Study On Bearing Mechanism And Optimization Design Of Bucket Foundation In Soft Clay

In early 1990's, bucket foundations emerge in the need of exploration of boundary oil fields. Lately, research on it has new development in China. The new kind of foundation makes up the defects of traditional pile platform and gravity platform. For the reasons of technology-owning country and special geology condition in our country, there is lots of work to be done for our engineering use.First, on the base of broadly researching work on this subject, this paper summarizes the current developments of bucket foundation in the world, especially to those in Norway. By contrast, our defects and direction of research on it are found. Furthermore, limit analysis theory and optimization design disciplines are discussed in detail.Secondly, large-scale experiment on bucket foundation is carried out for the first time in our country, including different scale indoor and outdoor model tests, which have reached the world level in the same type of experiment. Through these experiments, vertical and lateral bearing capacities which are used in numerical simulation are obtained. Besides, on the base of indoor and outdoor tests, through the analysis on the soil pressure variation inside and outside of bucket foundation and its displacement, the failure mode for bucket foundation subjected to lateral load in soft clay is proposed. In soft clay, when vertical load is larger than its ultimate bearing capacity, local shear failure is occurred on bucket foundation. Although translation and rotation will be occurred subjected to lateral load, rotation is dominant. Soil pressure has parabola-shaped distribution after bucket, and has triangle-shaped distribution before bucket.Thirdly, relations between internal dissipation power and external load are discussed in failure mechanism of soil slope. Upper-bound plastic limit analysis model of the laterally loaded bucket foundation structure embedded in soft clay is established based on the model tests results and plastic limit analysis theory. Six zones of internal energy dissipation, namely internal dissipation power induced by plastic deformation, dissipation power from outer soil, dissipation power from bucket wall, dissipation power from zone beyond rotation center of bucket, dissipation power from zone below rotation center of bucket and dissipation power from shear plane of bottom of bucket, are determined. Accordingly, mobile velocity field equation is established by solving their plastic dissipation power. For overcoming defects of traditional GA that search capacity is very strong in whole and very bad in local and premature is easily occurred, modified GA is adopted in the model. Through this model, the ultimate lateral load on the bucket structure can be obtained effectively avoiding complex and time-consuming nonlinear elastic-plastic FEM analysis on the all-stage loading history.In traditional Drucker-Prager model, the linear relation is adopted to simulate the effect of hydrostatic stress for soil yielding. It can simulate the effect of the soil deforming only when the hydrostatic stress is small for introducing the linear relation to simulate the effect of hydrostatic stress for soil yielding in the classic Drucker-Prager model. But the linear relation can't well simulate the soil deforming when the hydrostatic stress increases, for soil deforming under hydrostatic stress meets the linear relation in its initial stage and changes to nonlinear relation with the increasing of circle stress. In this paper, we establish a modified Drucker-Prager model which is fit for the soft clay on the basis of classic Drucker-Prager model by introducing the hardening curve and nonlinear relation under hydrostatic stress. Furthermore, on the platform of internationally common structure analysis software ANSYS, for the bucket foundation applied in our country's special geology condition, the 3-D elasto-plastic static FEM numerical model about the interaction system of bucket foundation in beach-shallow sea and the surrounding soil, is established, in which the modified D—P elasto-plastic constitutive model suited to soft clay's stress-strain relation from shallow water is adopted. Compared to experiment, the simulation model is rational. Through this model, the vertical and horizontal bearing capability of bucket foundation is analyzed. We can draw some conclusions that increasing diameter of bucket is more effective than increasing embedment for increasing horizontal bearing capacity. So, diameter of bucket foundation should be increased as soon as possible during design and ratio between diameter and embedment smaller than 1 should be adopted. Furthermore the displacement analysis and soil pressure of bucket foundation under lateral loads is done. The ultimate rotation angle of bucket foundation is obtained. This can provide help for engineering design.At last, structural optimization design is carried out on the bucket foundation structure. Traditional optimization design has two defects, namely its lower efficiency and designer's random. By defining aim function as least material usage, optimization model of bucket foundation subjected to lateral wave load is built. In this model, nonlinearity of wave load and soil can be simulated. The nonlinear two-directional spring element numerical analysis model of bucket foundation structure is established to simplify the structural optimization process. Variables of optimization design have diameter and embedment of bucket foundation, but excluding wall of bucket, for range of wall thickness is small. In solving process, initial values of design variables are supposed, then walking in space so as to get optimization dimension. According to the optimization mathematical model of bucket foundation structure under lateral loads, the optimal structure geometry, that's bucket diameter and embedded depth, can be obtained through the random walk optimization method. Thus the steel usage is greatly reduced and structural strength is fully mobilized, the structure's bearing status is improved as well.