A site in east London has broken the record for the UK’s most heavily loaded pile.
The pile was one of two which were being tested on the site of the prospective Spire skyscraper in Canary Wharf in London.
Using a maintained load static pile testing method, the load on the pile reached 108MN with the other reaching 103MN. Previously, the record is thought to be held by an 80MN pile on the Farringdon Crossrail site.
It is also thought to be the first time this type of test has been used on a pile reaching the chalk layer in London.
“As far as I’m aware, there are no other O-cell tests which have gone into the chalk,” said Robert Bird geotechnical engineering director Tim Hartlib. “So it provides us with an opportunity to learn from testing piles which have these loads in the chalk and review the data and feed that back via technical papers into the industry. So it allows us to give something back to the wider industry.”
The two 57m long piles are 2.1m in diameter and have been tested to verify the ground investigations. The team comprising consultant Robert Bird Group and specialist geotechnical contractor Bachy Soletanche will now be able to use the results of the tests to tailor the design of the piles on site.
Two Crossrail tunnels run beneath the site of the 67 storey building. One clips the corner of the site, however the other runs right through its centre. Because of strict loading restrictions for buildings above the tunnels, the building will be supported by a stiff, 4m thick reinforced concrete raft which bridges over the central tunnel.
The raft is then supported by 66 rotary bored piles piles on either side of the tunnel and around the site. These piles vary in diameter from 1.8m to 2.1m and are up to 67m deep. The working load on the piles is only around 40MN, but due to an applied factor of safety of two, the piles need to be tested to at least double this.
Due to the high loads involved the static method needed to be used in preference to a top down test where piles are loaded at ground level. However this method can only reach around a maximum of load of 30MN to 40MN.
In the tests carried out, a 600mm diameter O-cell – a hydraulic jack – was embedded in the pile 12m from its base. At this point the reinforcement cage is split in two, so the two ‘halves’ of the pile are free to move independently. When loaded, the jack works to push the two sections of the pile apart, allowing the capacity of the pile to be measured – the lower section working in end bearing and skin friction and the upper part in skin friction.
In both tests, the lower part of the pile settled by 20mm to 22mm with the upper part being pushed upwards by around 200mm reaching the maximum extents of the jack.
The second test which achieved 103MN, started on Tuesday morning and took around 48 hours to complete.
“The pile is loaded incrementally,” explained Hartlib. “Only when the settlement slows to less than 0.1mm/h can we then increase the load again. This can be up to three hours near the end of the test when it is reaching its capacity.”