Railway tunneling 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 crossed the railway line.The railway level was about 20 m below the surface of the ground.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.Time- and temperature-dependent mechanical parameters were determined for the actual soils and design temperatures.The analysis was carried out for 12 different phases during tunneling.The estimated displacements for the tunnel arch were negligible, if the temperatures were at or below -15 °C.The freezing was started in May 2002, and completed in September 2002.Tunneling was started in September, and it was completed in November 2002, following with the casting of the final liner. The application of ground freezing in deep excavations and tunnelling in soft soils is not a new idea.It has been widely discussed in ground freezing symposia since the 1970s.There are certain reasons why ground freezing has proved to be recommendable: the applications do not cause any environmental impacts on the site, freezing can be rationally planned, designed and controlled, and it uses the ground itself as a structural element.Ground freezing causes, of course, certain permanent changes to the freezing soils after thaw.Frost action causes ice segregation in the frost-susceptible soils, and results in consolidation settlements after thaw.These can be pre-estimated and evaluated in relation to neighbouring structures, if needed. As a geotechnical procedure, ground freezing has been economically used as an alternative strengthening method, or as the only possible method at many sites.The frozen soil has dramatically higher strength and stiffness than the unfrozen soil.The mechanical behavior is highly dependent on the temperature below freezing point.In frozen soils, the strength and deformations are also time dependent, which means that they have clearly plastic properties that become more and more dominating when frozen temperature rises.Experience, anyhow, has showed that these characteristics allow in normal cases the engineering application of frozen ground as a temporary support in geotechnical engineering, if the specific properties and conditions are properly verified and controlled for the site.The displacements must be in given limits, and the overall safety must be high enough to maintain the continuous construction procedure until the final support has been completed. The Stranneberg tunnel was located in Sweden, at the western coast of the Gulf of Bothnia, five kilometres along the Bothniabana railway line from Örnsköldsvik towards Umeå (Figures 1 and 2).Construction planning for the freezing started in November 2001.Freezing took place in summer 2002 and excavation in the autumn of the same year.The tunnel (100 meters long, 10 meters high and 10 meters wide) required 40,000 cubic metres of soil and weak rock mass to be frozen.Freezing work on the tunnel started in early 2002 with the installation of freezing columns, and the tunnel broke through the frozen section in December 2002.VTT developed and carried out the coupled thermal and mechanical analysis and design of the tunnel, including laboratory determination of design parametres, and monitoring temperatures and displacements during the operation.Lemcon Ltd., the contractor, carried out the ground freezing, excavation, insulation, preliminary shotcreting, and cast the final liner.