A coupled FE-model for tunneling by means of compressed air
This chapter deals with the development and the application of a coupled numerical model for tunnelling below the groundwater table, taking into account compressed air as a means for displacing the groundwater in the vicinity of the tunnel face. The coupl
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Introduction
The construction of shallow tunnels, located below the groundwater level, is an important topic in geotechnical engineering. In this case deformations of the ground and surface settlements are not only caused by the advance of the tunnel face but also by dewatering of the soil. Especially in urban areas the minimization of surface settlements during the construction of the tunnel plays a predominant role in order to avoid damage of existing buildings and infrastructure. For tunnels driven in aquifers the groundwater has to be displaced from the working area at the tunnel face. Compared to lowering the groundwater table with pumping wells and driving the tunnel under atmospheric conditions, the application of compressed air for displacing the groundwater yields smaller settlements as shown in a study conducted during the subway construction in Essen, Germany [19]. This advantage is due to the air pressure and the drag forces of the airflow in the soil which counteract the deformations caused by dewatering and tunnel excavation. This chapter deals with the development and the application of a coupled numerical model for tunnelling below the groundwater table which allows to take into account compressed air as a means for dewatering the soil in the vicinity of the tunnel face. The schematic diagram in Fig. 1 shows the loss of compressed air at the tunnel face and through cracks in the shotcrete lining as well as the flow of compressed air in the adjacent soil. G. Beer (ed.), Numerical Simulation in Tunnelling © Springer-Verlag/Wien 2003
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G. bttl, R.F. Stark, R. Stelzer, G. Hofstetter
Fig. 1. Tunnelling under compressed air
Basically, there exist two different solution strategies for the numerical simulation of tunnelling under compressed air, an uncoupled and a coupled approach. In the uncoupled approach the flow of water and compressed air in the soil and the deformations of the soil caused by dewatering as well as by the advance of the tunnel face are treated in two consecutive steps. In the first step the fluid flow in the soil is determined, assuming a rigid soil skeleton. To this end, numerical models based on the boundary element method [7] or on the finite element method [8] have been developed. In the second step the deformations of the soil are computed by making use of the results of the flow analysis. Consequently, interactions between the fluid flow through the pores and the deformations of the soil skeleton are neglected. On the other hand, in the coupled solution procedure adopted in this study the fluid flow in the soil and the deformations of the soil are determined simultaneously, treating the soil as a three-phase material. From a theoretical point of view, only this approach allows to properly take into account the intrinsic coupling of the process of dewatering with the deformations of the soil in a physically consistent manner. In addition, from a practical point of view the coupled model offers the advantage that all quantities are computed simultaneously on the basis of
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