Thousands of cars fly past 2 St Kilda Road heading south, just before it turns into Nepean Highway. Most wouldn’t even notice the sturdy and secure worksite fence, or even the tip-truck which waits on the footpath to receive its load of rock and dirt (the stuff that you end up with when you are digging an enormous hole).
Lost in the dulcet tones of The Ox and Marco on SEN Sports Radio, or listening to a bit of triple j, people wouldn’t realise the high-tech world that, these days, is the construction of a huge basement being prepared to support a high-rise building which is all going on behind that fence.
Peer over the top from the viewing platform, though, and you will see it is a massive excavation. And you will start to realise that the science of hole-digging is rather advanced, and that the engineering which is required is up-to-the-minute and sophisticated.
Which is why, on the morning when I am there, the University of Melbourne Civil Engineering school has brought 60 students on a site excursion to hear from Geotech’s Nic Morgan and to view the work in progress. This is all part of Geotech’s ongoing relationship with the university, which involves a number of visits each year.
Recent excursions have included field trips to the new Royal Children’s Hospital in Parkville and the Moonee Ponds Central shopping centre (both examples of basement works), and to the Royal Melbourne Hospital and Flemington Racecourse (tunnelling works).
Anyway, in the site office across the road, the scale model of the final building, a PaceDG development, shows 17 storeys above the ground (which will include 119 luxury apartments in a prime location) and a 5-level basement; all of this on 700 square metres, about the size of a suburban block.
Geotech is responsible for that basement, and with the work nearly completed, they have invited the engineering students along.
The students (in their Masters years) are attentive. Nic explains: “We get the harder job. The difficult stuff is below the ground. Above the ground is easy.”
The geotechnical conditions were established by three detailed high-quality bore logs which showed that the site is underlain by Tertiary age sedimentary deposits known as ‘Brighton Group’ material. It also showed sedimentary deposits of the Silurian age (“Dargile Formation”), some ferruginous sandstone, and some granite.
The water table was found to be at 11 metres, which had some impact in a basement built to 15 metres.
The initial design of the basement was the responsibility of Pace’s structural engineers. However, when those plans went before the Geotech designers, they were able to modify the approach in a commercially competitive way and Geotech were given responsibility for the design.
Nic explains to the students that this commercial imperative is a key part of contemporary engineering and that it is the amount of reinforcement in the piles which affects cost. The Geotech designers used geotechnical modelling software such as Wallop to help them save on those costs.
Despite the granite in the site and the position of the water table, this construction went smoothly, to plan, within budget and within timeframe.
The students are taken on a site tour and one of them noted sagely, “It is a big hole.”
They’re not far off being responsible for one just like it.
Scope of works:
- Design & construction of a 5-level basement on a very limited footprint of only 700m²
- Design & construction of 73 no. 750mm diam. cast in situ soldier piles, 18-21m deep
- Construction of 120 lineal metres of capping beam
- Design & installation of temporary ground anchors – 250 no. with loads 200-400kN
- Design & construction of shotcrete panels (approx. 1,000m²).
- Groundwater table at around 11m depth (retained height is typically 14-15m from St Kilda Road level)
- Very demanding program
- Variable ground conditions.