Load tests on a new style of threaded bored pile for stiff clays show dramatic increases in load carrying capacity, claims its designer Bachy Soletanche. A 43% load gain compared to a similar sized smooth bore pile, and a consequent "substantial" overall cost saving, were calculated by the test team on the North London trial site.
"This heralds a significant advance in the efficiency of piles bored through London and similar Southern England stiff clays" says 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 unlined bored piling. Once drilled, the soil around the open bore quickly de-stresses 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 unlined, newly drilled bore, increasing both pile adhesion and effective diameter. On the tested 750mm diameter pile, the 75mm deep V-shaped thread increased effective diameter to 900mm for just 5% moreconcrete.
The SolThread equipment, attached to a conventional crane mounted 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 a freshly augered bore where outer pads on thethreading tool hold it firmly against the shaft sides.