Underreamed bored piles have obvious advantages, but health and safety concerns over the need for manned descent means they have fallen out of use over the last decade.
A 2.1m diameter straight-sided bored pile in London typically has a maximum capacity of 1000 tonnes, but underream out a 6m-diameter base and the capacity rises to more than double that.
And in an age of increasing structural spans, higher column loads and heavily congested urban sites, the benefits of underreaming are greater than ever.
Traditionally piling contractors relied on manned descent to inspect the condition and proportions of the underream. The main concern is ensuring a clean base, because there is a risk of build up of remoulded clay, which gives markedly inferior pile performance.
But safety concerns about lowering people down pile shafts, which were formalised into HSE regulations in 1995, has led to a severe reduction in use – the piling industry has almost entirely reverted to straight shafts.
Although some contractors have offered remote inspection systems for more than a decade, until now they have not been accepted as good enough by consultants. The main problem, says Arup geotechnics associate director Tim Chapman, is that “testing devices available previously did not reliably distinguish between remould and intact clay.”
But Chapman says both Bachy-Soletanche and Expanded Piling have finally managed to crack the nut to Arup’s satisfaction. Bachy-Soletanche’s penetrometer-based system is being put through its paces at Sir Robert McAlpine’s Bishop’s Square project next to Liverpool Street Station. Meanwhile Expanded has successfully trialled its U100 (100mm diameter) tube sampling system at Cardinal Place in Victoria.
According to Chapman both have worked well and other foundation contractors, notably Stent, are known to be developing comparable systems. As Chapman says “designers can now specify underreams, knowing that safe and robust systems can be procured commercially.”
So a big pat on the back for the contractors, but Chapman raises the bar by adding: “The next challenge is for piling contractors to develop a way of installing a 20MN underream pile within a standard 10-hour working day.”