Advanced Ground Freezing at the Hallandsås Project, Sweden

An extensive fault zone within the Hallandsås Project Sweden, a railway tunnel project in southwest of Sweden, is subject of an advanced ground freezing operation in order to facilitate TBM excavation through this section. The rock mass within the fault zone consists of a completely decomposed gneiss and amphibolite in combination with 10 to 12 bar ground water pressure. Ground freezing was carried out in 16 horizontally drilled freeze holes with a length of 100 m. Within the 10.6 m diameter final tunnel section a small 5 m diameter pilot tunnel was excavated through the frozen area. The design and execution of the freezing and excavation operations have followed the observational method. This included prediction, monitoring and follow up of all parameters associated with design of frozen ground, such as strength and creep properties, as well as design aspects on the final segmental lining installed by the TBM. By thorough project management and active design it is possible to control the involved processes and achieve a transparent methodology in predicting and executing tunnelling using the freezing method.