk assessment in the deep-water slope area. The existing numerical models can’t reflect the whole evolution of the submarine landslide from slope deformation to slope failure to mass movement, which may lead to certain uncertainty for the pertaining scientific research and engineering evaluation. In this proposal, taking the Shenhu canyon area of the northern slope of the South China Sea as the target area, we focus on the cohesive soil submarine landslides process and its dynamics mechanism from the mechanics perspective. The coupled Eulerian- Lagrange finite element method rising in recent years is used to establish a new numerical process model for the cohesive soil submarine landslides. In this new model, the strain-softening and rate-dependency of the undrained shear strength of the cohesive soil, obtained from shear test under large shear displacement condition, is taken into account. The validity of the established model will be verified by comparison with the flume experiment result. According to the identification characteristics of typical submarine landslides located at different positions of the canyon, the quantitative study of the soil deformation characteristics and the influence of ambient water on the landslide mass are conducted by using numerical back-calculation method, and then the response relationship between deposition characteristics of submarine landslides and key engineering geological parameters can be setup based on the numerical results. Finally, the dynamics mechanism of the cohesive soil submarine landslides in the target area can be revealed through the above comprehensive analysis. The research results can provide theory and technique reserve for the quantitative evaluation and prevention of submarine landslide in the northern slope of the South China Sea.

海底滑坡过程是深水陆坡区块体搬运沉积体系研究与海底管道安全评价的关键内容，现有滑坡数值模型尚不能完全反映海底斜坡变形-破坏-滑动的整体过程，给科学研究与工程评价带来一定的不确定性。本项目针对我国南海北部陆坡神狐海域峡谷区，从力学角度开展黏性土的海底滑坡过程及其动力学机制研究。采用近年兴起的耦合欧拉-拉格朗日大变形有限元方法，基于大剪切位移条件下黏性土强度的应变软化与速率相关特性试验研究，构建一种新的黏性土海底滑坡全过程数值模型，并通过与水槽实验对比验证数值方案的有效性；针对峡谷不同位置典型海底滑坡识别特征开展数值反分析计算，定量研究滑坡过程中土体的变形特征以及环境水对滑坡体动态特性的影响规律，明确滑坡沉积特征对关键工程地质参数的响应关系，揭示目标区黏性土海底滑坡过程的动力学机制。项目研究成果可以为我国南海北部陆坡海底滑坡地质灾害定量评价与防治提供理论与技术储备。

南海北部陆坡神狐海域峡谷区海底滑坡地质灾害发育，海底滑坡滑动能够对海底工程设施安全带来较大威胁。目前海底滑坡研究主要从海底斜坡稳定性与划出后的滑动过程两个阶段开展研究。其中考虑强度剪切软化与剪切速率相关的滑坡模型多采用基于浮容重的单相土体滑动角度出发，忽略了滑坡体与海水的动态相互作用，而考虑水土耦合的两相流方法不能定量考虑滑坡体内部的强度软化及率相关效应。针对上述问题，课题首先设计开展了粘性土大位移剪切与流变测试试验，分析了粘性土的大位移剪切特性与流变特性。然后通过目标区典型海底斜坡的稳定性分析对比，算例分析表明土体强度剪切软化对目标区小型浅层斜坡稳定系数影响相对较小，但对土体灵敏度较大的大型海底斜坡，不考虑剪切软化一定程度上会导致稳定性分析结果偏危险；同时，为了考虑目标区土层抗剪强度的垂向空间分布对滑坡体形成规模的影响，在原有研究计划的基础上，增加了考虑抗剪强度分段线性增长的海底斜坡稳定性分析，以及潜在水合物开发分解对斜坡稳定性的影响规律分析。其次，基于耦合欧拉-拉格朗日大变形有限元方法，考虑滑坡体抗剪强度软化、率相关效应，以及滑坡体与环境水的动态相互作用，建立了黏性土海底滑坡的滑动过程数值模型，并通过与水槽试验以及已有数值模拟方法的分析结果对比，验证了所建立海底斜坡变形-破坏-滑动的全过程数值方案的可行性。通过典型海底滑坡过程的数值算例，并综合运用深度积分方法、两相流数值模型分析结果，开展了目标区黏性土海底滑坡的动力学机制研究，分析了影响目标区海底滑坡过程的主要影响因素。研究成果可以为我国南海北部陆坡区黏性土海底滑坡灾害的发生机制与定量评价提供一定程度的参考。