Paul Doughty from the Temporary Works Division at Sir Robert McAlpine describes groundworks at Glasgow School of Art redevelopment.
The £50m redevelopment of Glasgow’s School of Art involves construction of a new five-storey building in Renfrew Street, on a sloping site directly opposite the famous Grade A listed art school building designed by Charles Rennie Mackintosh.
The building’s double-storey basement, dug into stiff boulder clay, required extensive temporary propping during construction to support a 60m long, 34m wide and 10m deep excavation.
This size of basement would typically require two levels of propping, an upper and lower level, to provide stiff ground support but we determined that installation of lower level props would slow down the excavation and hinder concrete works – at a certain depth you have to stop to install walings and props, which slows things down.
Instead, McAlpine’s design team worked with Groundforce to design a robust upper level system of temporary works, which in conjunction with contiguous piles, would allow excavation to formation level with no lower level propping in place. The system utilises a GPRS wireless load-monitoring system, both within the site and on the adjacent buildings using a combination of load cells, prisms, tiltmeters and inclinometers to give an overall picture of prop loading and ground movements. Provided movement and prop loads were below certain trigger levels, agreed in advance with our engineer Arup, the lower level props could be omitted.
Groundforce was able to provide a cost effective alternative to fabricated steel propping for the length of time that the props were on site -— about 25 weeks maximum for the upper props and 10 weeks for the lower props, if used. This allowed the lower props to be procured and available in the depot should the trigger levels be reached.
Usefully, Groundforce’s load-monitoring system could be accessed in real time, 24 hours a day via a dedicated website.
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