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Creation of the Victoria Park Tunnel in Auckland is the final part of a plan to help a monster volume of traffic flow more smoothly between Newmarket and the Auckland Harbour Bridge. Not surprisingly, this “significant seven” roading project presents complex challenges.
“It is big and complex and it’s being built within a confined space in a very busy part of Auckland,” says the agency’s state-highway manager for Auckland and Northland,Tommy Parker The project addresses the last major bottleneck on the motorway network between Newmarket and the Auckland Harbour Bridge and will unlock the capacity of the recently completed Central Motorway Junction. The 2.4km-long project involves construction of a 450-metre cut-and-cover tunnel adjacent to the existing viaduct across Victoria Park to carry three northbound lanes. The viaduct will be reconfigured to carry four southbound lanes. Through St Mary’s Bay the motorway is being widened to provide five lanes in each direction plus a city-bound bus shoulder lane.
Urban design initiatives are included in the project, to ensure the Victoria Park and St Mary’s Bay areas are improved as places to live, work and play. These include a new public plaza around the historic Birdcage Tavern and see-through noise walls through St Mary’s Bay to preserve the iconic view of the St Mary’s Bay cliffs and pohutukawa and to improve the amenity of the St Mary’s Reserve that adjoins the motorway. Responsible for the project’s delivery is the Victoria Park Alliance comprising the NZ Transport Agency, Fletcher Construction, Beca Infrastructure and Higgins Contractors, with Parsons Brinckerhoff providing specialist tunnel mechanical-and-electrical design. The peak workforce is expected to be 450, though 1400 people have received a health-and-safety induction to allow them to do work on the site. Contributing to the project’s complexity is its proximity to major services and key traffic corridors. “There’s nothing linear about the construction programme. Rather, it’s multi-levelled with many things needed to be tidied away before we can proceed with the next activity,” Tommy Parker says.
Adding to this complexity is the project’s prominent location, close to established business and residential communities and passed every day by about 200,000 vehicles travelling to and from the Auckland Harbour Bridge. “A priority for the project is to manage the construction impacts on these key stakeholder groups.” While the project is due to be finished in mid-2012, it is planned to have the tunnel structure completed by the time of the 2011 Rugby World Cup. The alliance’s focus, immediately following last October’s start, was design and enabling works. In particular, a proliferation of services including local roads needed to be diverted from the tunnel site before any construction could begin. Alliance project manager John Burden says these services included the two watermains that supply the North Shore, the vital international communication cable which Auckland and New Zealand depend upon for communication with the outside world, the historic brick-lined Freeman’s Bay stormwater culvert (see page 38), and various local electricity, telecommunication, gas, and water service lines. Overhead gantries and median barriers on the motorway itself also had to be removed.
Attracting great public interest is the proposed relocation of the 1886-built Birdcage Tavern (originally called the Rob Roy Hotel) which sits on the site of the tunnel’s southern portal. It is being strengthened and “stiffened”, ready to be moved on beams 40 metres up Franklin Road in August-September. Once the southern portal is complete, the hotel will be moved back to sit on top of the portal as the focal point in a new public plaza. Construction of the cut-and-cover tunnel itself began in April this year. John Burdon says that under this method construction can be underway in several places at once. Also, it better suits the relatively shallow tunnel route. The tunnel width, measured between retaining wall faces, varies between 16 and 18 metres. The base of the tunnel is up to 12 metres below existing ground level.
This base slab will include up to 9500 cu metres of concrete. There are tension piles under the base slab which are 750mm in diameter and socketed into underlying sandstone. Props and inclined ground anchors/struts will also support the tunnel retaining walls. The roof consists of precast, pre-stressed, double hollow core beams (each 1150mm wide), with a 200mm thick cast in-situ concrete topping. Textured/architectural precast panels 150mm x 200mm) will also be used for the lower four metres to provide the desired aesthetic finish to the tunnel. The two different forms of construction are being used for the tunnel retaining walls (diaphragm and secant pile walls) to suit construction preferences, which are being driven primarily by the varied sandstone profile along the length of the tunnel. The reinforced concrete diaphragm walls are being used where the sandstone is relatively deep (southern end) and the secant pile walls used where the sandstone is shallower and trenching for diaphragm walls is considered less preferable (northern half). In addition the semi-contiguous bored pile walls are being utilised for the retaining walls in the approaches to the tunnel.
The tension piles are then drilled along the centre of the tunnel alignment. These will be tied into the base slab to hold the completed tunnel structure down against upward groundwater pressures. The diaphragm walls are excavated through bentonite, a clay slurry that is thixotrophic, meaning it can be pumped like water but rapidly sets to a gel to support the excavation when not agitated. Next, reinforcing steel cages are lowered into the excavated wall panel through the bentonite. Concrete is then pumped in from the bottom using a “tremmi tube” to displace the bentonite which is pumped back to the bentonite plant, cleaned and re-used. Capping beams are installed on the pile or panel walls to tie them together and to support the precast concrete roof slabs. Where the tunnel cuts through major local roads, roof slabs will be placed before the tunnel is excavated. Traffic, which has been diverted away from the construction, will then be diverted back to run on the roof so that tunnel construction can get underway beside the road.
The base slab is installed and tied into the tension piles anchored into bedrock up to 10 metres below the tunnel’s road surface. Wall props and the remaining roof slabs follow along with services such as fire protection, emergency exits, lighting, ventilation and traffic management. The final stage in the tunnel construction is reinstatement of the surface above. When completed, the Victoria Park Tunnel project is expected to deliver significant growth benefits to the region by improving access to markets and giving freight operators more reliable access across the Waitemata Harbour through to Auckland’s port and central business district. The Auckland Harbour Bridge is the primary freight route through the region, and the 8300 freight movements a day account for about five percent of the total traffic that uses the bridge. Most of this traffic continues across Victoria Park and through the Central Motorway Junction.
The agency brief also specifies maximum use of the local supply chain to stimulate and enhance growth, and a procurement plan that enhances the capability of New Zealand’s construction and construction-supply industries and incorporates a focus on the development of small and medium enterprises (SMEs). At the end of March, 31 of the 73 companies so far awarded work and supply packages were SMEs. Subcontractors include: Ward Demolition – demolition and other general works around site, ATF – fencing, Ross Roadmarkers – temporary linemarking, Hi Lift – crane hire, Auckland Cranes – crane hire, Pro-Drill (Auck) Ltd – geotech site investigations, Adams Landscapes – MSE walls, Southern Concrete Pumps – concrete pumping, City Contractors – watermain diversion, Pipeline & Civil – North Shore watermain, and United Group NZ – temporary street lighting. Major suppliers include: HEB Precast – DHC beams, Firth Industries – concrete supply, Stevensons – aggregate supply, Concrete Structures – retaining walls, Fletcher Reo – reinforcing supply, Steel and Tube – reinforcing supply, and Steelpipe NZ – pile casings. Historic stormwater culvert diverted
The pipeline was diverted through a concrete box culvert of up to 4.1 metres wide and 2.6 metres deep, installed at invert levels of almost six metres below ground level. A total of 109 units, each 1.55 metres long, were placed and had to traverse some tricky ground, starting at Victoria Park, crossing the very busy Victoria Street, then entering Franklin Road and passing by the historic Birdcage Tavern. For shoring, three modules of DGVP double-slide rail system were used with two 2.4 metre base panels and one 1.3 metre extension panel on each side to achieve the full depth. The system features double slide rails connected by a heavy-duty frame that slides up and down within the rails to give very large clearances. Corner slide rails enable a four-sided pit configuration. The system was supplied by Trench Shoring NZ through Humes Pipeline Systems and was installed by Brian Perry Civil, with initial guidance from Trench Shoring.
The box sections of the diverted culvert were laid at the rate of up to four a day. General bedding was placed and compacted in the excavation, followed by scoria fines bedding material which was spread and screeded by a small excavator lowered into the trench.
Contractor Vol.34 No.6 July 2010 |
Victoria Park Tunnel is one of seven roads of national significance identified by the Government as essential to support economic growth in New Zealand, and the first to be constructed. It is also one of the most difficult state-highway projects undertaken in the Auckland region, according to the NZ Transport Agency.
Total cost of the scheme, including consenting and land purchases, is $406 million. The actual construction cost is about $320 million.
“Also, every element of construction is inter-related with traffic management and services – nothing happens in isolation.”
Later in the project the Orakei sewer needs to be diverted to make way for the tunnel. This sewer carries 40 percent of Auckland’s wastewater and cannot be shut down.
Under the cut-and-cover method, the tunnel is being built from the top down in stages to a depth of about 16 metres using a range of piling techniques. The design consists of fully integral, reinforced concrete diaphragm walls (800mm thick and up to 8m x 15m panels), secant piles (900mm in diameter at 1.35m centres), bored contiguous piles (750mm, 900mm, 1050mm in diameter) and an 800mm thick base slab.
For diaphragm walls, 1.2-metre deep concrete walls are first installed on the line of the tunnel walls to guide an eight-tonne grab and keep it vertical as it excavates the wall panel. For secant piles the guide walls are simpler concrete structures with polystyrene “holes” that keep the pile vertical.
Excavation between the tunnel walls follows construction of the walls and will generally be top-down. Where the roof slabs have been placed, excavation will be from the side.
The project is also expected to deliver significant economic benefits to the region during construction. The Transport Agency’s brief to the alliance specifies an early start and early delivery, which results in a peak monthly spend nearly double that of any previous roading project – up to $30 million a month at one point.
Among the many services diverted from the Victoria Park Tunnel site before construction could begin was the historic brick-lined Freeman’s Bay stormwater culvert.
Brian Perry used a 45-tonne digger for the excavation and installation. A large crane also placed some of the DGVP components as well as the box sections. A small excavator was used for backfill and extraction of the DGVP system, which Trench Shoring general manager Frank Swanberg says was being used for just the fourth time in New Zealand.