Influence of Fluid Mechanics in Unsaturated Soils during Compressed Air Tunnelling by Means of the NATM

In shallow tunnelling below the groundwater table compressed air can be used for preventing water inflow into the tunnel. When using this method air loss takes place through both the unsupported tunnel face and the lining. Until today it has been very difficult to correctly estimate the amount of airloss during the design phase of a project, although it could be a significant factor concerning the total costs of the tunnel construction. To solve this problem, the multi-phase flow in the soil above of the tunnel has to be considered.The aim of this project is to develop a new approach, based on design principles and unsaturated soil constitutive models. At the Institute for Soil Mechanics and Foundation Engineering in Graz, large scale laboratory tests were conducted to simulate the air-permeability of the soil and the lining system.The experimental data is compared with results of numerical models. The models are based on existing constitutive laws to describe the mechanical behaviour of unsaturated soils. The results of the tests are discussed and analytical and numerical methodologies are deloped to estimate the amount of air loss during tunnel advance. In the numerical tunnel advance model both the influence of the dewatering process in the soil and the incresing length of the tunnel are considered. Different soils are used to calculate the air loss through the tunnel into the soil.