Bottlenecks around Adelaide’s O-Bahn Busway are set to dissipate with infrastructure works designed to ease congestion forging ahead following recent investment
Bottlenecks around Adelaide’s O-Bahn Busway are set to dissipate with infrastructure works designed to ease congestion forging ahead, following a major investment
A unique element of Adelaide’s public transport mix is its O-Bahn busway network with kerb-guided steering, allowing operation in a tight working envelope.
The dedicated busway allows buses to operate without the disruption and variability of travel on public roads.
The only problem with this system is that when the buses exit the dedicated network, they travel on congested public roads to reach and exit the CBD, creating bottlenecks during peak hours.
In an effort to address this, the South Australian Government initiated the $160 million O-Bahn City Access Project to extend the O-Bahn service, reducing traf c congestion and delays – particularly on the Inner Ring Route.
This extension, currently being constructed by McConnell Dowell (MCD), is scheduled for completion late this year and will create around 450 jobs over the life of the project. Signicant elements of the project include the provision of central inbound and outbound bus priority lanes on Hackney Road between Gilberton and the northern tunnel portal.
This requires the creation of a 650m tunnel from Hackney Road/Dequetteville Terrace under Rundle Park, Rundle Street and Rymill Park, emerging in Rymill Park to align with bus trafc on Grenfell Street.
The existing bridge over the Torrens River on Hackney Road will be widened, and construction of a separate shared-use bridge over the Torrens River is underway.
The guidance feature of the O-Bahn system actually reduces the cost of a tunnel compared to one that would be built to accommodate conventional buses. Because the buses travel along a track, the risk of colliding with the wall or an oncoming bus is eliminated.
This means that a narrower tunnel can be built, requiring only 800mm of separation between the opposing carriageways and 1000mm of clearance from the tunnel walls. No widening is required for travel around a curve.
However, the radius of curvature needs to take into account the line of sight for buses travelling in the same direction to provide adequate time for braking to avoid a rearend collision.
Given the location of the works in a long-established area close to the CBD, service location and relocation to provide a clear path for tunnel works to proceed was a signicant challenge. Services to be relocated and protected include water, sewer and gas mains; stormwater systems; and underground and overhead highvoltage cables.
The age of the area led to many unanticipated issues such as old tram footings beneath the road and redundant stormwater pits from a time when the road level was signicantly lower. While there were records of almost all of the utility services still in use, including sewer and water pipes that were more than 130 years old, the location of many of the services had to be re-surveyed.
A particular challenge was the existence of 66kV cables in Rundle Park. These cables were temporarily de-energised and a section of the tunnel was constructed while the cables were suspended above the excavation on a temporary bridge.
The bus tunnel is being completed using a mix of cut and cover in the parks and top-down construction in the road alignment.
The cut-and-cover method is ideal as it minimises the work area and the disruption to trafc. Road possession is required to install sheet piles for the side walls of the tunnel and roof beams over the top of the sheet piles.
Once the road is reinstated, excavation can take place between
the sheet piles beneath live trafc, and the tunnel oor and permanent walls can then be installed.
The sheet piling, roof beam installation and road reinstatement works for the Hackney Road/ Botanic Road intersection works took place as a series of night works in September-October. Cut-andcover works in Rundle Park and Rymill Park reached the 60 per cent completion milestone in November 2016 and are scheduled for structural completion in mid-2017.
The tunnel is predominantly a reinforced concrete structure, with the oor slab and most of the walls poured in situ. Other than where the tunnel crosses under the 66kV cables, the tunnel roof comprises pre-stressed concrete planks cast in Pooraka and transported to site.
Groundwater has been encountered within Rymill Park, Dequetteville Terrace and Hackney Road, but once construction is completed, the tunnel will be sealed to prevent groundwater ingress.
Because of heavy existing trafc levels, trafc management is a critical issue on this project to minimise disruption and allow safe entry and exit of construction vehicles, as well as provide a safe environment for workers.
A key step was to ban all right turns (except for buses and emergency vehicles turning right from Hackney Road to Botanic Road) at the intersection of Hackney Road, Botanic Road, Dequetteville Terrace and North Terrace.
This was heavily publicised in the media and through other channels prior to implementation. Congestion was successfully reduced by this measure.
These bans will remain in place until mid-2017.
Right turns have also been banned at other locations along Hackney Road, and some of these will remain in place, with U-turn facilities implemented to cover their loss.
A minimum of two lanes in each direction is being maintained along Hackney Road, Dequetteville Terrace, North Terrace and Botanic Road during peak periods (7am-10am and 3pm-7pm), and two inbound lanes and one outbound lane are being maintained along Rundle Road throughout construction of the tunnel.
A Trafc Liaison Group meets regularly to discuss and rene trafc plans. Members include the Department of Planning, Transport and Infrastructure (DPTI) bus operations, Trafc Management Centre (TMC), Torrens Transit, Adelaide City Council, Town of Walkerville, Norwood, Payneham and St Peters Council, Taxi Council, South Australian Road Transport Association (SARTA) and emergency services.
PROGRESS TO DATE
To date, East Terrace has been realigned and is open to traf c, parking has been removed from the western side of Hackney Road, central priority bus lanes are in operation on Hackney Road between Gilberton and the Botanic Road intersection (other than the outbound lane between the bridge over the Torrens River and the Gilberton portal), and a new southern parking area has been opened.
The beams for the pedestrian bridge over the Torrens River were placed in July 2016, with work on the exposed aggregate deck and balustrading completed late last year. The new shared-use path alongside the Botanic Garden is scheduled for opening in mid-2017.
Work on the civil aspects of the tunnel will continue through to mid-2017, and will be followed by tout, testing and commissioning for an opening in late this year.
Widening of the Hackney Road Bridge over the Torrens River will continue into early 2017. Its completion will allow the outbound bus priority lane to be completed
The guidance feature actually reduces the cost
through to the Gilberton portal of the existing O-Bahn.
Work on reinstatement of park lands is taking place progressively as the tunnel is completed and back lled. This includes the integration of a parcel of land freed up by the realignment of East Terrace, and the relocation of a fountain located in this precinct.
Works are undertaken as required to ensure that adequate access is maintained for major public events taking place in and around the park lands where construction is taking place.
Main picture: Adelaide’s O-Bahn has a unique design
Left: Bus transit is key in South Australia’s captial city
Above: You can’t get a better bus priority lane than this