A thesis proposal submitted in partial fulfillment of the requirements for the degree of Master of Engineering, School of Civil Engineering

Thesis (M.Eng.) - Asian Institute of Technology, 1999

Finite element analyses are used to study the performance of diaphragm walls for top-down deep excavation in Taipei subsoil. A two-dimensional finite element program, PLAXIS was used for back-analyzing the performance of a "Full Scale" top-down deep excavation in Taipei. In which, an undrained analysis incorporated with groundwater calculation was performed. The results of FEM prediction were found in close agreement with the field observation in term of lateral wall movement, ground surface settlement, lateral earth pressure distribution, bending moments of diaphragm wall and pore water pressure at various zones of excavation. Meanwhile, stress paths at various zones of excavation are established for deep excavation in Taipei subsoil. Finally, a series of parametric studies were carried out based on the back-analyzed parameters and average properties of Taipei subsoil to investigate the effects of wall and support stiffness, wall embedment depth, depth of unsupported excavation and surcharge load on the performance of diaphragm walls. The aspects of wall performance evaluated in the parametric studies including lateral wall movement, ground surface settlements and wall bending moments. The increasing of wall stiffness is not effective in limiting the development of wall and ground movement. The early installation of lateral support could minimize the potentially large wall and ground movements. Besides, an unsupported excavation could induce negative wall bending moment that is generally increased with the unsupported depth. Furthermore, the support stiffness has only a minor effect on the performance of a top-down deep excavation in Taipei subsoil. Moreover, increasing the wall penetration depth could not help in reducing the wall movement. Finally, the existence of surcharge adjacent to the excavation is necessary to be considered as it could significantly affect the magnitude and shape of wall deformation and ground settlement.