To excavate half a dozen 25m deep station boxes through city centre streets, and guarantee zero damage to hundreds of 200-year-old architecturally important timber piled buildings lying barely 1m from the cofferdams, was an exacting task. But to also be faced with a water table near the surface, and a need for excavations to remain dry, while timber piles stayed wet so they did not crack and trigger settlement, called for world leading geotechnical expertise.
Fortunately, Comet had geotechnical specialist Bachy Soletanche as part of the joint venture. Sides of most 60m long excavations were lined with 30m deep hard and soft secant piling to minimise disturbance. The contractor devised a $15M computerised dewatering and recharge system that ensured station boxes could be pumped dry at the same time as surrounding areas were recharged to keep the piles waterlogged.
The operation, claimed to be one of the most sophisticated water control regimes on any construction site, included monitoring groundwater levels beneath some 6,000 buildings deemed at risk.
"And it all worked exceptionally well," claims Jefferies. "Settlement beneath every at risk building was indeed a simple zero."
