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Displacement Theory

Soil displacement piling has many potential advantages, though engineers have been cautious. But recent load tests on the technique at a big site in the south-east have thrown up excellent results, surprising even leading advocate Cementation Skanska. Paul Wheeler reports.

Soil displacement piling was heralded as a breakthrough technology in the mid-1990s.

Its proponents pointed out that the piles had a higher load-bearing capacity than equivalently sized conventional bored piles; and the lack of soil arisings had obvious benefits, particularly given the then rapidly increasing cost of landfill.

Yet uptake, at least in the UK, did not match expectations.

This in part was because UK soils are generally “too good” for the technique, believes Cementation Skanska engineering director Jamie Dunbar.

In truth, the technique is best suited to loose gravels and soft alluvial soils.

These are ubiquitous in the Low Countries, where displacement piling emerged as a development of auger displacement “screw” piles.

In the UK though, such ground conditions are generally confined to large river estuaries.

And while Dunbar, who heads up the technical, resource and plant streams of the business, has run many successful soil displacement piling jobs alongside the Clyde, Mersey and Humber, projects in London and the Thames Estuary - the biggest potential market - have, until now, remained elusive.

A major project for a development near Woolwich in south-east London may change that, not least because, with a foundation contract value of £800,000, it is by far the largest soil displacement piling project that Cementation has undertaken.

“While we knew that you get more bang for your buck with soil displacement piles than other augered piles, the tests were nevertheless an eye opener”

More significantly, the project has included an extensive load testing programme that will help to allay the concerns of those design consultants who remain cautious regarding the high capacities reported to date.

Back analysis of recent jobs completed by Cementation in Glasgow provides design parameters that can make individuals very nervous, explains Dunbar.

Even so, with pile loads up to 1,300kN at the Woolwich job, he adds that Cementation too had some initial reservations over the ability of the 510mm diameter, 7m-long piles to carry the required loads within the settlement limitations.

In fact, the £60,000 pile testing programme proved the load capacity to be very much higher than their design predicted.

Maintained load piles tests achieved loads of up to 3,500kN - a mammoth capacity for such relatively slender and short pile.

And as Dunbar points out, the aim had been to test the piles to geotechnical failure, yet they all failed structurally first.

“While we knew that you get more bang for your buck with soil displacement piles than other augered piles, the tests were nevertheless an eye opener, with capacities correlating more closely to those of driven Franki piles,” says Dunbar.

This exceptional performance further increases the green credentials of soil displacement piles, in that for any given load, soil displacement auger piles are smaller than CFA piles.

There is less concrete going into the ground and fewer truck movements to the site; and with virtually no spoil arisings, truck movements from site and material to landfill are effectively eliminated.

Gloucester_piling15

According to Dunbar, clients are increasingly engaged in the sustainability agenda, and the potential to reduce concrete and wagon movements is starting to count when clients appraise one technique against another.

A further bonus, Dunbar points out, is that the technique does not generate the noise associated with most driven piling.

Notwithstanding these impressive attributes, Dunbar warns that soil displacement piling is not a panacea.

Not only is the process very ground dependent but, he says, you need to put a lot of engineering thought into its use.

“The parameters are not well understood,” he explains. “Indeed, it is questionable whether the current design parameters are the right ones; so it is a technique that needs a lot of engineering input.” Installation too needs closer and more experienced supervision than other techniques.

Nevertheless, Dunbar predicts that soil displacement piling will become a much bigger part of the market over the next 20 years.

Clients, he believes, will start questioning the economics of large bored piles where you simply do not get as much load for your pound.

In fact, Dunbar maintains that in an industry that has become more conservative in recent times - despite better technical understanding, better testing and better construction quality control - the drive towards better utilisation of materials could provide a catalyst for more novel and innovative foundation design in the future.

 

Displacement piling put to the test

Cementation’s largest soil displacement piling contract to date, in south-east London, has included nearly a thousand piles.

Some 840 of these are for buildings that include living units, education and sports facilities.

Main contractor is Skanska Building - London & South East.

Ground conditions are 0.5m of made ground, overlying soft alluvial clay to around a depth of 4.5m, with river terrace gravels below this providing a good bearing layer for the piles.

Pile loads vary from 350kN to 1,300kN and the majority of piles are just 510mm in diameter, increasing to 610mm where the piles are required to accommodate universal columns.

Despite the variation in load, piles uniformly extend to between 6.5m and 7m.

Under the testing programme, three pairs of piles of 410mm, 510mm and 610mm diameter were tested.

Maintained load tests were carried out on one pile of each diameter, and these results were correlated to non-destructive Statnamic tests carried out on the other of each pair.

The rationale behind the testing was that Skanska Building’s construction programme was very tight and Cementation wanted to confirm the pile design and performance across the site in advance of the main production.

Additionally, it wanted to investigate and evaluate Statnamic as a relatively rapid and cost-effective alternative to conventional static load testing.

To the surprise of some within Cementation, the correlation between the Statnamic and maintained load testing was good.

Engineering director Jamie Dunbar says that although the two methods measured slightly different displacements, the shape of the graphs was almost identical and the responses were consistent.

So much so that “all are satisfied that there is a correlation” and many previous concerns over the validity of Statnamic have been allayed.

 

A history of displacement piling

The first screw piles were developed in the US and used as foundations for seaside piers in the 1830s.

The process was basically analogous to a wood screw, creating piles with characteristic flights.

Screw displacement piling evolved into systems such as the Atlas screw pile, which proved particularly popular in the Netherlands and Belgium from the early 1960s. Clever tool design in the Lowlands led to a big evolution on the screw displacement process and the birth of soil displacement piling in the 1990s.

Soil displacement piles are essentially “forced” CFA piles, in which the soil is displaced radially around the bore.

This leads to some soil improvement and increased end bearing and frictional resistance. It is particularly suited to granular soils and very soft clays.

In the mid-1990s installation pushed commonly used rigs to the limits, resulting in excessive wear.

Since then, piling machines have got bigger and more powerful, with the trade off that today’s 22 to 25tm rigs need a properly engineered working platform.

Wear on tools remains high and must be properly accounted for both economically and logistically.

Cementation uses Germany’s Bauer Maschinen’s full displacement process (FDP) system.

It is suitable for pile loads up to 1,500kN and diameters from 410mm to 610mm.

The ratio of torque to resistance, or the alpha value, is the key installation measure, similar to the set on a driven pile.

The installation process itself is automated, with the rig operator driving to computer, giving consistency and quality of product.

However, soil conditions have to be right.

Bell_Marsh_0021_safety_checked

As a rule of thumb you cannot penetrate granular soils with an N value over 30, while in clays the undrained cohesion must be less than 100kN/m2.

So, as Cementation’s Jamie Dunbar puts it: “The penetration determines the load capability, not the other way round.”

The displacement tool is screwed into the ground by a powerful, high torque piling rig.

The ground is displaced laterally and downwards, resulting in very high-end bearing and shaft friction values from much shorter piles than a CFA design in the same ground conditions.

As the auger is withdrawn, concrete is pumped down the hollow auger stem, in the same way as a CFA pile is formed.

The pile formed is a cylindrical shaft into which a circular section reinforcement cage can be inserted.

Because the soil is displaced sideways and downwards there are essentially no spoil arisings.

For the Woolwich project, Cementation developed a new flared head detail.

Flared pile heads have emerged as a way of simplifying the connection between the pile and the ground beams.

Normally flared heads are V-shaped, but Cementation’s new tool creates a cylindrical head of about a metre in diameter and half a metre deep, exactly centred over the pile shaft.

Its more substantial mass enabled main contractor Skanska Building - London & South East to place pre-cast beams directly on the pile heads without having to break out the pile caps at all, greatly speeding up the next stage of construction.

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