Tunneling through soft clay

A new railway line was under construction in Northern Sweden, along the shoreline of the Gulf of Bothnia, between Umea and Ornskoldsvik. The ground conditions varied dramatically: Postglacial clay and silt valleys with intermediate ridges of crystalline, Precambrian bedrock, crossing the railway line. Within the stretch in question, the railway line level was about 20m below ground surface in soft organic silt. For the design of freezing, a freezing analysis was carried out to determine the practical freezing plan with pipeline depths and distances, to estimate the necessary time for freezing and to determine temperatures within the frozen zone. The frozen soil in the tunnel was planned to be cut by blasting, and the tunnel walls and arches were planned to be strengthened temporarily with shotcrete, that should work the period of cutting, about 3 months. To estimate the deformations, displacements and safety level, a mechanical analysis of the tunnel cross-section was carried out, applying the FEM-program PLAXIS. The analysis resulted stresses and displacements before shotcrete cover (3 days), and long-term dis-placements and factors of safety for the shotcrete-supported tunnel. The estimated displacements for the tunnel arch were small, if the temperatures were at or below -15oC. The freezing was started in May 2002, and completed in September 2002. The tunnelling was started in September, and it was anticipated to be open in November 2002. The tunnelling proved to be technically and economically successful, and was carried out in the planned schedule.