KEY POINTS:
Work on the country's biggest rail project since much of the North Island main trunk line was electrified in the 1980s got off to a ripping start in Auckland yesterday.
With a fleet of digging machines, contractors to rail agency Ontrack moved in to a 1.1km stretch of Auckland's western line, which is to be duplicated and sunk into a trench up to 8m deep, through the heart of New Lynn.
Formidable 800m retaining walls will be built on each side of the line next year before the main "digout" of about 90,000 cu m of earth begins for the $100 million-plus project, due for completion in 2009.
Duplicate tracks will be laid for trains over a concrete slab along the bottom of the trench, in which a new station will be built below a surface-level bus interchange and road crossings.
Ontrack is replacing level-crossings with bridges at Veronica St and the difficult Clark St-Rankin Ave traffic roundabout. Waitakere City Council is also considering adding two new surface crossings, at Hetana St and Memorial Dr.
That will allow pedestrians easier access between the two sides of a redeveloped New Lynn town centre, as well as removing a notorious bottleneck which planners warned would otherwise have become intolerable as more trains competed with extra traffic arriving from the motorway extension through Mt Roskill.
A level-crossing will remain further east at Portage Rd, but will be lowered by about a metre as the railway line enters the trench.
Although the main construction phase will not start until March, contractor Fletcher Construction is taking advantage of a Christmas shut-down of the western line to erect safety barriers and strip away vegetation to make room for large pile-driving cranes.
About 500mm of topsoil also has to be removed to a managed landfill before the Auckland Regional Council determines whether the site is free of contamination.
Ontrack has shut the line until New Year's Eve, primarily to lower rail tracks for a $70 million redevelopment of the Newmarket rail junction, but also to remedy drainage problems east of the Whau River bridge, which is soon to be replaced and duplicated.
But the agency's New Lynn project manager, Peter King, said he wanted to use the temporary absence of trains to get as much enabling work as possible done.
He said the existing single track would be moved about 6m west to make room for three sets of piling machines including 100-tonne cranes between the Whau River bridge and Titirangi Rd.
"There will be a lot of heavy equipment next to the rail track - it is extremely challenging from a construction planning perspective."
Piles would be driven to depths of up to 40m, into rock beneath clay and peak layers, to build a largely impermeable wall on the northern side of the railway line by next September.
The line would then be moved so that trains would run along the top of that structure, providing room for a similar wall on the southern side.
Four months of excavations would follow in which about 90,000 cu m of earth would be dug from between the two walls, leaving a trench 20m wide, along which duplicate railway tracks would be laid.
Mr King said although the project would provide New Lynn with long-term transport improvements, the town was in for about 18 months of traffic restrictions, starting on January 7 with the installation of a new stormwater pipe along Totara Ave.
RAIL TRENCH
* Length: 1080m
* Depth: Up to 8m
* Cost: $100m-$120m
* Completion date: Mid-2009
