Barrett Byrd Associates

Displacement piling was pioneered for use on contaminated sites but the innovative technique is proving equally popular for quietly fast tracking construction on squeaky clean developments. Claire Symes visited a site in Abingdon to find out why.

Abingdon’s Milton Park is filled with aesthetically pleasing modern offices and tidy access roads but it is still under development. A 50,000m³ complex of small business units is currently being built at the park using specially selected techniques that are minimising the impact of the construction on existing tenants.

Piling on the site has just been completed by foundation specialist Roger Bullivant and is an example of construction manager Glanville Projects’ commitment to maintain ‘business as usual’ at the park. Bullivant used its continuous helical displacement (CHD) piling at the site to keep noise and vibration levels low and avoid the cost, and potential mess, of taking spoil off site.

“CHD piling was initially developed for contaminated sites to reduce the risk and cost of disposing of spoil and minimise exposure of workers to potentially hazardous materials,” says Bullivant piling contracts manager Jim Bourke. “Milton Park is not a typical candidate for CHD piling because there is no soil contamination at the site.

“Ground investigations had already confirmed the site to be clean, but Glanville opted for the CHD technique because of its performance benefits. If the ground conditions are right, and there are no concrete delivery hold ups, we can install up to 100 CHD piles in a day, which is very quick.”

Milton Park has been gradually developed by its owner MEPC Milton Park over the last 15 years and the main access road to the current building site from the busy A34 is through the established business park. “Our client was keen to avoid disturbing the site’s neighbours with excessive noise and vibration during the construction,” says Glanville Projects director Bill Brown. “Using displacement piling techniques has saved around 30 lorry loads of spoil being transported through the park. This has not only reduced landfill costs but also removes the potential for material to be dropped on the roadways during transportation.”

The CHD technique, pioneered by Bullivant, uses a bullet ended hollow stem shaft with a specially designed 600mm diameter helical auger section to displace spoil laterally instead of bringing it to the surface. Once the stem has been augered to the required depth, concrete is pumped down the 300mm diameter hollow shaft and the stem withdrawn while rotating in the reverse direction to form a solid pile. Once the concrete is in place a steel rebar is added and pile caps or ground beams can be formed later.

Bullivant installed 131 CHD piles at 127 Milton Park to a maximum depth of 13m below ground level through around 1m of fill to found in the underlying Gault Clay. The 500kN capacity piles have been constructed on a regular grid pattern to support three buildings at the site which will be split into small business units varying in size from 150 to 200m2.

“CHD piling is best suited to cohesive soils and the ground at Milton Park was near perfect for the technique. We managed to complete the piling work at the site in just three days,” says Bourke. “I’ve been in the piling industry for almost 40 years and this system is probably one of the quickest and most effective techniques I have ever seen.”

Use of a piling technique developed for contaminated soil on a clean site was not the only unusual aspect of the contract at Milton Park. Procurement of the work was also undertaken in a less conventional manner but according to Brown, the method benefits everyone involved. “There is no main contractor for the development at 127 Milton Park,” he says.

“Glanville Projects is acting as construction manager for MEPC Milton Park and all the design consultants and trade contractors, from architects through to foundations specialists, are employed directly by the client. We were responsible for fast tracking the design and procurement phases of the work, eventually overlapping the two, to give significant time savings on the overall project programme.”

Now piling has been completed at the site, MEPC Milton Park’s groundworks contractor is hard at work building cast in situ ground beams. Steel work contractors will follow on to install the unit's steel framework and steel liner tray walls which will be insulated then rendered. “The steel frame and liner are quick to install and very lightweight. The only block work on the development will be used on the stair cores to enhance fire safety and durability,” says Brown. “Selection of fast construction techniques for every phase of this development has enabled us to speed up delivery of the scheme. The first tenants will be moving in in September this year – less than nine months after work began.”

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