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Waste of time

Unexpected clues from the past informed the choice for ground improvement at a Welsh site, says Derek Taylor.

Leckworth Road, Cardiff, is the current hub of engineering activity in the Welsh capital. It is an area well-known to Keller Ground Engineering. Currently carrying out piling work there for a new Asda superstore, the firm had previously installed piles on land adjacent for FA Cup finalists Cardiff City's new stadium on a major contract for Laing O'Rourke. The area is also home to an almost-completed non-food retail park.

Much of the site was an old landfill site and is underlain by refuse ranging in thickness from 2m to 5.3m overlying soft to firm alluvial clays. These are in turn underlain by river gravels and then siltstone.

The chosen solution for the New Ninian Park Stadium was a Junntan PM25, which has spent much of the winter installing 340mm and 430mm diameter driven cast in-situ piles into the gravels.

"It's the pile of choice in Cardiff," claims Keller business development manager Rob Millar, referring to several kilometres of piles installed during various developments around "The Bay" during the 1990s.

"This system generates no spoil and carries loads efficiently into the gravel. Importantly, on a site like this it gives a good seal through the clay preventing downward migration of any leachate within the made ground."

Although Asda's existing store at Ferry Road, Cardiff, has sat happily since the mid-1990s on Keller's driven cast in-situ piles, Peter Gower of structural engineer GDP, and main contractor Pearce were looking for an alternative at Leckwith Road.

The answer lay in the age of the made ground. During the 1950s and 1960s coal fires were the norm and households burnt most of their waste. The tough post-war years were times of little food waste and what went to landfill tended to be ash, bottles, ceramics and metal – all material with little potential for degradation. This means there is less danger of methane generation on these sites than on made ground created after the two post-Second World War Clean Air Acts, which contains higher contents of degradable materials.

Keller carried out a programme of trial pits to satisfy itself about the percentage of degradable material within the fill for the Asda site. Approximate date of deposition was reached by way of tiny remnants of a 1961 Daily Express.

These partially burnt pages showed an advert for Ford's re-styled Anglia – the car seen in the Harry Potter films – and an advertisment for Fairy Liquid at 9d (pre-1971 currency before the UK introduced decimalisation). These clues about the age of the refuse suggested there would be very little degradable and combustible material, and Keller confirmed it could treat the site.

The trial pits also confirmed the presence of underlying firm clay and a lack of water suspected to be found perched on top of the clay. This enabled the choice of the correct vibro system.Traditionally, where high water is present in made ground or weak soils, Keller adopts a bottom-feed vibro approach. The system uses more sophisticated rigs with special vibrators that deliver the stone to the base of the column while the vibrator remains in the ground via a tremie tube. This gives a high integrity column in wet or soft soils.

At Cardiff, the absence of water in the loose made ground meant a conventional top-feed approach would still allow the formation of high-integrity columns.

Vibro offered a number of advantages including speed of construction, lower cost than piling and importantly, sustainability. Vibro sites traditionally are easily redeveloped and with the average life of retail parks being about 15 to 20 years, this is increasingly why some developers choose a ground improvement solution.

Stone columns on this site meant that shallow foundations, designed for 125kN/m2, could be used together with a ground bearings slab. This is much simpler construction than pile caps and suspended floors.

The performance of the vibro was confirmed by carrying out two zone tests using kentledge (concrete blocks) to load a 1.5m2 plate to 1.5 times the design load.

The choice of system enabled Pearce project manager Simon Philips to gain a fast start on the foundation pads ready for the imminent
steelwork erection without waiting for piles or caps to cure.

Keller's work on its £130,000 contract began in March and ended last month.

Derek Taylor is sales director at Keller Ground Engineering.

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