吸力式沉箱基础极限拉拔承载力的数值分析

作为新型的深水海洋基础型式，吸力式沉箱基础被广泛地用于系泊深水海洋设施中，从而承受巨大的倾斜上拔荷载。在上拔荷载水平分量与竖向分量的共同作用下，吸力式沉箱的承载特性及其工作性能评价是海洋工程设计与建设中的关键技术问题之一。然而现有的理论分析与试验研究并不能满足工程实践的需要，因此，对吸力式沉箱基础的极限承载力分析建立了有限元数值计算方法。当沉箱基础在快速拔出过程中，正常固结黏土处于完全不排水状态，沉箱基础发生整体破坏时表现出反向地基承载力失稳模式，此时沉箱基础所发挥的极限承载能力往往最大。为此，在数值计算中直接假定沉箱基础及其周围土体处于完全不排水状态，针对不同的沉箱长径比，分别确定了在竖向上拔荷载和水平拉拔的单独作用下沉箱基础极限承载力。对比发现：竖向上拔极限承载力有限元解能够较好地与理论计算结果相符合，而水平极限承载力解与理论计算结果存在一定的差异。

Numerical analysis of ultimate uplift bearing capacity behavior of suction caisson foundations

As a new type of deep-water offshore foundations, suction caisson foundation is widely used to fix offshore structure or facilities. Under such a circumstance, suction caisson foundations are frequently subjected to pullout forces. Therefore the ultimate bearing capacity and performance of such foundations under combined application of both vertical and horizontal components of pullout are one of the key issues in the design and construction of offshore structures and facilities. However, the performance of such foundations has been not well understood; and current methods of evaluation and design cannot meet the increasing requirement of engineering practice. Therefore a numerical procedure based on FEM for evaluating the uplift bearing capacity behavior of suction caisson foundation is developed. On the basis of the multi-purpose finite element analysis software ABAQUS, step-by-step loading procedure is numerically implemented to simulate the progressive deformation process and failure mechanism. During the pullout of caisson foundation, the normal consolidated clay is under fully undrained state. The general shear failure of caisson foundation is characterized by an ultimate collapse pattern due to bearing capacity which is opposite to traditional failure mode of foundation subjected to vertical compressive load. It is assumed that the soils surrounding the caisson foundations are in undrained condition during pullout and their original strength is fully mobilized. Then comparative numerical computations are performed for different aspect ratios of caisson foundation to determine the ultimate bearing capacity of caisson foundations under single action of vertical pullout or horizontal pullout. The authors’ numerical results are compared with the solutions obtained by current theoretical or numerical analyses. It indicates that the uplift ultimate bearing capacity by finite element numerical method is accordance with by theoretical method, and the horizontal ultimate bearing capacity has small variant.