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Deadline Buster

Ahead of schedule excavation for the basement of a Coventry office block is now underway thanks to the performance of one of Britain’s most advanced piling rigs.

Installing piles up to three times faster than programmed, with a sophisticated computer-controlled rig, has helped contractors complete a seven-storey Coventry office block bristling early.

A pre-contract understanding between main contractor BAM Construction and its chosen geotechnical specialist Expanded Piling allowed early pile design plus a flying day-one start.

BAM successfully tendered for the £40M shell and core contract to build Severn Trent Water’s new operations centre 1km from Coventry city centre. Beneath the building will lie a three-level, 80m-long rectangular basement car park. Expanded’s task was to design and build a watertight piled wall forming the basement perimeter and offering minimum obstruction during follow-on bulk excavation for the car park.

By the time BAM had signed the main contract, Expanded had already designed the average 17m-long secant piles as full, prop-free cantilevers. Detailed discussions between BAM, Expanded and other selected subcontractors had also smoothed possible work interfaces between the several overlapping trades that would be needed on the tight, congested site. "We even had the luxury of getting onto site a week early," recalls Expanded senior project manager Rob Howarth. "When our £1.5M contract officially started, we already had the rig erected and ready to go from day one."

Key to the piling’s rapid progress has been Expanded’s £1M Bauer BG28 rig boasting a 28tm torque and designed specifically to install fully-cased secant piles. Early discussions had looked at a contiguous piled basement wall, which would have worked out 30% cheaper. But Severn Trent, and its development partner Stoford Developments, opted for the more expensive secant design to ensure they got a waterproof wall.

The resulting high verticality tolerance demanded, and secure overlap needed between male and female piles to maximise water tightness, pushed Expanded’s versatile BG28 into pole position. The rig incorporates twin rotary heads on its mast. The lower head supports the single, maximum 19m-long pile casing, while the second, 1m higher, guides the follow-on auger.

At Coventry it was possible to complete a single-cased 880mm diameter pile in just 30 minutes. Howarth estimates this was as much as twice as fast as installing a conventional segmental multi-cased pile. And achieving a 1 in 200 verticality tolerance – 30% above the contracted specification – further ensured the watertight wall requested. Except for the upper 3m of made ground, the entire pile bore lay through sandstone with strengths up to 36mPa.

Initial concerns that such hard rock was close to the upper limit of the machine’s capability were quickly dispelled when the rig operated virtually non-stop, and at full torque, throughout the 10 hour shifts. High running costs associated with this type of plant were considered more than offset by the rig’s performance.

The programmed rate of completing five piles per 10 hour shift was nearly tripled to an average of 14 per day, so the option of bringing in Expanded’s second CSP rig never really got past first base.

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