Three Dimensional Limit Analysis Of Suction Bucket Foundations

Suction bucket foundation, being an innovative foundation, is more and more commonly used for a variety of coastal and offshore structures, due to its high bearing capacity under extreme environmental conditions, and cost effectiveness of being recycled and so on. The dissertation presents a study of 2- and 3-dimensional (2-D and 3-D) limit analyses of two single-bucket foundations and a three-bucket foundation under the join action of lateral load and moment, using an upper bound limit analysis method developed by the Institute of Water Conservancy and Hydroelectric Power Research. The research work is based on previous centrifuge modeling, and is supported by National Natural Science Foundation of China (50679035) and Tsinghua Basic Research Foundation (Project No. JCxx2005067). The main achievements are as follows.1) Suction bucket foundation of a tension leg platform is under the combined action of large lateral load and moment, creating loading conditions much different from slopes and inland foundations which are commonly under vertical loads. In the centrifuge tests, the lateral loading points are respectively 6.4m and 18.4m above the ground surface of a saturated silty sand. A virtual soil layer was assumed overlying the silty sand with a thickness equal to the vertical distance between the loading point and foundation surface. The unit weight and shear strength of the virtual soil is set to be zero. By doing so, the combined action of lateral load and moment is applied to the foundations. The failure modes of the foundation are deduced from the centrifuge study. Finally, numerical models of the suction bucket foundation are established for both 2- and 3- dimensional limit analysis.2) The two-dimensional limit analysis is performed on the two single-bucket and three-bucket foundations in saturate silty sand, under the limit lateral load. The factors of safety are obtained. The results show that the failure surfaces determined by optimization agree well with Rankine's theory regarding failure patterns of soils behind a retaining wall. The soil surrounding the bucket is primarily divided into an active zone and a positive zone. The limit load calculated by 2-D limit analysis is smaller than that from the centrifuge model tests. The 2-D limit analysis can be used for preliminary analysis of quick identifying potential failure of the foundation. Since the soil-foundation structure is not a two dimensional system, the 2-D limit analysis method has its limitation associated with the simulations of geometry and geological conditions.3) The three-dimensional limit analysis is performed also on the two single-buckets and three-bucket foundations in saturate silty sand, under the limit lateral static load. For the soil-foundation system subjected to lateral load, the failure mass is approximated by a series of prisms having rectangular side faces forming two different basal surfaces: a partly elliptical failure surface and an artificial defined failure surface. The factors of safety calculated by the 3-D limit analysis vary from 0.7 to 1.2, most of which are reasonable. Additionally, the results also show that the artificial defined failure surface may provide more reasonable predictions of the stability of the foundation than the partly elliptical failure surface, with respect to the results from centrifuge modeling.4) A parametric study associated with the controlling points on the basal failure surface is performed to assess the effects of the failure mechanism on the stability of the foundation. The results show that the assumption of the controlling points has great effect on the stability of the foundation, especially controlling points in the active zone.5) The three-dimensional limit analysis method, based on upper bound plastic limit theory, can be used in the stability analysis of suction bucket foundation under combined action of lateral static load and moment, based on reasonable assumptions of the failure mechanism. Since it is a multi-variable dependent problem, further investigation is needed for searching the critical failure mechanism and corresponding factor of safety.