First use of a new piling system for High Speed 2 offices in London could deliver 80% of the building’s heating and hot water needs as well as supporting the structure. 

Once complete, Mace Dragados’ new site offices for staff working on the construction of High Speed 2’s (HS2’s) Euston terminal in London will look striking compared to normal site cabins. 

But the architecturally designed temporary structure’s most striking design feature – the foundations – will soon be hidden from view. They will have a lasting impact on the building beyond their normal support role.

The new building is the first live site to use Keltbray Piling’s Hiper Pile system, which it has jointly developed with City University of London using funding from Innovate UK. 

It has been many years in the making (see box). The pile’s developers believe that it could offer a step change in sustainability and programme certainty, as well as providing the building with geothermal heat, reducing operational carbon.

The Hiper Pile brings together a number innovations but it is essentially a hollow pile that can be used to produce geothermal energy. Its outer face has protrusions, known as impressions, that generate greater skin friction – to mobilise greater load capacity – than a standard smooth faced pile.

We use a special casing and formwork to cast the pile

Keltbray’s piling work started just before Christmas and was due to finish in January.

A total of 40 Hiper Piles are being installed – 35 are cast insitu piles and five are precast concrete. All of which will become geothermal piles supplying up to 80% of the new offices’ hot water and heating needs.

Mace construction lead and operations director Steve Burrowes says he expected the site team to complete two to three cast insitu piles per day once the site team’s technique became established. 

For the Mace Dragados office, the piles are being installed to 22m below ground level and are predominantly in the London Clay. They just toe into the underlying clays, silts and sands of the Lambeth Group. The top 3m to 5m of ground at the site is formed by made ground and gravels and the piles are cased to that depth to prevent groundwater inflow. The impressions are formed in the clay formations below the casing depth.

This technology creates the opportunity to fundamentally change the sector

The main piling work on site follows on from the installation of a cast insitu test pile on the site in late November last year, but the technique is still very new. 

The cast insitu piles are rotary bored. The impressions are made within the London Clay using a specially designed tool with hydraulic rams that push horizontally into the clay to create the profile.

Four rows of impressions around the circumference of the 900mm pile are formed with each lift of the tool. Each impression is 125mm by 50mm square and 70mm deep and the rows are 700mm apart vertically.

“We use a special casing and formwork to cast the pile,” explains Keltbray Piling technical manager Asha Panchal. 

Modular construction and the hi-tech piles mean the site offices are a far cry from normal site cabins

“First the liner, used to form the 500mm diameter hollow middle section, is placed into the bore. We splice together two 1.5mm thick galvanised steel tubes which are normally used in culverts. Spacers are attached to the outside to keep the liner central in the bore.

“The base plug is then cast and we use a hot water DC3 concrete with accelerant for this and place around 1m3 to form a solid base approximately 1.5m high. This creates the formation level for us to cast the hollow pile annulus from.

“The concrete for the annulus is a C32/40 strength flow mix that is placed with a tremie to pile cage toe level. The cage is then placed and we flow the concrete around the remaining section of the pile.”

The final five piles for the project will trial the precast Hiper Pile solution. 

Panchal says this could have programme advantages as well as offering factory controlled quality.

She says that the precast piles will be installed to the same finished depth and diameter as the cast insitu ones but the 800mm diameter precast units have a 200mm diameter annulus and will be grouted into place.

Installing the piles is not the end of the story. Around 10% will undergo integrity testing and ongoing analysis is planned as the piles are put into service.

“Low impact strain testing wouldn’t work with these piles as the signal would just bounce due to the hollow design,” explains Panchal. “Sensors have been installed in four piles so that we can carry out the testing using thermal integrity profiling (TIP).”

Looking down the pile to the impressions attachment

TIP uses the heat generated as the concrete cures to assess the integrity and quality of drilled shafts.

“Strain gauges have also been installed for long term testing,” says Panchal.

The completed piles must also be filled with water to turn them into geothermal piles and that work will be undertaken by G-Core during the groundworks phase.

Although it will be a while before the full results of the work are known, the excitement of Keltbray’s site team to be putting the system into use is palpable and they are clearly proud to have got to this stage with their research.

The six storey building the Hiper Piles will support is using the latest offsite construction techniques with units manufactured by Premier Modular in East Yorkshire. They have been designed by Fraser Brown Mackenna Architects.

Once complete, the 5,500m2 building will accommodate up to 2,500 staff working on Mace Dragados contract to build the new HS2 terminus at Euston station.

Despite being described as temporary, the offices are likely to be used for up to 10 years, says Burrowes. They will replace the existing units, which look like more commonly seen temporary site offices.

There is much to do on site before the first workers move in this summer. 

Keltbray is not just delivering the piling work. It will also undertake the follow on groundworks ahead of the installation of the new office. This is expected to start in June with completion due in August.

Groundwork for the site also includes installing a utility corridor, to the north of the office piling site. Through this a water main will be diverted along with other services to simplify other elements of taking HS2 into Euston.

The material that currently forms the piling mat for the Hiper Pile installation will not be removed from the site when piling finishes, but will instead be reused by Keltbray to bring ground levels up around the new utility corridor.

The HS2 project is the first live scheme to use the Hiper Pile technique which brings together a number of innovations. 

The pile has been the central focus of Keltbray Piling technical manager Asha Panchal’s career. She originally started on the initiative during her days as a student at City. 

The name explains the different aspects – Hiper stands for hollow, impression, precast, energy-generating and reusable.

According to HS2 innovation manager Heather Donald, the combination of five innovations makes the technology particularly interesting. “The piling industry has been slow to innovate in general but this technology creates the opportunity to fundamentally change the sector,” she says. 

There are a number of drivers for the Hiper Pile innovations. The hollow design reduces the amount of material needed by up to 70%, while the impressions improve shaft friction capacity – the load the pile can take – by up to 40%. The hollow centre can be used as a geothermal pile and Keltbray’s analysis suggest that this approach delivers a 60% increase in thermal conductivity compared to piles with cast-in geothermal pipework.

The hollow design means that the pile can also be bored through in the future if a deeper pile is needed to support more load when the building is redeveloped. The precast pile could also be removed and reused.

“It is the five innovations that are brought together that makes Hiper Pile truly transformative,” adds Donald.

Panchal says using Hiper Pile on the HS2 project is only part of what the system can offer and she is eager to demonstrate its capabilities. It is a desire that Donald shares and she believes that the system will be put into action on the main HS2 Euston station, as well as other stations on HS2  phases 1 and 2.

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Tagged with: Energy hs2 Low carbon Piling

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