Britomart
Construction Issues
The location in the middle of downtown Auckland meant difficult access to the site, and much potential for upsetting the neighbours. "There were homes and businesses all around us," says Mr Burtenshaw, "and trying not to be a nuisance was difficult at times." They concentrated the work in the noisier parts of the day, and stepped up monitoring to minimise the effect on people.
A major construction headache was controlling water draw-down in the surrounding area. New and old buildings surround the site, some as close as 15 metres, and the majority of them are extremely sensitive to settlement. With the underground station's platforms 12 metres below ground-level, and eight to nine metres below sea level, there was a considerable amount to excavate – 200,000 cubic metres of spoil, to be exact. A long-buried valley to the back of the CPO was filled with insecure, soft material, including consolidated clays, marine deposits and rubbish; and a flood plain draining Queen Street further complicated the equation.
COP Outcome development and evaluation
To solve the problem at the western end of the site, hundreds of secant piles, varying in diameter from 900mm to 1,200mm, were constructed to create watertight retaining walls that could withstand the hydrostatic pressures. "The secant pile walls provided an impermeable barrier that prevented draw-down and settlement on the outside," explained Opus project director Melvyn Maylin (MIPENZ). "They enabled us to meet the maximum horizontal displacement limits of 25mm at the west end, down to 12.5mm immediately adjacent to the CPO."The bored concrete piles were embedded a minimum of four metres into the Waitemata bedrock. Beneath the base of the station, 600mm tension piles had to go eight metres into the rock to secure against the uplift pressure of the water below. At the eastern end of the site, the bedrock is considerably closer to the surface, and more conventional construction techniques could be used. Temporary sheet-pile walls could be built, the hole excavated and then the station constructed from the bottom up.
Forming the secant piles was in itself a multi-stage operation. "We put in a guide beam at ground level," says Mr Maylin, "through which we drilled the pile, and filled with 20mPa concrete. We then over-drilled every second pile and filled with reinforced 35mPa concrete." Very precise drilling was called for.
Secant piles were further used to provide additional support to the new station wall behind the CPO. "We used them like buttresses," continues Mr Maylin. "The design drew on our experiences on the Jubilee Line extension in London and the new airport railway in Hong Kong. It was nice to bring an innovation back to New Zealand having practised it first!"
