ABSTRACT
The design and construction of infrastructures, whether it is in a recently reclaimed land or in a congested urban environment, are becoming more and more complex due to excavation of the deep grounds in soft and heterogeneous soils. To arriving a safe and economic design where the ground responses can be fully comprehended in a 3-dimensional configuration, 3D geotechnical modeling of soil-structures interaction has become popular and a tool that is indispensable.
The use of a peanut-shaped cofferdam for deep excavation of the cut-and-cover tunnel is gaining popularity. The cofferdam consisted of 3 numbers of truncated circular cells of 21 m long and 22 m wide for each of the cell connecting together. Each cell is formed by connecting perimeter D-wall panels in an arc shape which resists the lateral earth pressure by hoop forces. The hoop forces induced on the perimeter D-wall panels are then transferred at the junction of the cells to some heavy-duty D-walls designated as Y-Panels, which are transversely supported by some reinforced concrete struts and cross-walls.
A 3D finite element computer program, RS3, is used in this paper to model the construction sequences with generic soil properties adopted to illustrate the key lessons learned from excavation involving 3-dimensional geometry. The bending moment and deformation of the D-wall panels, Y-Panels, and struts are presented in this paper to illustrate the mechanism developed in the composite structure.
In this study, it is found that the soil spring constant deduced from the simulation is variable at different unloading stages and will approach a relatively constant value which is higher than the prescribed value usually adopted in structural analysis.
