Author(s): R. F. Stark; G. Oettl; G. Hofstetter; G. Kammerer; S. Semprich
Linked Author(s):
Keywords: Compressed air; Three-phase model; New Austrian Tunnelling Method; NATM; Air loss; Dewatering; Tunnelling under compressed air; Pneumatic tunnelling; Finite element modelling
Abstract: In this paper practical experiences with tunnelling under compressed air as well as related field and laboratory tests are presented and a mathematical model capable to simulate the dewatering process of soil subjected to compressed air is described. Ongoing experimental and theoretical investigations described here are part of an Austrian joint research initiative on" Numerical Simulation in Tunnelling" supported by the Austrian Science Fund. For the construction of section 30 of the subway in Essen, Germany, the New Austrian Tunnelling Method (NATM) in combination with compressed air was applied. On site measurements revealed the air loss through the shotcrete lining to be an important design parameter. Additionally, measured surface settlements turned out to be smaller compared with driving under atmospheric conditions. At a full-scale field test the dependence of the air permeability on the degree of water saturation and the influence of the flow of air on the soil deformations were studied by pumping compressed air into a bore hole. Currently laboratory tests are conducted in order to investigate the influence of the air permeability of cracks in the shotcrete lining on the development of the flow field of compressed air in the surrounding soil. In particular, the volume of air loss through a crack in a shotcrete lining element and the adjacent ground, the pressure distribution in the ground and the intensities of the flow forces are measured. Theoretical investigations currently focus on the development of a numerical model which allows to predict deformations in the soil and ground settlements due to both soil dewatering and tunnel excavation. The model can be described as an extension of Biot' s consolidation theory by modelling the soil skeleton as a deformable elastoplastic medium and taking into account both the seepage of water and the flow of air through the soil skeleton. The mathematical description of the soil as a three-phase medium relies on averaged values for the density of the three-phase mixture, for the stresses, acting on the surfaces of a differential volume element of the mixture, and for the velocities of the fluid phases relative to the soil skeleton. In addition the dependence of water permeability and air permeability of the soil on the degree of water saturation of the soil is taken into account.
Year: 1999