Compressed Air Tunneling

Abstract

For the first time in the Netherlands, the construction method top-down cut-and-cover with compressed air will be applied as primary construction method for the construction of two tunnels in deltaic soil conditions. Nowadays a low degree and a short period of disturbance is more and highly valued when it comes to construction methods. Cut-and-cover with compressed air differs from traditional construction methods in that during excavation and construction of the tunnel floor, compressed air is used to maintain horizontal and vertical equilibrium. Instead of working completely top-down, the tunnel is constructed beneath the tunnel roof in a train like motion, called the construction cycle consisting of (1) Groundwater control, (2) Excavation and (3) Construction tunnel floor. The feasibility of the cut-and-cover with compressed air construction method depends on the construction schedule, a faster construction schedule will increase the feasibility. Wet excavation with this construction method is not desirable. To achieve a fast construction schedule, dry excavation is vital. This research focuses on the groundwater control methods that are feasible to apply for the construction of cut-and-cover tunnels with compressed air to ensure dry excavation. The feasibility of a groundwater control method depends on the period necessary for groundwater control and the deformations induced by the groundwater control method itself. Important soil parameters influencing the result of groundwater control are the resistance of the soil and the corresponding Soil-Water Characteristic Curve. The application of compressed air will lead to an increase in the water volume released from the soil, due to a shift in the Soil-Water Characteristic Curve. Compressed air as an exclusion method is not a feasible groundwater control method in low permeable soils. The groundwater control period with compressed air is higher than the resistance of the soil deposits. The resistance value of low permeable soils are often significantly higher than the construction cycle schedule. Suitable groundwater control methods studied in this research are compressed air with vertical drains, vertical dewatering, horizontal dewatering and combinations of dewatering with compressed air. For the case study groundwater control methods are analyzed. The groundwater flow calculations are performed with a coupled SEEP/W and AIR/W calculation to determine the groundwater control period. The deformation analysis is performed with Plaxis 2D. Groundwater control with compressed air with vertical drains has the most effect on the volume of water discharged during the 12 days groundwater control period. With the study is determined that in between 33% and 43% of the required volume for dry excavation is removed in 12 days. The resistance value becomes less important with a decrease in center-to-center distance of the vertical drains. Deformations that occur during the groundwater control period do not have an influence on the total deformation during the final stage of construction. It is recommended to perform a field test to generate data that can lead can lead to more knowledge and therefore a more feasible construction method.