A large excavation in Adelaide required an expert shoring solution.

The Project
 

The ‘217 East’ project in Adelaide is a luxury residential development that will be six storeys high, contain 14 apartments and include a basement car park. It is due to be completed in early 2020.

The Problem

The new building requires an excavation for the basement, around which is a contiguous pile retention wall to create the underground car park. The retention wall extends to the southern boundary of the new building and abuts an existing building. This neighbouring building was constructed on relatively shallow foundations, which imposes pressure on the new retention wall. And the retention wall is not authorised to be ‘anchored’ into the neighbouring land.

An alternative would be to prop the basement’s retention walls internally using structural steel beams. But the excavation has a span of 19.2 metres between the retention walls, and to prop them using structural steel struts would require supports at their centres. This does not suit the project because there has to be room for workers to move in the excavation, and strut-supports would limit the work area.

The Solution

The construction group, Minuzzo Project Management, and its consulting engineers, Wallbridge Gilbert Aztec (WGA), needed an internal shoring solution to safely secure the retaining walls while not requiring central supports. They engaged Coates Hire, which designs, fabricates and installs modular hydraulic shoring solutions that span large excavations without a central support.

Coates Hire Engineer – Temporary Works, Douglas Norton, says the final shoring designs on the 217 East basement used four MP 150 hydraulic struts, each of which span 19.2 metres.

“They are the longest unsupported hydraulic shoring struts yet used in Adelaide,” says Norton, of the struts that were installed in early January 2019. “The MP 150s have been designed for this sort of project.”
He says the contract hinged on some challenging problems, including the weather and the size of the struts, which weigh five tonnes each when fully assembled.

“The struts were designed and assembled by Coates Hire in Melbourne, and trucked to Adelaide. They were too long for the trucks so they had to be transported in sections and assembled on closed roads in Adelaide before being craned into place.”

The installation also faced a heat wave. “The temperature on the day of installation was 42 degrees Celsius, but it was going to rise through the week to 46 degrees,” says Norton. “So we decided to get it done in one day.”
He says the job was technically difficult but also a management challenge, given that six different companies had to be co-ordinated at the installation: construction, engineering, Coates Hire, traffic management, steel fixers and the crane company.

Project Manager at Minuzzo, Hubert Dybiec, says the complications of the adjoining building and the fact that the new structure will take up most of the site, meant innovation was required.

“Due to the complexity, our engineers reviewed the scenario and looked at the option of using props,” says Dybiec. “The Coates Hire solution provided the best option for propping, given the loads required to be restrained without using temporary propping pads.”

He says Minuzzo and WGA worked with Coates Hire to create the propping solution. “Coates Hire were very helpful, working closely with our engineers and project team to finalise the design and refine the offer.”
Senior Structural Engineer at Wallbridge Gilbert Aztec, Clement Ao, says with the option of ground anchors no longer available for the retaining wall, the design team selected horizontal struts to stabilise the retention piles during excavation.

“The removal of highly-loaded conventional steel struts in an open excavation would have been a challenge and posed serious safety concerns,” says Ao. “The hydraulic strut system enables a progressive and safe stress relief to the struts before removal, while the modular assemblies can be easily pulled apart and transported off-site.”
He says Coates Hire included a thorough presentation of the hydraulic prop system to the clients, the builders and the consultant team, “which delivered a coherent understanding for each team member, eliminating misunderstandings and confusion on-site.”

Douglas Norton says that as well as delivering engineering precision, Coates Hire’s shoring solutions must also deliver 100 per cent safety for everyone on-site.

“At 7 am we did an induction for all the workers, and I held a toolbox chat where we walked through how the installation was going to work, and who had to be where. We have the best gear, but you also have to communicate with people about the part they play. These shoring solutions hold back thousands of tonnes of weight from a work site – it’s all about safety.”
 

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