ABSTRACT
In recent years, large investments have been made to improve and expand the public transport capacity in the city of Stockholm, such as the extension of the blue metro line. One of the most technically challenging parts in this project was the excavation through the fault zone in Mårtensdal, where two parallel tunnels were excavated with low rock cover. This fault zone is overlayed by a shallow layer of basal moraine and more than 30 m of permeable glacial deposits. In addition, the area hosts a pile-driven bridge foundation. The uncertainties associated with the critical site conditions resulted in the decision of temporarily freezing the rock and moraine before tunnelling through the fault zone. This paper presents the utilized methodology to design the temporary rock support using the freezing method, and the back-analysis of the rock mass properties based on observations made during the excavation phase.
