Big dig is all done

HAPPINESS is a great big hole in the ground for KiwiRail project manager Paul Crawford.

The trench 300 metres long by seven deep and up to 18 metres wide is the result of a year's work on the $42 million Manukau rail link.

And now comes the best part of the job – seeing the concrete slab poured as the base for the twin platforms for the new Manukau train station.

"It's been a long time coming," he says.

"The excavation has been a mammoth task."

It's taken three months to dig out more than 30,000 cubic metres of silt from the trench running from Davies Ave to just beyond the Lambie Drive bridge.

Now revealed are the 733 overlapping secant piles along the walls, the concrete props and capping beams all previously poured at ground level.

Mr Crawford says the 500mm thick base slab is being poured in "patchwork" sections to allow for shrinkage when the concrete's cured.

"Once all the slabs have cured and shrunk, we'll put in expansive concrete to push it out towards the walls," he says.

The base slab is held in place by 158 tension piles driven from the surface deep into the Waitemata sandstone.

"It's to stop it lifting because we're four metres below the water table here."

A few cylinders of concrete jumbled on the trench floor are remnants of the tension piles, lopped off during the excavation.

Contractors are now preparing areas for escalators, lifts and stairs up to the Davies Ave bus interchange which will also lead into Manukau Institute of Technology's planned tertiary campus.

The next stage will be covering the secant pile walls with 180 pre-cast, six-metre-high panels of honed concrete.

"They will be polished and it will look nice," Mr Crawford says.

The panels will cover the damp patches on the walls, evidence of the "minimal seepage" from the surrounding water table which will be drained into a tank near the Lambie Drive bridge.

The 250 cubic metre tank is still being built and will cope with a one in 100-year storm, he says.

"The whole trench slopes by 1 percent over its length so everything drains."

Soon contractors will start work on the walls for the pair of 160-metre platforms, then on the platforms them-selves.

"By the end of September we should have finished the civil works to the stage where the platforms are back-filled," Mr Crawford says.

In October KiwiRail will fill the trench with ballast and start laying the 2km of tracks out to the southern line.

The Auckland Regional Transport Authority will surface the platforms and hang canopies from the capping beams to protect travellers from the weather.

In February work is due to start on the bus interchange building, a joint venture between the regional transport authority and Manukau City Council.

About 600,000 train passengers each year are expected to use the rail station when it opens next year.

About 1.2 million people are expected to use the bus interchange each year.

- © Fairfax NZ News