To meet the sustainable development of shipping industry and the need for large-scale shipbuilding, effective development and rational utilization of marine deep water space is the top priority of the future shipping industry. However, deep-water marine environment is extremely complex. Traditional structures are unfavorable for deep water structure neither economically nor technically. Therefore, in order to meet the development trends towards deep water, there is an urgent need to develop a new port structure that adapts deep water environment, deep soft foundation and is of good stability and rapid construction.
Plug-in large diameter steel cylinder structure is a space shell structure with no cover, no bottom and no partition. It works as a whole together with the backfill sand inside the cylinder. From the point of mechanics characteristics and work mechanism, the work mechanism of shallow plug-in steel cylinder structure is close to gravity structure. However, when the soft soil layer is thick, the buried depth of cylinder is deep and the soil layer under the bottom of structure is not hard, there is no definite bearing point at the front toe of steel cylinder. The center of gravity is unable to achieve a stable torque. The stability is mainly maintained by lateral friction of soil inside and outside the cylinder wall and embedding action of soil before and behind structure, i.e., the cylinder must be inserted to a certain depth in order to achieve stability. In this case, the plug-in cylinder structure takes into account the force characteristics of both gravity structure and pile foundation structure. From the point of foundation adaptability, deep plug-in steel cylinder structure is very adaptable to deep soft soil foundation environment. The cylinder can be directly sank into soft soil to reach the bearing stratum. It also can be inserted to a certain depth of soft soil with the foundation before and after cylinder being properly dealt with according to demand. From the point of construction process, using plug-in steel cylinder structure in the deep sea area may not form the foundation bed, which speeds up the construction progress and saves costs. To sum up, compared to traditional structure, deep plug-in steel cylinder structure has advantages such as simple form, good mechanical condition, short construction period, economic material usage, strong foundation adaptability, small wave reflectivity and so on. Hence, it is more suitable for harsh sea conditions especially on the silt soft soil foundation. Such a structure will have broad prospects of application and should be further studied and promoted.
Whereas, as a new structure in harbor engineering, deep plug-in steel cylinder structure is still in its infancy with limited design experience and imperfect calculation theory in the world. So far, there is no unified recognition of calculation theory and method for structure stability and displacement in the industry, nor relevant standards and norms. At present, insiders have limited knowledge about the mechanical condition, the failure form, the complex interaction between cylinder and soil, the selection of the calculation parameters and the simplification of the model of deep plug-in steel cylinder structure. Deep researches on mechanism and design method of deep plug-in steel cylinder structure need to be did to solve the important issues of principle dimensions, stability and instability criteria which is related to project technical feasibility.
In this paper, by using three-dimensional finite element numerical simulation method, working mechanism of deep plug-in steel cylinder structure has been studied through sensitivity analysis on core parameters. Plane equivalent calculation method that simplify the spatial structure into a planar model of deep plug-in steel cylinder structure is proposed. Interaction model of soil and steel cylinder is identified. Distribution rule and calculation method of soil pressure are presented. Referring to field data measured from east and west artificial island project of Hong Kong-Zhuhai-Macao Bridge in which deep plug-in steel cylinder structure were used to build islands, model selection, border setting and choosing principles for parameters of spatial finite element numerical simulation for stability calculation are studied as well as the structural stability criteria. Plane equivalent calculation method for stability of plug-in steel cylinder structure is also verified and perfected.