Construction of a merging section of Kariba Tunnel #3 without traffic suspension

Abstract

The disused section of #3 Kariba Tunnel, in Hokkaido Island, Japan, runs through a coastal area prone to rockfall and collapse. A new tunnel section was built to detour around that dangerous area of cliffs and merge with part of the old tunnel. Because there are no alternative routes in the environs of the tunnel, construction on a section where the new tunnel merges with the existing tunnel was done while the road was kept open to traffic. Reconstruction of the existing tunnel accounts for one-third of the merging section length. In that one-third, one lane was kept open to traffic during construction, which was performed concurrently with the construction of the new tunnel section. It was attempted to reduce the construction duration by completing the merging section before the excavation of the new tunnel section reached the merging section. Construction of the old section employed two methods: use of towed longitudinal/transverse movable protectors and use of concrete lining of high-strength thin precast RC plate. The methods and technologies introduced in this paper were developed as among the best options for maintaining traffic safely while widening an existing tunnel with a comparatively small cross-section and shortening the construction period. They were first used for the #3 Kariba Tunnel and have been effectively applied to other tunnel design and construction projects that have conditions similar to those of #3 Kariba Tunnel. Detailed improvement of the methods will facilitate various uses of the technology. The following are proposals to improve the methods and technologies to facilitate higher operational efficiency based on the experience during construction. Protector: (1) For swifter movement and installation, it is necessary either to install a driving mechanism or to lighten the structure to facilitate manual movement. (2) To meet variations in tunnel width, a width-adjusting capability needs to be added. Precast lining: (1) Lightweight plates of a different shape need to be developed to facilitate swift erection and application to a larger cross-section of tunnel. These methods are expected to be improved to facilitate automated tunnel excavation and high-speed operation, which also offer an improved working environment. To this end, it will be necessary to make these improvements to the protector and precast secondary lining, and to coordinate their operation using a tunnel workstation (TWS) that also has the ability to protect passing vehicles. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.