The record-breaking test took place during groundworks for a new high rise development in London, UK. Fugro used Osterberg Cell (O-cell) bi-directional testing to achieve and sustain a maximum gross load of just over 16MN in a 900 mm-diameter CFA pile. At 33.61m long, it is also one of the deepest CFA piles of this diameter to be installed and tested in the UK, said the company.
The breakthrough came during preliminary pile testing to validate and optimise the geotechnical design and pile installation method for the East London development. Fugro proposed the use of O-cell bi directional testing, which is frequently preferred for piles in clay and soft soils typical of the prevailing ground conditions.
In contrast to traditional top down testing, the O-cell static load test method works in two directions, with the O-cell assembly positioned at a balance point between the upper and lower pile elements. This allows the generation of test loads in excess of conventional loading capacities without the need for anchor piles and reaction beams at ground level, giving scope to increase the test load limits.
The concrete stresses during loading are half those necessary in a traditional test where the load is applied at the pile head. The technique also involves minimal surface equipment, making it ideal for sites with restricted access or headroom and where safety might be of concern, such as beside roads or railway lines.
The pile was installed within an excavation and the load test performed from 3m above the piling platform level after backfilling. A single 620mm-diameter O-cell was installed at a design depth where approximately equal reaction would be available above and below the O-cell assembly within the London Clay deposits.
“Long CFA piles with diameters larger than 600mm are considered above average, making the900 mm-diameter pile on this project especially challenging,” said Dr Melvin England, manager of foundation testing at Fugro.
Fugro undertakes around 400 O-cell tests a year around the world, citing safety and adaptability among the advantages, in addition to significant cost savings as the test load and pile dimensions increase.
A CFA pile solution is generally more economic than a bored pile for large diameter piles (<1200mm); if the required pile capacity can be achieved, then the CFA installation method may be used to advantage. Since bi-directional tests become more cost-effective with increasing test load, O-cell testing adds value engineering by being able to verify pile capacity in the most efficient manner.
“The use of CFA combined with O-cell testing in soft soils can also reveal the end bearing in a much better way than traditional loading methods,” said England.
In evaluating the geotechnical behaviour of piles in soft soils, traditional top down methods do not always give full credit for the capacity contribution of the end bearing. However, Fugro said that the O-cell is ideally suited to soft ground analysis as the loading schedule can ensure the test measures both the skin friction and end bearing with more certainty.
“Based on our long experience and the accuracy of engineering data delivered, this approach can provide the confidence to optimise piling design across a project,” adds England.