Wembley Stadium may be finished but the regeneration of the surrounding area continues apace - with displacement piling in high demand.
Roger Bullivant south east area operations manager Dominic Lovelock is working flat out - with four big auger rigs installing 1,000 continuous flight auger (CFA) piles a month and a lot of continuous helical displacement (CHD) piles besides, in and around London.
One rig and its crew are in Wembley on a job near Stonebridge Park and the North Circular Road, where Rydon Construction is building a multistorey housing, community and health centre development.
Bullivant was contracted to provide a grid of 400 CHD piles across the 80m by 60m site.
Its helical pile is a quick method of getting a large number of piles with high load capacity into the ground. The absence of muck-away costs - up to nearly £250 per load - have added to the cost-efficiency of the CHD system.
fiThere is virtually no spoil arisings from a CHD pile, whereas about 3m 3 comes up from a CFA equivalent and the quantities to remove get pretty big where sites have several hundred pile locations. CHD piling is particularly good where there is a risk of contaminated ground and high disposal cost. fl Bullivant's Wembley site called for more than just CHD piling.
The site was sloping when Lovelock's team arrived to first install a 105m long and 12m deep contiguous piled wall along one side. That was done during December last year - with a continuous capping beam cast in situ - before Rydon excavated the site from about 5m beside the contiguous wall, to less than a metre at the opposite end to produce a flat site.
Rydon had removed 2,000 loads of muck from site by the time Bullivant returned in February to start CHD piling, which was able to progress continuously without being held up by spoil clearance.
Pile design, to Rydon's given loadings, was done by Bullivant.
According to Lovelock, the CHD system is limited to a maximum depth of pile of 19.5m. But this is more than compensated for by the relatively high surface area of the helical pile.
fiThe increased surface area of the pile's helical flight gives a far better loading capacity than a conventional displacement pile, fl Lovelock says. fiWe limit ourselves to a 1000kN load on our 300/600 pile (a 300mm diameter core with an overall flight diameter of 600mm), but can go up to 1350kN with a 400/700 bullet. fl Both types of 'bullet' - the CHD pile cutting head - have been used at Wembley to form 600mm and 700mm diameter piles, to depths varying from 11m to 16.5m across the site.
The process is quick - at its peak Bullivant was finishing 30 to 35 piles a day.
According to Lovelock the three weeks needed to install 400 CHD piles using its 7001 machine compares with a five-week programme using a conventional rig.
fiIt is like driving a car, in the sense that you get a feel for when the extra torque is needed, fl says Bullivant's 7001 rig operator at Wembley, James Jackson. fiTo begin with, the bullet goes down rapidly at about 40 rotations per minute, then, you drop down a gear and go to high torque and low speed, at about 4m down on this site as the stiffer clay is reached. fl It takes only a couple of minutes to get down to 16m, but the process is a little slower as the pile is formed, with concrete pumped through the bullet head as it is withdrawn.
To date there have been zero filost time accidentsfl at Stonebridge Park. Roger Bullivant has a CSCS-Gold Standard accredited training centre which ensures that all site personnel are properly trained to work safely on construction sites throughout the UK.