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Threaded piles show load advantage in stiff clay

CONTRACTS

LOAD TESTS on a new style of threaded bored pile for stiff clays show dramatic increases in load carrying capacity, claims designer Bachy Soletanche.

A 43% load gain compared with a similar size smooth bore pile, and a consequent 'substantial' overall cost saving, were calculated by the test team on the north London trial site last month.

'This heralds a significant advance in the efficiency of piles bored through London and similar southern England stiff clays, ' said Bachy Soletanche chief engineer David Puller. 'The piling market in these areas alone is worth £10M a year and rising.'

London Clay, cohesive and overconsolidated, creates a particular challenge for uncased bored piling. Once drilled, the soil around the open bore quickly destresses and softens to lower the pile's shaft friction.

Central to Bachy Soletanche's SolThread technique is cutting a helical screw thread right up the uncased, newly drilled bore, increasing both pile adhesion and effective diameter. On the tested 750mm diameter pile, the 75mm deep V-shape thread increased effective diameter to 900mm for only 5% more concrete.

The SolThread equipment, attached to a conventional cranemounted rig, consists of a 3m long threading tool, with retractable cutting teeth either side, and a cylindrical crumb bucket mounted directly beneath.

Both are lowered to the base of an augered bore where outer pads on the threading tool hold it firmly against the shaft sides.

Slowly rotating the tool triggers release of the cutting teeth which spring outward. As the still rotating tool is withdrawn, these teeth cut the 75mm deep helical thread into the shaft perimeter.

Soil cuttings which, if allowed to fall to the pile bottom, could reduce end bearing capacity drop instead directly into the crumb bucket beneath the cutting tool.

Last month's load test to failure compared smooth and threaded piles bored in London Clay just metres apart. A down-the-hole video camera recorded a clean, fully formed thread which remained open until the pile was formed using a standard, high slump concrete mix.

Puller calculated that the alpha adhesion coefficient, which for the adjacent smooth bore pile was 0.5, had risen to 0.8 in the SolThread pile.

'The technique's 'double whammy' of this 60% increase in shaft friction, plus larger effective diameter, offers a more efficient pile for depths, easily up to 35m, ' he said. 'We hope to employ the design on a suitable contract in London this summer.'

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