Stability Analysis For New Types Of Breakwaters On Soft Foundation

With the development of ports in China, the new breakwaters are built farther and farther from shore, where soft clay constitutes the majority of the foundations. The wave load in this area is much more powerful than that in the shallow-water area. Under this condition, conventional gravity structures are not suitable for soft-clay foundation and the semicircular breakwater, the large cylindrical breakwater and the box and bucket foundations breakwater may be good choices. However, the design theories for these three new type breakwaters are far from engineering practising, especially those for the box and bucket foundations breakwater because its working mechanism and failure pattern remain unclear. There is no rule to follow for analyzing the stability of the box and bucket foundations breakwater. Moreover, the stability analysis method for new type breakwaters, in which the cyclic weakening effect of soft-clay foundation is considered, still needs improving. Therefore, in this thesis paper, a series of research work have been carried out on the stability analysis of new type breakwaters, in combination with engineering practices. The main contents and conclusions are as follows:1. A 3D elastic-plastic finite element model was established for stability analysis of the box and bucket foundations breakwater. Furthermore, four criterions to judge the stability of this new structure were proposed. Based on the displacement field of the structure in a critical state, the instability mode can be concluded as the box and bucket foundations breakwater rotating around a point below its foundation base. Also, earth pressure on box and bucket foundations breakwater was studied. According to the displacement mode of the box and bucket foundations breakwater, the earth pressure distribution is divided into three regions, i.e., active, passive and static regions. Simplified calculation methods of earth pressures for different regions are suggested.2. A quasi-static finite element method for stability analysis of the large coastal structure considering the cyclic weakening effect of soft-clay foundation under wave load was developed by combining the concept of cyclic shear strength with D-P yield criterion. Then the proposed method was numerically implemented in the framework of the general-purpose FEM software ABAQUS. The cyclic shear strength curve for soft clay of Tianjin port was concluded from results of related dynamic triaxial tests. 3. On the basis of motion mode of the box and bucket foundations breakwater in critical state calculated by finite element method, a critical equilibrium method was established in consideration of the structure's geometry, force characteristics and rotation point position. Compared with finite element method, the critical equilibrium method was consistent in calculation results. But the critical equilibrium method has faster calculation speed.4. For the purpose of engineering application, a simplified calculation method with high accuracy for anti-overturning analysis is presented here. This method is based on the gravity structure stability checking mode, finite element method and the critical equilibrium method. Moreover, a simplified calculation method for anti-sliding analysis was established. Considering the stucture's geometry and force characteristics, a simplified method for vertical bearing capacity calculation, based on classical bearing capacity theories, was established.5. Modal analysis on the interaction between the box and bucket foundations breakwater and the foundation was performed by applying the Lanczos method. Dynamic characteristics of the box and bucket foundations breakwater were analyzed by using a 3D dynamic explicit elastic-plastic finite element method. Then, criterions to judge the dynamic stability were suggested.