Recent Advances and Encountered Problems in Computing Air Losses in Compressed Air Tunneling by Consideration of Unsaturated Soil Mechanics

Compressed air tunneling is one of the rare cases in applied unsaturated soil mechanics, where air has to be considered as an active fluid phase. An excess air pressure inside the tunnel is applied to prevent groundwater inflow. As soil and also the shotcrete lining of the tunnel are permeable materials, a transient air flow from the tunnel into the surrounding ground takes place. In order to allow an accurate prediction of the amount of air losses in the design process and to provide pore pressure distributions for coupled flow-deformation analyses, a 3-D numerical tool considering unsaturated multiphase flow has been developed. The results of conducted case studies show a very good agreement with measured air losses for tunnels driven in sandy soils, whereas a big discrepancy for tunnels driven in silty soils was experienced. Recently performed laboratory tests are indicating that permeability increasing macro flow paths are likely to develop in cohesive soils. Therefore, a continuum mechanic based numerical approach to predict the air losses in cohesive soils requires the use of adapted relative permeability functions.