A Study On Working Mechanism And Bearing Capacity Performance Of Bucket Foundation In Soft Ground Under Lateral Loads

As a new type of foundation in shallow water, bucket foundation emerges in the need of exploration of boundary oil fields. The bucket is a large diameter cylinder, opened at the bottom and closed at the top. It is usually installed by applying underpressure to its interior after it is allowed to penetrate under its own weight. Taking the reverse process, the bucket will be pulled out and transferred to other sea areas for recycle. This new kind of foundation makes up the defects of traditional gravity foundation and pile foundation, which has great gravity and causes high engineering cost. It is especially suitable for complex conditions such as soft clay ground and worse sea environments. Therefore, bucket foundation is praised as the dawn of skirted foundation by Norway experts. With the development of our national marine exploitations in Bohai, Donghai and Nanhai, bucket foundation must have an extensive prospect in the future because of its own advantages. During working process, bucket foundation is not only subjected to the long-term action of vertical load induced by all weights of platform and equipment, but is also imposed by lateral loads and moments caused by wind, wave, current and ice. According to engineering experience, the lateral bearing capacity is more important to the stability of bucket foundations. So far, the studies of bearing capacity behavior of bucket foundation in soft ground subjected to lateral loads have been not clarified well and there are a lot of works needed to be done for engineering use. Therefore, it is much necessary to examine the working and failure mechanism of bucket foundation under lateral loads in order to find out effective methods for evaluating the lateral bearing capacity of it in soft ground. In this dissertation, studies are emphasized on numerical methods for evaluating the bearing capacity and failure mechanism of bucket foundation in soft ground under lateral loads. The main investigations consist of the following parts:1. Based on the general-purpose finite element analysis package ABAQUS, the finite element computational model for single-bucket foundation in soft ground under lateral loads is established.In order to examine the mechanical behavior on the interfaces and establish a finite element model that considering the potential crack at active zone in soft ground under lateral loads, the advanced master-slaver contact pair algorithm is utilized. At the same time, friction contact between the inner surface, the top surface as well as the bottom surface of bucket and neighboring soil surfaces is considered. Therefore, the deficiency of conventional finite element analysis which seldom considers the potential crack between the outer surface and soil surface or less takes account of the friction contact between the inner surface and soil surface is avoided and the detailed soil-structure interaction mechanism can be simulated in a more rational way. On the basis of finite element analysis, it is concluded that the structure tends to rotate around a certain point which is located below the ground surface and above the bucket bottom. In addition, there is an individual wedge in passive zone and a crack in active zone as failure happening. This failure mechanism is remarkably different from that of traditional gravity structure and pile foundation.By second-phase development of ABAQUS and parametric computations, the effects of undrained shear strength of the soil S_u, effective unit weightγ' slenderness ratio of the bucket LID and height of the load point above the mudline h on the lateral bearing capacity of single-bucket foundation are discussed. Based on the numerical results, it indicates that the failure mechanism and earth pressure distribution obtained by finite element analyses show good agreement with that got from existing model tests, from which the rationality and validity of finite element method are verified.2. Based on the failure mechanism of single-bucket foundation in soft clay ground under lateral load and earth pressure distribution observed by FEM, a limit equilibrium method for evaluating the lateral bearing capacity of single-bucket foundation is proposed. Comparison with finite element results obtained by ABAQUS shows that the results of limit equilibrium method can agree with that from FEM within an acceptable accuracy and the suggested method is applicable for directing engineering practice.3. On the base of the cyclic shear strength concept proposed by Andersen et al for considering cyclic softening behavior of soft clay seabed, which is characterized by strength weakening and stiffness degradation induced possibly by wave cyclic loading, a nonlinear elasto-plastic-cyclic strength model is suggested and embedded into the finite element analysis pack ABAQUS through second-phase development, then a quasi-static finite element method for assessing the cyclic bearing capacity of single-bucket foundation is developed. The computational results indicate that the failure mode of cyclic instability and distribution of equivalent plastic strain in soft ground are somewhat different from those of ultimate failure state under static loading. In the failure mode of cyclic instability, the rotation center deviates obviously from the symmetry axis of the bucket and near to the loading side of the structure and the sliding surface at the bucket bottom turns to be a more complex spacial surface instead of a spherical surface which center is located on the rotation center of the structure. Parametric studies show that the lateral cyclic bearing capacity of single-bucket foundation is influenced by some factors such as the embedded depth, height of load point and cyclic loading number to failure etc. Moreover, variating the height of load point only, decreased degree of the lateral cyclic bearing capacities of single-bucket foundation in soft ground compared with those under static loading are approximately same.4. According to the actuality that most studies pay their attention on single-bucket foundation but little on multi-bucket foundation for which the influence of bucket group effect on lateral bearing capacity should be not overlooked, parametric computations and comparative analyses on the lateral bearing capacity of multi-bucket foundation (four buckets) are carried out using ABAQUS based on the research experience of single-bucket foundation in this paper. It is found that the lateral bearing capacity of multi-bucket foundation increases at first, and then remains constant mostly with the increase of B. Keeping B invariable, it indicates that the lateral bearing capacity of multi-bucket foundation increases with the increase of connection stiffness, specifically, no connection