Coates provides shoring solution for deep, sandy excavation in Sydney

When anchors couldn't be used to shore a deep excavation with significant groundwater, a safe and effective alternative was needed

The project

The redevelopment of Telstra’s exchange in Manly, Sydney, is called ‘Twenty95’. Construction Profile is building the $21 million mixed-use development featuring 700 m2 of retail space, 26 apartments, a multi-level automated car stacker in the basement and the refurbishment of a live Telstra exchange. 

The challenge

The Twenty95 development incorporates a deep basement built in sand, where the water table starts at 4 metres below ground level. Because of the sand, the deep section of the basement which was 30m x 15m could not be ‘anchored’ into surrounding ground. Instead, the basement walls had to be shored with hydraulic strutting, from inside the site.

Twenty95 is also a ‘live exchange’, so the exchange had to operate as normal throughout the refurbishment and construction, and movement in the surrounding structures was unacceptable.

To hold back the walls of this deep and complex excavation – and allow work to be completed safely – required a turnkey engineering solution.

The solution

Coates Engineering Solutions was contracted to complete the design and install the hydraulic shoring system, which consisted of a steel-pile perimeter wall and four levels of hydraulic struts and walers, with each level using eight hydraulic struts.

Neil Gujare, Engineering Manager - Installations at Coates, says the complexity of the site was such that the design work began more than a year before it was installed, and customised components were fabricated specially for the job at Manly.

“The excavation was deep and the soil was sandy and water-charged,” says Gujare. “So our shoring solution had to be designed to hold the large soil pressures, and the water pressure.”

The shoring had to account for variability in the soil, so Coates used its wireless Load Monitoring telemetry on the central struts so the construction company could verify the loads remotely on their own laptops.

“The project managers were given our monitoring system so they could see if the struts were working to 80% of their capacity – as agreed – or if they had risen to 90 or 95%,” says Gujare.

The impact

Twenty95 was an example of the turnkey engineering solution that Coates offers to its shoring, propping and dewatering clients.

“We design the solution with our engineers, we install it and test it and then have an engineer certify the system, at every level,” says Gujare. “We can keep the entire design and installation in-house, so the client has one point of contact.”

De-installation of shoring can also be complicated and clients prefer the speed and safety of one contractor managing de-installations, he adds.

Neil Denton, Managing Director of Construction Profile, says the shoring had to be of high quality to allow a safe work environment and minimise the risk of movement in surrounding properties, including the Telstra exchange which was being supported.

“Anchoring was looked at originally, but the model showed failure due to extreme loads. We used the Coates Mega Brace system because it allowed us to stress the internal steel struts before we started each stage of excavation – that meant a lot of co-ordination between us and Coates,” Denton says.

There were four rows of Mega Brace shoring in the Twenty95 excavation, each using eight Mega Brace hydraulic struts.

“I’m a believer in using the right system for each project, rather than the same system for every job. On this project we used sheet-piling because we could weld it and create a cofferdam against the ground water, and the Mega Brace system held up the cofferdam from the inside,” says Denton.

Denton says Coates has a reputation of taking on complex jobs and designing a solution that works. “The Coates engineers worked with our design and provided a solution. It all worked very well."

Need an innovative shoring solution for a complex excavation? Contact Coates for an engineered solution