Railway tunnelling in frozen ground on Bothniabana

A new railway line was under construction in North-eastern Sweden, along the shoreline of the Gulf of Bothnia. The terrain consisted of Postglacial clay and silt valleys, and ridges of Precambrian bedrock, crossing the railway line. The railway level was about 20m below the ground surface. For the design of freezing, a freezing analysis was carried out to determine the pipeline depths and distances, to estimate the necessary time for freezing and to determine temperatures within the frozen zone. To estimate the deformations, displacements and safety level, a mechanical analysis of the tunnel cross-section was carried out. As input, the geometry of the soil layers, bedrock and temperature zones were given. Time- and temperature-dependent mechanical parameters were estimated for different soils and temperatures. The analysis was carried out for 12 phases simulating different phases during tunneling. The estimated displacements for the tunnel arch were negligible, if the temperatures were at or below -15oC. The freezing was started in May 2002, and completed in September 2002. The tunneling was started in September, and it was completed in November 2002, following with the casting of final liner. According to the experiences until now, the work has progressed successfully.