|
|
A technically challenging kilometre-long rail trench and station, and improvements to surrounding roads and streetscapes, are paving the way for New Lynn to develop into one of the most vibrant suburbs in Auckland. BY GAVIN RILEY
The $300 million project will see improvements to rail infrastructure, local roads and the town centre and is unusual for the number of public funders involved. KiwiRail is providing $140 million for undergrounding and double-tracking the existing rail line, Waitakere City Council $69 million for the rail and road upgrades, Auckland Regional Transport Authority $13.6 million and Waitakere City $22 million for a new rail station, and the NZ Transport Agency and Watercare are contributing the balance of the required funds. This massive boost will result in New Lynn (pop. 56,000) having an integrated public-transport system and pedestrian-friendly core, and will enable the suburb to cater for an expected doubling in size of its business hub and number of town-centre residents over the next 20 years. The outstanding feature of the rail upgrade is the digging of a kilometre-long trench up to eight metres deep through central New Lynn to underground the line. The open-cut trench will mean dispensing with two existing level crossings, lowering a third, and building four road bridges with spans of 12 to 18 metres. With the addition of the double-tracking and an enclosed station with an island platform, the undertaking is New Zealand’s largest railway project for several decades and is part of KiwiRail’s $600 milllion Project DART (Developing Auckland’s Rail Transport).
Quite apart from the trench involving major building on a narrow strip of land in a busy town centre, and the need to keep trains running and road traffic moving, it is one of the most technically challenging of the DART projects. Much preparation work was required to move services such as water and gas away from the area designated for the trench. Then the existing single rail track had to be moved to the south side of the rail corridor to enable Fletcher to build the trench’s northern wall. In late March the track was moved onto the completed northern wall so the southern wall could be built. In late May, the first of 46 permanent precast props was placed to strut the trench walls apart as the material between them was excavated. Three of the 30-tonne props were positioned during each scheduled week-night rail closure. The trench excavation began in June 2009 and was completed in early December. Some 89,000 cubic metres of predominantly clay soil were excavated two months ahead of schedule in a restricted area where movement of heavy trucks and machinery was difficult. The bulk dig work was subcontracted to Vuksich & Borich, which removed up to 1000 cubic metres a day and transported it to cleanfill at Puketutu Island.
A temporary station platform is in use adjacent to the trench until completion of the new station and in-trench island platform, which will have stair, lift and escalator links to an interchange and bus bays up above. The trains now running along the track atop the northern trench wall are scheduled to be moved into the trench around March-April and the second in-trench track will be commissioned half way through the year. That will officially complete the rail-infrastructure project, though some work on the steel-and-glass signature station building and precinct may continue into the third quarter of the year. (A signature station is one that is more than just a bare platform but has its own distinctive features, such as Britomart or Newmarket.) Innovative diaphragm wallConstruction of the trench walls has involved a diaphragm-wall technique. It is the first time the technique has been used on a major project in New Zealand, though it is common enough in Europe and the United States in situations which require working closely against structures, because it supports the ground and minimises settlement.
Once the construction team has dug the full depth and length, a pre-fabricated reinforced steel cage is lowered into the bentonite-filled slot. A tremmie concrete pour introduces concrete in the bottom of the slot, with the displaced bentonite being pumped out to leave a reinforced concrete panel. The process is repeated along the length of the trench with a vertical water-stop between each panel. The bentonite plant was designed to cope with three diaphragm wall panels and one or two tension piles a day, but the Fletcher team has completed up to four panels and three to four piles daily. This doesn’t mean progress has been straightforward. Fletcher Construction’s project manager Malcolm McDonald says everything about the rail contract has been challenging, including having to position up to 10 cranes (the largest of which was 120 tonnes) in a rail corridor only 20 metres wide, six metres of which is reserved for the trains which have had to keep running during construction.
A major challenge on a project of this scale has been the logistics. “At the height of the diaphragm-wall work we had to take out 400 cubic metres of muck a day, bring in six to eight reinforced cages each seven tonnes every day, and place about 400 cubic metres of concrete a day,” Malcolm says. “To do that and minimise the effect on the trains we had a round-the-clock operation. We dug and mucked out during the day, placed rebar cages after the train services had diminished in the early evening, then concreted all night so that we could get the concrete trucks through the traffic outside the commercial and retail area’s daytime traffic peak. “We’ve had traffic-management issues to deal with too. We had to maintain three road crossings of the trench as we were building the trench, and with the transport hub and high-density housing nearby we had a lot of pedestrians to manage as well.” Associated worksTraffic-management ranks high on the list of challenges faced by Dempsey Wood Civil Contractors, which is using up to 20 staff plus a small number of subcontractors to carry out a $13 million road-improvement and streetscape-upgrade contract in New Lynn for Waitakere City Council. The project covers upgrades on roads that surround the rail trench.
Dempsey Wood’s scope of work includes service diversions, deep drainage, road widening and reconstruction, intersection improvements, construction of a walking and cycle lane,installation of lighting, signage and parking facilities, and planting of trees. The work is being undertaken in sections to minimise impacts on local businesses, road users and pedestrians. Accesses to business premises are being maintained and temporary pedestrian walkways are in place. Full-time traffic management and pedestrian-support staff are on site to ensure safety through the works area, restrict disruption and maintain a “business as usual” environment as much as possible. As well as carrying out road reconstructions and streetscape upgrades, Dempsey Wood is responsible for the traffic management of Harker Underground Construction’s installation of a 600mm-diameter, deep sewer main for Watercare under one of the busiest streets. Infrastructure investment creates opportunitiesThese investments in rail and road infrastructure by the Government and Waitakere City Council have unlocked exciting development opportunities in the New Lynn town centre, notably the Merchants’ Quarter and Crown Lynn Precinct.
The Merchants’ Quarter will provide a heart for New Lynn. Centred around the transport interchange, it will rejuvenate the precinct which has a long history as a trade, banking and retail area offering all the important services one would expect in a town centre, including post office, police station, library and community centre. The mixed-use environment of the new hub will build on this character and heritage and support a variety of public open spaces and retail outlets, including cafes and restaurants. Interconnected spaces will be created to make the area very pedestrian-friendly. Teamwork is keyWith Waitakere City Council predicting the revitalisation of New Lynn will make it one of the most vibrant town centres in the Auckland region, Fletcher Construction’s Malcolm McDonald is proud of his team’s ability to cope with the “hard graft” of the rail trench. The team consists of up to 250 long-term and some relatively new Fletcher employees, young graduates, recent South African and British immigrants, and a smattering of key people (plus equipment) from Trevi, the Italian foundation-engineering company which worked with Fletcher on the Clyde Dam and, three years ago, on the Arapuni Dam repair.
“The diversity of people and the experience they bring have added significantly to the capabilities we have in New Zealand. A knowledge of diaphragm-wall work is one of those additions. Everybody has contributed really well to what has become a successful job.” But, as noted earlier, not an easy job. Malcolm, who has spent 12 of his 25 years in the construction industry with Fletcher, is asked to rate the project on a degree-of-difficulty scale of one to 10. “Eight or nine, I would say,” he replies, after due consideration. “In terms of grand achievement it would be up there in my top two, the other being my last job which was an incrementally launched bridge in Suva and was technically quite challenging.”
Contractor Vol.34 No.1 February 2010 |
Many millions of central and regional government dollars are being spent to realise west Auckland suburb New Lynn’s dream of one day rivalling Newmarket as one of the most prestigious town centres in the region.
The rail trench is a design-and-build contract being undertaken by a Fletcher Construction-Beca-Synergine consortium.
While excavation was underway, the reinforced concrete base slabs for the floor of the trench were steadily being placed. In total, the trench construction has required 7500 tonnes of reinforced bar and 50,000 cubic metres of concrete.
This is how the diaphragm-wall system has worked in the New Lynn rail trench. A specialised grab scoops out the soft clay to create a slot 16 metres deep, 800mm thick and seven metres long. As that is happening the slot is filled up with bentonite slurry to maintain support of the trench walls.
“We had a 14-metre-wide corridor to construct everything in, and cranes lifting tail loads in positions which had the potential to foul the tracks had things gone wrong,” he says.
Work began in September and is on schedule for completion in late 2010. The programme is responsive to the wider project and the need to synchronise tie-ins and traffic management with Fletcher Construction’s rail contract and other, separate contractors.
Crown Lynn Precinct is envisaged as a high-density, sustainable, residential-led neighbourhood built around a park. It is an important part of a total regeneration package in which one target is 20,000 people living within walking distance of the transport interchange.
“We have a great team here who are all working towards the common goal,” Malcolm says.