Short and sweet: new Banora to border highway is a beauty

June Cummings reports

The 2.5km new highway alignment sweeps north from the Barney’s Point Bridge Tweed River crossing, across a fill embankment, over a viaduct in the central valley, pushing through a large cutting in Sexton Hill, a land bridge underpass and causeway before linking up with the Tweed Heads cross-border motorway.
The existing highway and local roads will be upgraded. With the existing highway becoming a local road, and with new and upgraded interchanges at the northern and southern ends, two light signal intersections on upgraded local roads, traffic under/overpasses and upgraded pedestrian/cyclist routes, a much safer interconnected local community road traffic network will result.
The $359m project is part of the Ballina to the border section of the overall $3.6bn NSW and federal government funded 5-year NSW Pacific Highway Upgrade Program due for completion by 2014.
While short in distance, the Banora Point project is in a highly trafficked narrow urban residential and commercial corridor with terrain ranging from basalt rock, agglomerate, Cudgen clays, indurated sands, soft soil swamps.
Due to its complexity and to meet time and cost constraints, RTA formed an alliance with Abigroup Contractors, Seymour Whyte Constructions and SMEC to design and build the upgrade. Much refining of the original plan took place before all stakeholder needs were met in the final planning solution.
Not the least was the RTA’s strategies for scenic highways that minimize impact on the environment and communities, to have rest stops, a consistency of approach for interchanges and exit roads that encourage regional development on the east coast. Highways are designed to encourage travelers to make side trips, explore and then rejoin the highway, the RTA says.
The Banora Point upgrade more than lives up to RTA strategies. The cut through Sexton Hill offers spectacular ‘gateway’ views north to the Gold Coast skyline and south to the Pacific Ocean and Tweed River. A Banora Point green corridor is preserved with a vegetated land bridge linking existing parklands either side of the new highway and pedestrian/cyclist pathways, by over and underpasses. The new northern interchange connectivity with Tweed Heads shopping district offers another urban design experience.
Alliance manager Steve Lambert of Abigroup, says with gradients in the new highway half that of the old highway, total fill on the project amounts to 340,000m³ made up of cut and fill, 200,000m³, import 90,000m³ and blast rock 35,000m³. Most rock has been sourced from the large cut through Sexton Hill, which is right up against the existing highway in hard basalt lenses at 300Mpa and vertically cut 18m at its deepest point. Blasted rock is used for select materials and aggregates. With no storage on site, most recovered material will be placed.
The project’s infrastructure includes seven bridges - all piled, a cut and cover section 75m long with cantilevered piled walls with anchors, a twin 330m viaduct bridge on 15m high twin blade columns, major retaining structures against the existing highway with some 9m to 10m high constructed near live traffic, two trafficked signal intersections on local roads, interchanges north and south, embankments and major services relocations.
Work is being carried out in four stages – 1/ offline work, 2/ preparing southbound alignment, 3/ preparing northbound alignment, 4/ local roads upgrades and tie-ins with each stage taking about 8 months to complete. The new highway should open to traffic mid 2012 with local roads upgrades completed December 2012.
Construction
Bridge construction consists of super T bridges and some plank bridges.
The large northern interchange with a 60m diameter roundabout connecting the highway to upgraded local roads, is largely on ground with reinforced soil walls predominantly making up the ramp structures, that lead onto or off the new highway alignment.
Embankment is predominantly reinforced soil block walls up to 10m high against the adjacent highway on the main carriageway.
A new 15m long cut and cover underpass in the northern area is reinforced soil walls with reinforced concrete plank and girders of 18m spans over the top. It links east to west Banora under the new Highway. An interconnecting ‘green’ landbridge has a vegetated 75m x 250m roof park.
Other major retaining structures are cantilevered piling walls with anchors and causeway type construction 10m above ground.
For piling in basalt areas, a 1500mm diameter cluster hammer is being used to lessen noise and vibration and speed up drilling in hard rock. Blasting is shallow for minimum noise and impact.
Soft soils
With the water table a metre below ground level in the northern part of the project, one quarter of the project is in swampy, sandy conditions mainly around Lake Kimberley. Large areas have to be dewatered to place underground services and drainage, using spears or well points, then reinjected.
Before construction, for environmental protection, sediment basins and protection were constructed to control discharge and surface water runoffs. Works close to residents also include vibration and noise limiting devices.
Finishes
Concrete noise walls varying in size – reinforced soil walls with concrete panels - have a dark rough textured finish. The cut through Sexton Hill has architectural linings running through it with a ‘gateway’ theme expressing the NSW/Queensland border crossing.
Native species vegetation and turf have been used throughout, with the vegetated land bridge adding to the ‘green’ theme. Pavement is a flexible asphalt composite pavement – stone mastic asphalt - with a concrete lean mix sub base. Architectural lighting has been used on some bridge elements and cut and cover sections.
Challenges and innovations
Alliance manager Steve Lambert says with 50,000 cars a day passing through the construction site, a big focus of the alliance was on traffic management – planning work in advance and the safety issues involved.
Working in a brownfields environment meant houses in close proximity, traffic, noise and vibration, big structures against existing roads and night work issues.
But, delivering on the expectations of the community, refining the concept design to meet budget and time constraints as well as functionality, getting the buy-in of all the stakeholders is what drove value for money innovations, he says.
Design innovations have successfully connected east and west Banora Point, separated local roads from the highway, staged construction to minimize disruption to traffic and integrated art with engineering.