Easing the squeeze

Wedged between two halves of Hutt City’s suburbs, State Highway 2 north of Wellington is a natural bottleneck.   HUGH DE LACY explains how it’s being uncorked.

One of the country’s biggest ever mechanically stabilised earthwall (MSE) projects is the key to freeing up traffic flows on the congested Dowse Drive to Petone section of the State Highway 2 arterial route out of Wellington through the Hutt Valley.

Designed by Beca Carter for Transit New Zealand, 12,000 square metres of MSEs involving 80,000 cubic metres of earthmoving are the centrepiece of the $65 million operation being carried out by a special project joint venture between Fletcher Construction and the Higgins Group.

The completion of the Ngauranga Gorge interchange in the early 1990s freed up the State Highway 1 traffic flows from the capital up the western side of the island, but flows due north up State Highway 2 to the sprawling suburbs of the Hutt Valley and the farmland of the Wairarapa have posed a separate set of problems.

Traffic flows have been controlled by three sets of lights along a 2.7 kilometre stretch of the state highway at the intersections with suburban access roads. Southbound traffic flows of 23,000 units and northbound flows of 20,000 units every 24 hours had reduced that stretch of road to an accident-prone bottleneck that in 2001 prompted mitigation planning to get under way.

What Transit came up with was a staged reconstruction of the congested strip of State Highway 2 beginning with a series of elevated interchanges between Melling and Petone, and referred to as D2P. The second stage, involving another

interchange at Melling, is being investigated as part of Transit’s 10-year State Highway forecast. Finally, further 

construction likely to be implemented in 2009-10 involves the remaining traffic signal-controlled intersection at Kennedy Good Bridge and the Haywards Hill exit onto State Highway 58.

The D2P project’s physical constraints – the narrowness of the stripo of land between the western escarpment, formed by the Wellington earthquake faultline, and the railway to the east – are compounded by environmental ones of noise and visual effects, as well as the closeness to the work-site of the Percy Scenic Reserve, a haven for rare and endangered plants.

A dozen private industrial buildings had to be bought up for demolition, and by the time the project was ready to begin early in 2007, it had had to gain more than 400 separate resource consents from the Hutt City Council and the Greater Wellington Regional Council.

Developing what had previously been a track parallel to State Highwat 2 into the new Pito-One service Road through the Korokoro industrial area required the demolition of a number of prominent buildings, including the landmark Horizon Paving premises. Earlier, more buildings on Hutt Road between Hume and Wakefield Streets had to be removed to make way for site preparations.

Work finally began on the first of the two areas of the project last August. The first comprises a full grade-separated interchange at the Dowse Drive intersection, with a roundabout sitting above the highway connecting to a series of on-ramps and off-ramps accessing the urban roads.

One of the connections is to a second overbridge to the east of the highway to take traffic, by way of a newly-constructed roundabout, through the Korokoro industrial area for distribution by way of a roundabout to the Lower Hutt CBD. This allows traffic from the western hill suburbs to cross the highway and access the Hutt without running the gauntlet of traffic lights.

Improving access to the Percy Scenic Reserve involved installing a new culvert over Percy’s Stream just south of the Dowse Drive intersection, then channelling the stream itself under State Highway 2 by way of a 250 metre length of 1.8 metre-diameter pipes.

To the south at the Korokoro intersection, another set of traffic lights is being replaced by the lowering of the state highway and construction of an overbridge which will connect Korokoro and its industrial area to Hutt Road.

An existing footbridge giving access across the highway to Petone station had to be upgraded by replacing the existing spans. Concurrent earthworks firstly created a new temporary car-park adjacent to the station for park-and-drive commuters, while the permanent one to follow will be bigger than the original.

Included in this is the new road-bridge providing access for commuters to the Petone station park-and-ride facility. While the southbound access to the carpark will remain, all other access will be by way of this new bridge across what will be known as Mackenzie Avenue.

In purely construction terms the extent of the MSE work – including no fewer than nine walls – on the off-ramps and on-ramps has set the project apart from other recent large-scale operations around the country, but to Fletcher-Higgins JV contract manager Andre van Heerden by far the greatest challenge has been traffic management.

“You’re working on a live motorway, you have to build the structures over the motorway, and you have to build a new line and level right through the 2.7 kilometres – and the biggest thing is you have to do is keep the traffic flowing at all times,” he told Contractor.  Accordingly a huge amount of planning went into creating deviations through and around the work-sites.

Even so State Highway 2 had to be closed twice earlier this year. The first time was for 12 hours in January when the existing spans were removed from the Petone Station footbridge, and the new spans set up on temporary piles to keep the footbridge open. The following month there was another closure late on a Saturday night while the new spans were lifted into position.

A major milestone was reached in February this year when 18 Super T beams making up the main span of the Dowse interchange bridge were hoisted into place over the railway line at Melling on a single Saturday. The work had been expected to take the whole weekend, but it was just one of those days when everything went right.

The favourable autumn construction weather ensured the Fletcher-Higgins JV made good progress on D2P to that point, and even the onset of a series of savage southerlies in late June does not seem to have threatened the completion of the project on time in early 2010. The Horokiwi Quarry on the western hills is supplying the 40,000 cubic metres of gravel and aggregate that should keep the feet of the 100-120 staff out of the mud until the winter’s end.

Most of the machinery needed for the job has been conventional, but the drilling for bridge piles has allowed Fletcher subsidiary Brian Perry to show off  its new Soilmec 615, one of the biggest drilling machines in the country. Supplemented by several Higgins machines to drill the 90 piles scattered around the eight sites, the Soilmec has been the star of the show for the tens of thousands of motorists cruising daily through the work-sites.

While the Wellington region’s roading attention will now be drawn to the billion-dollar Transmission Gully bypass on State Highway 1, the completion of D2P will remove one of the major blockage on the  arterial route connecting Wellington with the Wairarapa. More work is planned to provide a seamless expressway from Ngauranga to Upper Hutt and all points north up the eastern coast of the island. And for the commuters of the Hutt Valley, no less than the lifestylers north of the Rimutaka Hill, it can’t come soon enough.