Unexpected items below the ground can be a headache for the unprepared – just ask the construction team working at London City Airport recently, who discovered an unexploded wartime bomb.
Laing O’Rourke’s business leader at Expanded Geotechnical, John Chick, has extensive experience. “Your biggest asset is knowledge. You need to know what's there before you start,” says John.
“Venture in unprepared and you risk coming up against, groundwater, swallow holes, hard or soft ground conditions, or contamination. This can lead to a whole host of issues which create a lack of confidence in the price or design of a project, or even in the entire programme. Sometimes the viability of the whole project can suddenly be called into question.”
So what can be done to mitigate such risks? “Comprehensive geotechnical site investigations, are critical before projects start on site. If you drill boreholes down to 30m, but your pile design goes to 40m, you are missing critical information. Problems have emerged in the past where sites were sold on, but designers still relied on the site investigation work from a previous scheme.”
At the Edinburgh St James Retail centre project, deep piles are being made into strong sandstone to form the basement of the new shopping centre, as John explains. “It’s very difficult to assess how rock will react when bored by a large auger, which is basically a tool like a large corkscrew, used for boring holes in the ground. Our accurate assessment of ground conditions in Edinburgh is vital as very specialist equipment may be required and has to be manufactured with long lead times”.
“Pile depths of over 55m are common,” adds John. “They can also have a total capacity of 2500 tonnes each, which is the same as the entire weight of Nelsons column, per pile. If that pile can only settle by 10mm when it is fully loaded, we have to understand the ground and know what we are doing”.
The Expanded Geotech team often have to gain permission to work within critical underground exclusion zones, while permits are granted dependent on the client having confidence in the precision of the team’s work. At the Southbank House development, a key structure required them to install deep, large diameter piles within 900mm of a Bakerloo line underground tunnel. Accuracy of installation was crucial, with detailed knowledge of underground structures also critical.
Piling rigs are large, tall and sometimes quite noisy, meaning that the team’s work is often conspicuous on city centre sites. At the new Google Headquarters being constructed in London, Expanded are working between Kings Cross and St Pancras station buildings in full public view, right outside the windows of existing offices. High professional standards are expected of the teams who work in these busy and sensitive environments.
Building for the future
Working on brownfield land can often mean working out the likely presence of contamination and the locations of existing foundations or piles. The sustainably minded can seek to reuse these structures to help support a new development, but this process is not always straightforward.
“It can be difficult to get a proper warranty on original piles that you don't have full records for, and detailed technical information for anything over 15 years old is very sparse,” says John.
“We also have the occasional brush with history, when we find timber jetties from the Middle Ages, or bell pits and flint mining shafts in chalk. Most pilers have a collection of unusual objects found in borespoil, from unusual bottles and oyster shells, to belt-buckles and cannonballs. At Laing O’Rourke, we're building for the future, but at the same time we must do everything we can to protect the past,” says John.