Design of artificial ground freezing for an access tunnel of a railway station in Switzerland

Abstract

The paper presents the static and thermal design of an artificial ground freezing (AGF) measure with brine for an access tunnel of a railway station in central Switzerland. The geology of the first 45 m of the access tunnel consist of sediments (silty sand to sandy silt) followed by sandstone. The tunnel crosses under the railway lines and train platforms of one of the train stations with the highest traffic. At any time during construction of the tunnel traffic of trains cannot stopped. Due to the sediments aforementioned and since the water table lies slightly above the crown of the tunnel, AGF is the only suitable auxiliary measure for the tunnel. For achieving an adequate frozen body above the crown water table must be raise up by watering the ground locally with drains. The paper presents also the dimensioning and optimization of these drains. The strength parameters were estimated from literature and verified with uniaxial tests with frozen soil samples under constant temperature. Due to project requirements the core of the tunnel is also frozen. Implementation of the AGF measures starts from a shaft. Due to spatial conditions given by the location of reinforcement of the diaphragm wall of the shaft and of a reinforcement beam the distribution of the freeze pipes was demanding. The goals of the thermal design were the generation of a frozen body with uniform thickness around the tunnel and to avoid generation of excess pore pressure in the core of the tunnel during freezing.