Planning and construction of an underground station with a large arch cross section

The MM21 Line in the south-west of Yokohama City plays an important part in the city's New City Center Comprehensive Development Plan and it is planned to construct the central station of this line, Minato Mirai Central Station (provisional name) in an area at the heart of a network of commercial, business and cultural facilities. This central station is a standard, underground, three-level, three-path and RC structure of total length 250 In with total excavation volume 145,000 m(3). The 70 m length of this is to be made into an "urban gallery" space, a large arch cross section unusual in conventional underground stations. The arch is designed to replace two levels of frame structure and it has a width of 23.0 m, a height of 9.4 m and an inner volume of approximately 10,000 m(3). As for the geological conditions, the top soil consists of soft alluvial clayey sediment, and soft mudstone with the N value of 50 or more takes over from a depth of GL-15 m and below. In view of the need to be able to easily cut off water because of a high groundwater level of GL-3 m, the cheapest and safest column underground continuous wall (SMW) construction method has been adopted. By replacing underground beams with the ground anchor construction method for a 120 m section including the arch, the construction of the arch portion has been made safer than ever. One of the most difficult technology is the section where the structure temporarily supports a common-use conduit (2,300 tons) containing the utilities and telephone lines of existing buildings and hotels in the surrounding area. This paper introduces the details of the planning and construction of this large-scale underground station.