Finite element study of deep excavation construction processes

Excavations for deep basements in urban areas present complex design questions, e.g. related to the selection of retaining systems and the specification of construction processes. Consideration must be given to the design of structural systems to support the excavation and the mitigation of any impact that the construction might have on any nearby infrastructure. The finite element analysis is routinely used (in design and research) for the analysis of this type of complex soil-structure interaction problem. Care is needed to ensure that representations of the construction processes, soil and structural behaviour are incorporated in the finite element model at an appropriate level of detail. This paper addresses the implementation of construction procedures in a finite element analysis of a deep excavation in Shanghai. Initially, an analysis is developed to model the construction processes actually employed in the project. The results of this model compare favourably with data measured during construction. The model is then developed to investigate the influence of certain aspects of the construction processes on the computed results. The results indicate the following: (i) the construction sequence for the floor slabs does not have a significant influence on the computed deformations in the retaining walls or the nearby ground at the end of construction; (ii) earth beams (used as temporary supports) are effective in reducing the computed wall and ground movements, and (iii) neglecting the presence of openings in the floor slabs may lead to unconservative calculations of the retaining wall and ground movements.