An extensive piling programme on a High Speed 2 London site is being eased by an alternative to traditional pile cropping.

Neatly aligned rows of straight reinforcement bars run between temporary sheet piles along the High Speed 2 (HS2) site in Camden, north London.

These exposed cages indicate the positions of a small fraction of the rotary bored piles that are being installed along what will form the project’s Euston approach.

This is the section of HS2 track that will take the high speed rail line from the Euston tunnel south portal into Euston Station.

The piles are being installed to support and strengthen the tunnels, concrete box, headhouses, bridges and retaining walls which form the structure and foundations of the new railway.

Overall, more than 1,700 piles will be used at the tunnel portal and shaft sites.

HS2's Euston approaches site

Traditionally, the construction of these foundations would involve overpouring concrete to cast the piles. This is necessary to ensure sound, homogenous concrete at the required cut off level. When constructing piles using the traditional method, the concrete slumps when the temporary casing is removed.

As a result the pile must be overpoured to ensure the made ground does not penetrate the wet concrete below the cut off level. Excess concrete would then have to be trimmed down to the required cut off level.

Pile head cropping is carried out once the poured concrete has cured. Traditionally this has been done by operatives using jackhammers.

But the manual work of trimming the piles can cause health problems such as hand-arm vibration syndrome, hearing problems and silicosis.

Mechanical pile breaking methods are also available and offer some benefits over the traditional method of using a jackhammer. But often these still use some form of drilling or equipment such as excavators or cranes to assist with the cropping.

To avoid using these techniques altogether, a new so-called “zero trim” technique has been developed by the team currently delivering the HS2 Euston tunnels and approaches and the Northolt tunnels in west London.

While this method still involves overpouring the pile, the team no longer has to wait for the concrete to harden. Instead, the excess concrete is removed by a vacuum excavator known as a “vac-ex” while it is still wet.

Cementation Skanska project director Deon Louw explains the process: “What we do is, once the pile is bored to the correct depth, we put the reinforcement cage in and then we concrete up.

“At that point we use the vac-ex to hoover out the waste polymer – we’re using support fluid on these piles because they’re so deep at 48m below piling platform level – and concrete down to the right level.

“The following day when the concrete is set, we just crack the seal on the casing, backfill it with shingles and pull the [temporary] casing out.”

The sheer number of piles needed for the project prompted the project team to seek improvements to the piling method.

The team decided to focus on the key challenges relating to the risks and quality issues that can occur during pile cropping. These include noise during drilling and breaking, as well as health and safety risks associated with cropping using hand held equipment.

In addition, traditionally when a pile is cast, the concrete in the top layer is contaminated with polymer or bentonite support fluid that is used to retain granular soil. So the concrete is overpoured and “flushed over” the top of the temporary casing until sound, homogenous concrete is seen. This process creates a mess on the piling platform and wastes more concrete.

To prevent these issues, main contractor SCS Railways, piling subcontractor Cementation Skanska, plant supplier Hercules Site Services and designer Design House collaborated on the alternative “vac-ex” method.

SCS Railways comprises Skanska, Costain and Strabag, while Design House is made up of Arup, Typsa and Strabag.

SCS Railways senior works superintendent Lee Piper explains: “There are not many jobs that you’re on the project piling for three years. Traditionally, you would be on a project piling for six months maximum; even on big retail projects you’re not piling for a long time. But we’ve got the time here to invest in this and it’s the right thing for us to do. And obviously our stakeholders are very close to us. So, from a noise mitigation point of view, everything made sense.”

Design House was supportive of the proposal to reduce the need for breaking down concrete to cut off levels for the piles.

But before Cementation Skanska was allowed to use the method on site, the designer first wanted proof that the concept worked.

Getting Hercules on board enabled the project to go ahead. On a previous project where a similar approach could have been taken, Louw said the team had been unable to find a willing equipment partner. The perception had previously been that the vacuum excavation of concrete could damage the suction excavator. Hercules, however, was happy to give it a go.

SCS initially fine-tuned its proposal to make it more robust and compliant with the Institution of Civil Engineers Specification for Piling and Embedded Retaining Walls.

Following this, Design House said that there remained some uncertainty about the effectiveness of the method, and it wanted to see it explored through a phased trial.

The first phase of the proof of concept was an off-site trial carried out in November 2020 at Hercules’ yard in the Cotswolds.

The goal was to demonstrate the effectiveness of the vacuum excavator at removing the concrete and to calibrate the power required so as to avoid over-vacuuming.

To do this, three 1,200mm diameter manhole rings were sunk 1m below ground level on the site and filled with concrete to replicate rotary bored piles.

“In the first one we put a bit of polymer in the bottom and poured concrete on top of it to mix it up. For the second one, we had a small dummy cage made and we put that in, and then we concreted that and put a bit of polymer on top of it. And the third one, we concreted up with just a bit of polymer on the top,” Louw says. The concrete was then removed with the vacuum excavator.

“This was because Design House wanted to make sure that the machine had the capability to actually remove the concrete from all three piles.”

The trials were a success.

The zero trim piling method in use at the Euston site

The next step was to test the method on two contiguous bored piles at the Euston site. Additional testing was also done following vacuuming to check the quality and integrity of the piles.

Concrete bleeding in bored piles, when water from the concrete mix rises to the surface, can cause defects. To test for bleeding, bucket samples were taken from the top of the piles to check for cement and water content.

The results showed signs of bleed but within the expected range and were therefore acceptable. It was also noted that the bleed remained at the same level over time. In a second test, a 1.5m long core sample was taken from the centre of each pile by diamond drilling seven days after the pour. This was then tested for segregation and compressive strength to make sure there was sound concrete at the cut off level. The results confirmed that both were within the specific tolerances.

Pile exposure was also tested by excavating 1.5m below cut off level to allow inspection of the cover zone, or the side of the pile, again to make sure there was no bleed or segregation.

“The designer was also concerned that the annulus around the outside of the reinforcement was where concrete was going to be trapped. But the concrete is wet anyway – fairly high slump – and as we were sucking it, it was just pulled into the vacuum and was fine,” Piper notes. “We found out that the vac-ex could only suck roughly 6m of overhead, but we developed an answer to that,” he adds.

“We’ve come up with an ingenious design to attach onto the vacuum excavator to try to get it to 18m, which we’re trying out.”

This way, in addition to the contiguous external wall piles, the zero trim method could also be adopted for the tension piles on site, for which the cut off level is 18m below the piling platform level.

Louw adds: “We have done some further trials on deep cut off piles and the signs are positive that concrete can be removed from a depth of circa 18m below piling platform level.”

Overall, the technique can be applied to other foundation types than the rotary bored piles it has been used on so far. It can also be applied in different ground conditions.

Increased accuracy of achieving the cut off level is a key benefit of the zero trim pile method. The way this is achieved during the vacuuming process, however, is quite simple – by using a tape measure.

“We still have the temporary casing in the ground, and we try to get the toe or bottom of the casing just surrounded by the cut off level. Then we’ll take another level at the top of the casing, and we work out how far down we need to vac-ex it. And we just keep checking the tape,” Louw explains.

The "vac-ex" and tape measure at work

“We leave it about 70mm above cut off level, so that when you 'hydroscab'  it, you’re still within the tolerance that’s required for the embedment into the capping beam.”

After blinding and hydro-scabbling with a high-pressure water jet you are then left with a clean result, with even concrete and straight steel reinforcement cage bars.

“Integrating the capping beam reinforcement is thus made easier,” notes Louw. “Because the bars are nice and straight, it’s easier to feed the bar for the capping beam through them.”

Piper adds: “Traditionally when piling, we’ve worked out that there’d be 10% non-conformance. So on 100 piles, probably for 10 of them you would have to either redrill a reinforcement bar or the integrity of the pile might be affected.”

For the Euston approaches site, SCS has calculated carbon savings of more than 840,000kg as the method uses less concrete and steel.

“The concrete, as detailed in the specification, we would normally be overpouring say for instance by 1.2m on a 1.2m diameter pile; we’re not putting in 1.2m overpour now, because we’re confident in the concrete and are able to only overpour by 400mm. So, we’re saving 800mm of concrete per 1.2m pile on 1,700 piles. When you calculate that in vehicle movements and in the production of concrete, it’s a significant amount,” Piper says.

“When the concrete gets sucked out from the vac-ex, it will also go into a recycler. The recycler will split the cement and then the aggregate will go back in the pile.”

Zero trim piling typically adds between 30 and 60 minutes to the piling process, depending on the diameter of the pile and its depth below the piling platform.

But so far the method has added no additional time to the piling programme, and there are expected to be time savings in the pile breakdown process. The team has successfully installed 456 piles using the zero trim method.

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