The new brand of continuous helical displacement piles, developed by specialist foundations engineering contractor Roger Bullivant, is unlikely to be put to a more severe test than providing the support for the 'Hangover', a 5M white knuckle rollercoaster ride which opens in Southport, Lancashire next year. The plethora of Bullivant's vertical and raking CHD piles will cater for rapidly fluctuating compressive and tensile loads generated by the suspended rollercoaster train as it travels at high speed along the 680m of stomach-churning looping and twisting track.
'We are asking quite a lot from the CHD piles,' says project civil engineering consultant Harrison & Taylor engineer Neil Cartmell. iThe ride will take around 103 seconds, excluding unloading, with the train producing dynamic loads several times the dead load of the structure.
'Some piles will have to accommodate a rapid reversal of loading from peak tension to compression and vice versa from peak compression to tensile loading in only a few seconds. It is quite an onerous loading '
The unique feature of a completed Bullivant CHD pile is the helical concrete flange which surrounds the central concrete column. Key is the special drill bit or 'bullet head', which resembles the point of a bradawl.
A short, hollow tapered steel former complete with a larger diameter helical flange, the bullet head is fixed to a hollow drill pipe which is connected to a high torque rotary head running up and down the mast of a special rig designed and built at Bullivant's main office and workshops at Burton-on-Trent, Staffordshire. Bullivant has based the all hydraulic, self erecting rig on a modified Swedish Akerman H16B hydraulic excavator.
Bullivant, operating from its north west of England area office as the nominated piling subcontractor to main contractor Shepherd Construction, worked closely with Blackpool-based consulting structural engineer Harrison & Taylor at the foundations design stage. After competitive tendering, where contractors had to propose pile type and piling method, Bullivant was awarded the 150,000 lump sum subcontract based on its CHD technique.
The contaminated site was peppered with 392 dual purpose CHD piles and 258 driven precast concrete piles. 'The CHD system has several major advantages over continuous flight auger and other types of piling,' says Bullivant area director David Roberts.
'It displaces and improves the ground and there is no muck away - especially useful when working in contaminated ground. CHD piles are usually shorter than other bored piles of the same capacity. They are also very good in tension and very fast to install.'
'Bullivant has installed up to 45 CHD piles a day,' Shepherd Construction site agent Peter Magee reports. 'It's a very good, fast, clean system. There is the added benefit of no spoil removal or costly tipping charges or the need to have an excavator to clean up around the pile tops, which makes the technique very attractive, especially for working in contaminated ground.
To install a CHD pile, the hollow bullet head is plugged with a disposable bung and, rotated by the rotary head and drill pipe, screws and pulls itself into the ground to a required penetration or torque setting. The bullet head, working just like a wood screw, displaces, compacts and improves the surrounding material as it screws itself into the ground.
Concrete is then pumped through the rotary head down the hollow drill pipe and pressurised to blow off the bullet head's expendable bung. The bullet head is slowly unscrewed and withdrawn out of the ground without removing any spoil.
Simultaneously, concrete is pumped in to fill the cylindrical and helical void left behind to form a concrete column with a continuous spiralling helical concrete flange. Suitable reinforcement is then pushed into the fresh concrete to complete the CHD pile.
The rig is fully instrumented with all relevant piling parameters, such as torque, penetration and withdrawal speed, penetration, concrete pressure, flow and consumption displayed on a screen in the driver's cab. All data is stored and can be downloaded for future interrogation .
Different size tapered formers are available to suit different sites' requirements and varied ground conditions. At Southport, Bullivant installed 275/600 piles, the first figure relating to the diameter, in millimetres, of the central solid core and the second figure to the outer diameter of the continuous helical concrete flange.
All CHD piles at Southport are between 8m to 10.5m long and driven through 2m to 3m of contaminated fill and into underlying medium dense brown silty sand. The vertical and up to 15degrees raking piles, reinforced to full depth with a single T32 bar, are arranged in capped clusters of four to 98 piles with many of the reinforced caps, ranging from 2.3m square up to 22m by 10m, linked together with tie beams.
Bullivant was able to cut a week off its original five week programme. The company replaced nearly 40% of those original 650 CHD piles, designed just for compressive loading, with 200mm square section driven precast concrete piles. London-based Geotechnical Consulting Group was retained by Bullivant to check and approve its CHD pile option, which was tested on site prior to starting on the revised four week scheme.
Both the vertical and raking CHD piles were designed for a compressive working load up to 400kN and 200kN in tension. 'The test pile only deflected 2.38mm with twice its compressive working load and extended 2.44mm at one and a half times the tensile working load,' Roberts says. RB