There are 7 locations where the rock cover is significantly less than the tunnel spans on the CLEM7 Tunnel (formerly known as the North South Bypass Tunnel) in Brisbane, Australia. The twin highway tunnels are each approximately 5 km long with wide on and off ramps at both ends and at mid-point of the tunnels, two wide underground intersection chambers and several service and utility tunnels. A total of 8,550 m of the tunnels were excavated by two 12.4 m diameter double shield TBMs and the remaining 3,400 m of the main tunnels and the ramps and utility tunnels were excavated by 8 road headers. Where the traffic converges from 3 lanes to 2 lanes at both ends of the tunnels, the rock cover varies from about from 0 m to 6 m while the tunnel spans are from 17 m to 13 m. The intersection chambers near the mid-point of the main tunnels are up to 26.5 m wide and at one location have access ramp tunnels passing over them with clearance of less than 3 m locally. Also in this same area, there are cross passages between the main spans where there is less than 15 m of low quality rock cover. Where the main tunnels pass under the Brisbane River, the rock cover is less than 10 m where there is approximately 55 m of head. The rock mass characterization, analyses and rock support designs were initiated and advanced during the tender design stage. Upon award of the design-build project, the analyses and rock support classes were finalized for these particularly critical areas along with the balance of the tunnels. This paper describes the analyses, designs, support and construction methods that were used to construct the tunnels in the different ground conditions where the rock cover is low compared to the wide spans of the tunnels.
The North-South Bypass Tunnel recently opened in Brisbane, Australia and is now known as the CLEM7 Tunnel. It extends from the northern suburb of Bowen Hills under the Brisbane River to Woolloongabba in the south. The project was designed and constructed by the Leighton Contractors and Baulderstone Hornibrook Bilfinger Berger Joint Venture (LBBJV) as a subcontractor to the River City Motorway consortium that was awarded the concession to design, build, operate and maintain the tunnel. Overall, the underground works of the project consist of 4.8 km of twin, two-lane tunnels excavated with 12.4 m diameter double shield TBMs; 2 km of two and three lane tunnels excavated with large road headers at the north and south ends of the tunnels; in aggregate, 370 m of cross passages (41 x 9 m), four substations (consisting of four individual cross passage excavation for each), two ventilation tunnels (approximately 540 m in total length) and on and off ramps with associated underground merges at Shafston Avenue south of the river totaling approximately 1350 m in length. The overall layout of the tunnel is shown on Figure 1.
