RISBANE is big on tunnels, but the first cab off the rank is probably the most complex and unheralded – the Inner Northern Busway (INB) Stage 1 package of works.
It is a $333 million, 1.2 km dedicated busway incorporating a 500m tunnel running beneath the very heart of the Brisbane CBD and will provide the vital link to the city’s northern and southern busways in the Translink public transport system.
The expensive piece of infrastructure includes a new King George Square underground bus station and another on the surface at Roma Street. In a first for an Australian capital city, a CBD bicycle centre with showers, lockers, change rooms and a bicycle lock-up. Post construction, affected city streets will be tree-lined and landscaped and the city’s King George Square given a re-design facelift.
The project is funded by the state government with Brisbane City Council contributing 400 spaces from its city centre underground car park.
The busway link was stalled for some time because the former council refused to relinquish the revenue earning car park land.
The INB Alliance partners for the project are Queensland Transport, Leighton Contractors, Maunsell, Coffey, Bligh Voller Nield and Edaw Gillespies.
The tunnel is approximately 500m portal to portal and starts underground from the Queen Street Bus Station running in a westerly direction under King George Square and daylighting at the Roma Street parklands.
Rectangular envelope
It is not a bored tunnel, but typically cut and cover construction with bored piled walls, excavated out to roof level, and a cast insitu roof, to form a rectangular envelope to a maximum depth of 13m to 14m to its base.
It is in three sections with the first a 150m cut and cover tunnel from the existing Queen Street Bus Station to the new King George Square station of approximately 140m, which requires the partial demolition of the car park, then a top down construction tunnel across Ann Street followed by open cut technique where it emerges at the Roma Street forum.
Work started in May with concrete saw cutting into the pavement of King George Square to prepare an access shaft to the basement of the car park and the modification of its traffic circulation.
The car park has split levels either side and level F and portions of level D will be demolished for the bus station with its platform levels on level H. The shell of the tunnel will be formed with steel girders and beams.
The elevation of the tunnel is governed by the location of the King George Square Station on the level H car park, so there is a 10 per cent grade from the Queen Street Station to the new station, which runs horizontally for its platforms lengths. It then rises upwards to a point where it can be piled out for open cut. The tunnel roof follows the base of excavation.
With cut and cover, piles are installed from the surface continuously to effectively form a solid wall of bored piles, which is then excavated out to the underside of the roof.
The roof is a combination of 600 mm precast reinforced concrete planks placed in position on top of a concrete topping slab 200 mm thick, and backfilled with general fill material. Other roof construction is cast insitu concrete approximately 1m thick.
Excavation equipment is lowered through access points in the roof and if necessary, excavation for the tunnel is stabilised using rock bolting and extra piles. Excavated fill is fed out through the access holes.
Location program
Where there is better access at the final elevation at the Roma Street forum portal, the tunnel will be cast insitu with 30m to 40m of piling and open cut technique. The tunnel is generally two lanes wide – 12m, broadening to two lanes either way plus 6m platforms at bus stations, 32m wide.
An extensive location program of Brisbane’s CBD services – water mains, deep sewers, electricity and telecommunication cabling, medium and high pressure gas mains – is a major part of preparatory work. The walls of the tunnel will be mostly lined with reflective acoustic panelling currently used in the busways, which aids lighting levels and maintenance.
The portals are part of the ventilation system as well as a new outlet point at Turbot Street 10m above existing ground level and the existing ventilation point for the King George Square car park. The air conditioned station platforms have a different venting system to the tunnel.
Low impact construction techniques are being used with conventional bored piling rigs, excavators and small dozer to transport soil to trucks. The tunnel section of the INB Stage 1 is due for completion in early 2008 with the final link section to the existing INB, late 2008.
Source: Construction Contractor