Fugro has raised the bar in CFA (continuous flight auger) pile testing during groundworks for a new high rise development in London, UK.
Using Osterberg Cell (O-cell®) bi-directional testing, Fugro achieved and sustained a maximum gross load of just over 16 MN in a 900 mm-diameter CFA pile. Believed to be a new test load record for CFA piling in the UK and Europe, at 33.61 m long, it is also one of the deepest CFA piles of this diameter to be installed and tested in the UK.
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 3 m above the piling platform level after backfilling. A single 620 mm-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 600 mm are considered above average, making the 900 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 (<1200 mm); 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, 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.
As the leading proponent of O-cell pile testing for over 25 years, Fugro has achieved other landmark feats using the method in demanding applications at test loads not previously considered possible.
Working for Bachy Soletanche at the 67-storey Spire London project, Fugro recently set another UK record, this time for static load testing of piled foundations. Using the O-cell, they achieved a maximum loading force in excess of 100MN at what will be the tallest residential tower in Western Europe standing 235 m high.
Elsewhere, Fugro undertook its deepest CFA pile test to date of over 50 m for a project in Miami, Florida (USA). They have mobilised over 300 MN in bored piles at several locations around the world and over 360 MN in a series of barrette tests in Dubai (UAE).
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