The Norwegian National Rail Administration started in autumn 2011 the building of a new double-track railway tunnel through Holmestrand, located to the southwest of the capitol Oslo. The tunnel includes a 260 m long station hall with a span of 34.5 m. At both ends of the station, the 34.5 m span is reduced to a normal double-railed tunnel of 14.5 m, resulting in a 34.5–30.5 m spanned cavern with a total length of about 600 m. Upon completion in 2016, the underground railway station will be one of the largest in the world. The station hall is to be constructed in basalt lavas that contain three sub-vertical joints sets and one sub-horizontal joint set. To the south of the railway station, a 30 m wide tunnel section will be excavated through a major fault. The central part of the fault is expected to influence the stability and excavation methods over an axial length of up to 100 m. Stress measurements indicate only gravitational stresses in this area. This paper addresses the main challenges in the project, the design of rock support in the large spanned cavern and in the 30.5 m cavern through the fault. Numerical modeling using 3DEC has been carried out in order to investigate the modes of failure, and modeling using Phase2 has been used to assess the expected necessary rock support. Design of excavation method through the major fault is also described.
This section will include a 12 km railway tunnel, which will include an underground railway station in the town of Holmestrand. The area above the railway station is a residential area, called the "Holmestrand plateau" (Fig. 2). Excavation and grouting thus must be carried out under strict requirements.