The project is the latest phase of a residential development by Erith Hills LLP, made up of Anderson and L&Q, comprising 849 new homes and a new primary school.
Called simply The Quarry, the scheme occupies a disused 50-acre quarry in Erith, abandoned for more than 30 years.
Central Piling has been working on site since 2021, installing foundations to support the new development. The ground conditions are variable, with large areas of mixed fill reaching depths of up to 30 metres in some places. The challenge is to install piles that are deep enough to reach firm ground – which means piles up to 40 metres deep.
“Rotary piling wasn’t feasible due to the depth of casing required and driven piling might not have penetrated harder elements within the fill,” said Central Piling managing director Steve Hadley.
Instead, Central Piling chose CFA techniques. But that gave rise to another challenge: few CFA piling rigs can drill down beyond 30 metres. “There are rigs that can drill to about 36 metres but they are huge, 120-tonne machines,” he said.
The larger of the two rigs is used to install the first stage of the pile. “This is the main machine,” Steve Hadley said. “It can drill down to 29.5 metres”.
Once the SF65 has drilled to about 24 metres, it decouples and backs away to allow the smaller SF50 to position itself over the first auger, to which it couples its own auger. This machine then continues the boring process to reach the required depth.
“This presented a safer and higher-production solution than adding single six-metre sections of auger to a single rig,” he explained.
To make this system work, Central Piling had to develop a tool for supporting the drilling tools as the rigs were swapped during the concreting stage of the operation. The team also configured the drum and pump set-up with a valve that allowed concrete to be shared between the two machines.
Even if a 120-tonne rig had been available, Steve Hadley doubts that it would have proved suitable for this site. “It’s quite tight in some places but having two machines that can move independently means we can get into those spaces,” he said.
He added: “I’ve never heard of anybody using two rigs in tandem like this – as far as I know, it’s a first.”