A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering in Geotechnical and Earth Resources Engineering, School of Engineering and Technology

Thesis (M. Eng.) - Asian Institute of Technology, 2013

Mechanized tunneling is always feasible and justified with its boring length and in case of short length tunnels or cross cuts between two tunnels or a non circular tunnels required then the excavation method is to be changed from mechanized to the conventional excavation method. A numerical study is performed for assessment of applicability of sequential tunnel excavation in Bangkok subsoil condition. Three schemes of sequential excavation are investigated using the Mohr Coulomb and the Hardening soil model. Three water pressure conditions in the soil layers are defined to explore the effect on tunneling. The undrained effective stress analysis is performed using effective strength and stiffness parameters. The 2D plane strain model is implemented with consideration of the 3D face effect by stress reduction method. Shotcrete lining is considered with two different stiffness values to incorporate approximate effect of shotcrete aging. The results demonstrate that sequential tunnel excavation is not feasible in Bangkok subsoil. Two mechanisms of failure are found first is failure of soil cover above roof and second is the base heave. However, with improved ground it is possible to excavate the tunnel sequentially. Increase in dimension or strength of improved ground results in reduction in lining loads and surface settlement. Hardening soil model results in narrow and deeper surface settlement troughs if compared to the Mohr Coulomb model result. Water pressure variation results in higher bending moment in lining.