Jet grouted underpinning columns, micropiles and multi-strand soil anchors have been used by Keller to redmediate the foundations on a historic building in Australia.
The mid-1800s building at the Scottish Hospital in the Paddington district of Sydney was to be the focal point of a new over 55s residential development but Multiplex had concerns over the foundations.
Investigations early on in the project showed that the Terraces’ original footings were deteriorating and Multiplex brought in Keller to find a solution to preserve the building.
A complicated series of props needed to stay in place during the entire works and the proposed renovations increased the existing loads considerably, so it was a difficult design,” says Keller senior project engineer Damien Bray.
“Because we had to work around the structural props and in tight spaces, including doorways and metre-wide hallways, it was necessary to use the smallest jet-grouting and micropile rigs available,” he explains. These included a KB1 jet grout rig, designed and built in house by KGS. As the works were on the client’s critical path it was necessary to perform the micropiling and jet grouting concurrently, requiring careful planning and complicated material handling.”
The work included over a hundred jet grout underpinning columns, 60 vertical and inclined retaining micropiles, 51 foundation micropiles and a number of multi-strand soil anchors. Due to height constraints, the team could only drill 1m lengths for the majority of the site.
During the works it was also discovered that the rock level was up to double the depth to that originally designed for. This made constructing and installing the micropiles more complex, especially when working around structural props. Ingeniously, to overcome this, micropiles were specially designed to be coupled together on site before being installed using a limited access crane.
“Due to the sensitivity of the structure it was specified that no more than 20mm settlement and 10mm lateral movement could occur,” said Bray. “As you can imagine, that’s difficult when you’re jet grouting and temporally liquifying the soil under a partially collapsed building. With little room for error, it was necessary to understand if any movement was occurring during or after any of the works had been completed. To reduce risk we brought in our sister company Getec from the UK to travel to Sydney to install a number of hydrostatic levelling cells.”
The specialist instrumentation was linked to a web-based system, providing real-time vertical movement monitoring during and post installation works. This introduced a level of confidence and understanding that all works were performed safely and without further damage to the building.