Alpurt B2 Northern Motorway Realignment, New Zealand

Over the last ten years New Zealand has been spending more and more public and private money on the upgrade and extension of its motorway system. One of the largest engineering projects ever undertaken in New Zealand is the 7.5km SH1 Northern Motorway Extension (also known as the ALPURT B2). The new four-lane motorway will be the country's first state highway that charges a toll for use.

This project represents the last stage of the Northern Motorway project between Albany and Puhoi. The current section (7.5km) under construction extends north from Grand Drive in Orewa to connect with the existing State Highway 1 route at Titford's Bridge, Puhoi.

The new motorway will also allow state highway traffic to bypass the town of Orewa, easing traffic congestion, and will provide a safer route between Auckland and Northland.

The motorway is being constructed for the government agency Transit New Zealand. The project is being undertaken under a public private partnership (PPP) arrangement and has an estimated cost of $360m.

CONSTRUCTION

The planning, design, construction and project management of the motorway and associated structures is being carried out by the Northern Gateway Alliance, which includes: Transit New Zealand, Fulton Hogan, Leighton Contractors, URS New Zealand (engineering and project management), VSL International (bridge engineering), Tonkin & Taylor (engineering geology and pavement design) and Boffa Miskell (environmental).

Other contractors include Halcrow for design services on tunnels, SIKA NZ and Allied Concrete Ltd (tunnel concrete) and Allianz Global Corporate & Specialty – Pacific for insurance of the project.

Construction began in December 2004 following the release of an initial $20m grant by Land Transport New Zealand. The motorway is expected to be completed by mid-2009 and will involve 130,000m² of pavement.

The motorway project is also involving the construction of five culverts, twin tunnels at Johnstone's Hill, and a series of bridges and viaducts including three large bridges at Waiwera, Nukumea and Otanerua. The project is being carried out over rough terrain and it is anticipated that four seasons of earthwork will be required involving the removal and dispersion of 3 million cubic metres of earth.

PROJECTS

The structures along the motorway route will include a full diamond interchange at Orewa (68m long, 10.25m wide and four spans) incorporating a bridge (one lane in each direction and a footpath). There will also be two eco-viaducts at Nukumea and Otanerua, providing ecological corridors beneath the structure to allow movement of wildlife.

The Nukumea Eco-viaduct (180m long, 23m wide and six 30m spans) will cross the northern arm of the Nukumea Stream and provide an ecological corridor under the motorway for wildlife. Similarly, the Otanerua Viaduct (256m long, 25m wide with a clearance of 32m over the stream and eight spans of 32m) will span across the northern tributary of the Otanerua Stream and also be an eco-viaduct.

The largest bridge in the project will be the twin bridge Waiwera Viaduct (537m long with span lengths of 76m).

The Hillcrest Bridge will be an overbridge providing access to the west of the motorway from Hillcrest Road in Orewa. Finally, there will be the Johnstone's Hill Tunnels (345m long and 12m wide).

WAIWERA VIADUCT

The 537m-long Waiwera Viaduct is being constructed as two independent structures, which will have a separation varying from 2.5m to 11m. The variable separation is necessary to split the northbound and southbound lanes before entering the tunnels which are situated immediately north of the viaduct.

The bridge superstructure is the tried and tested pre-stressed concrete box girder and is being constructed using a precast segmental balanced cantilever method using an overhead gantry. This method was chosen since it allows the superstructure to be constructed without access from ground level; the ground is off-limits in case habitats are disturbed or damaged.

The piers are being constructed from reinforced concrete, incorporating a microsilica additive for protection from the marine environment. The viaduct has a design life of 100 years.

JOHNSTONE'S HILL TUNNELS

The Johnstone's tunnels are two parallel tunnels 15m apart; they were the preferred option over a deep cut through the hills as they will cause less environmental damage.

Each of the tunnels is 345m long and 12m wide with a height of 9m and there are two cross tunnels to provide escape routes if either tunnel has accident involvement.

The tunnel systems will include: lane/traffic control signs, fire control systems, ventilation fans in tunnel crown, two emergency walkways, a 10m × 6m oversize traffic envelope full concrete lining, two 3.5m traffic lanes and one 2.5m hard shoulder. Initially, each tunnel will only have a single lane until additional road construction north of the project has been completed.

Construction has been undertaken using a 50t road header to excavate through inter-bedded sandstone and siltstone (30MPa hardness). The initial tunnel support uses a system of sprayed concrete with steel fibres embedded provided by Sika NZ and includes the Sika ViscoCrete and SikaTard systems to allow the concrete to be sprayed without setting too quickly.

In the tunnel construction the northern portals have been cut and covered, making it easier to restore the natural contours of the hill after construction is complete. The southern portals are located in an environmentally sensitive area, so the construction team has planned to dig both tunnels from the north and just do the minimum of work from the south. This will reduce the work station needed on the southern slope and also minimise potential damage.