【作者】 肖忠；

【导师】 王元战；

【作者基本信息】 天津大学 ， 港口、海岸及近海工程， 2009， 博士

【摘要】 随着港口规模的扩大,新建的防波堤工程离岸越来越远,波浪条件变得更加恶劣,地基多为软黏土。传统重力式防波堤已不适用于软土地基,半圆型防波堤、大圆筒防波堤和箱筒型基础防波堤是适用于软土地基的新型防波堤。然而,新型防波堤的设计理论远落后于工程实践,尤其是箱筒型基础防波堤,其在波浪荷载作用下的承载机理、破坏模式尚不明确,在使用期的稳定性分析方法尚无规范或标准可循。另外,考虑软基循环弱化效应时新型防波堤结构的稳定性计算方法尚需完善。为此,本文结合实际工程,对软基上新型防波堤的稳定性分析开展了一系列的研究工作,主要内容和结论如下:1.建立了箱筒型基础新型防波堤稳定性分析的三维弹塑性有限元模型,建议了四种结构稳定性判别标准,通过分析极限状态下结构位移场的分布,得出这种新型防波堤的失稳模式为绕基础筒底以下某点发生转动失稳。研究了箱筒型基础防波堤结构上的土压力分布性状,根据箱筒型基础防波堤的位移模式,将土压力分为主动、被动和静止3个区域,建议了不同区域土压力的简化计算方法。2.在大型通用有限元软件ABAQUS平台上,通过二次开发,将软黏土的循环强度与D-P屈服准则相结合,基于拟静力分析建立了波浪循环作用下考虑软土地基弱化效应的大型海岸建筑物的稳定性计算模型;在相关动三轴试验数据的基础上整理出了天津港软黏土的循环强度曲线。3.在有限元方法计算得出的箱筒型基础防波堤极限状态下结构运动模式的基础上,并考虑箱筒型基础防波堤的空间几何特性、受力特点、极限状态转动点的位置等因素,建立了箱筒型基础防波堤稳定性计算的极限平衡法,通过与有限元结果对比,相互验证了正确性,与有限元法相比,极限平衡法计算速度快。4.为便于工程设计人员应用,在有限元法和极限平衡法的基础上,建立了基于重力式结构稳定性验算模式的箱筒型基础防波堤抗倾稳定性分析简化模型。此模型不但简单,而且具有很高的精度;同时建立了抗滑稳定性简化计算模型;结合箱筒型基础防波堤的空间几何特性和受力特点,根据经典地基承载力理论建立了箱筒型基础防波堤竖向承载力的简化计算模型。5.采用Lanczos法对箱筒型基础防波堤结构和地基相互作用系统进行了模态分析,采用三维弹塑性显式动力有限元法对其在波浪循环荷载下的动力特性进行了分析,建立了箱筒型基础防波堤动力失稳判别方法。更多还原

【Abstract】 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.更多还原