The HIPER® pile was designed as the combined foundation and heating/cooling system for the HS2 Euston Station Welfare building, as part of the HS2 Euston station development. Wentworth House Partnership (WHP) was appointed by Mace-Dragados Joint Venture (MDJV) as the Structural Engineers, providing the engineering design for the HIPER® piled-raft foundation, with Keltbray employed as the specialist contractor. This was the first ever full-scale deployment of the HIPER® pile, utilising all elements of the innovation. HIPER is an acronym and stands for Hollow, Impression, Precast, Energy-generating and Reusable pile.
Scope of works
The building comprises modular units, supported by a structural steel frame, all of which is supported by a 600mm thick slab partially cast in 90m3 Earth Friendly Concrete (EFC). The slab was founded on 40no, 900mm dia HIPER® piles. The scheme was developed to protect a number of sensitive existing underground services beneath the footprint of the site.
A total of 35no piles were constructed as cast-in-situ HIPER® piles and the remaining 5no piles were constructed as pre-cast HIPER® piles. All piles were constructed as 900mm, typically with a 500mm central void to depth.
The HIPER® piles reduced the volume of concrete used by 41% compared with a conventional solid pile scheme with the impression element of the piles reducing the pile length by 15%. Overall, the reduction in embodied carbon from the original scheme intent was 35%.
All piles were impressed with Keltbray’s bespoke impression tool which increased the shaft capacity of the piles by 40% over and above a standard bored pile design, founding within the London Clay and Lambeth Group formations.
The hollow piles were designed to support the ground source heat pump scheme with all piles being used as thermal heat exchangers. Four HDPE u-loops were installed within the void of each HIPER pile after the piles had been exposed by the groundworks team. The voids were filled with either water or a non-cementitious thermal grout with the pre-assembled thermal GSHP loops being lowered to the toe of the piles. Once this had been completed the GSHP loops were thermally welded to a single flow and return circuit at ground level and fed to the manifold underneath the raft. This method of converting a hollow pile to a thermal pile removed all risk of damage to the loops which would otherwise be present during the construction of a conventional solid cast in situ thermal pile.
The thermal & structural piles will be monitored throughout their working lifetime (~10 years) and data collected in order to improve the understanding of their performance for integration into future schemes.
Challenges and Solutions
This project marked the first HIPER® Pile contract and offered all elements of the technology. Monitoring and instrumentation equipment was also deployed to support the ten-year monitoring plan. The installation of these innovative and instrumented piles required significant planning and coordination between the delivery team, site logistics team and a wide range of supply chain partners.
Ground investigation reports revealed that the competent London Clay was located approximately 5m below platform level and the Lambeth Clay at 23m depth. The piles were designed with an impression shaft to increase the shaft capacity by 40% and therefore, shorten the piles to avoid the risk of encountering water bearing strata.
The piling mat was heavily constrained with a retaining wall immediately to the north, a public school building neighbouring the southern edge of the site and a live services corridor to the west. The piling platform itself was approximately 75m wide by 22m long.
THAMES WATER AND OTHER THIRD PARTY STAKEHOLDERS
A live Victorian brick sewer was located through the middle of the piling platform orientated east to west. A 2m exclusion zone either side of the sewer was imposed on all large plant which restricted the sequencing, access and positioning of plant during the piling activities.
Keltbray manufactured the pre-cast HIPER® piles at our facility in Silvertown and delivered them to site on a just-in-time basis.
Initial thermal tests on the piles have shown a thermal conductivity ranging from 2.30W/mK (thermal grout filled) to 2.70W/mK (water filled). These results have shown that the water filled void is better at dispersing heat (high conductivity, lower thermal storage potential) and the grout filled void is the opposite (lower conductivity but higher thermal storage potential).