Once complete, the project at 405 Bourke Street will be a 40-storey tower built on four 2.1m and one 2.5m diameter mega piles, which will carry a load of up to 140MN.

Speaking about the project, Sophie Rex, the senior project engineer, said: "Due to the size constraints of the site - roughly 50m by 20m - the mega piles had to be constructed from the basement, two levels down. Because there was no room for an access ramp, everything had to be craned into the site from the street, which added logistical complexity to the job.

As well as the mega piles, the scope also included the design and construction of 47 bored retention piles (750mm diameter) and 51 bored foundation piles (900mm and 1,200mm).

"Just the space required to store the large-diameter tooling, cages and casings took up about a third of the site. The 80t crane, Bauer BG30 drilling rig and other plant took up another third, which didn't leave a lot of spare space.

"The planning process was very detailed and extensive, with a lot of back and forth between our technical team and the structural engineers on how the piles should be designed, and a lot of meetings with our client - and their client - to plan the logistics and sequencing of events. But we have excellent long-standing relationships with the builder, Multiplex, and the civil contractor, Delta Group."

To confirm the quality of the mega piles, the team used new technology from America. The Shaft Quantitative Inspection Device - or SQUID - which was attached to a Kelly bar and lowered into the pile to collect real-time data on the thickness and strength of the debris on the base of the pile. That data was recorded on a tablet and enabled the team to make quick decisions on the integrity of the pile base.

"This is the first time SQUID has been used in Australia, so an expert came over from the US to show us how to use it and interpret the data," added Rex. "Testing only took about 30 minutes whereas the alternative would have taken hours."