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Geotech special: Under floor heating

Geothermal energy solutions could be a major weapon in the fight against climate change. Gemma Goldfingle visits Bolton Sixth Form College to see how relatively new geothermal piles are installed.

Bolton Sixth Form College’s new £15M city centre campus, being built by BAM Construction, will be a modern purpose-built education facility with the latest cutting-edge equipment and technology for its 1,250 students, including an underfloor geothermal energy heating and cooling system.

Like many local authorities, Bolton Council has adopted its own version of the Merton Rule, a planning requirement developed by the London Borough of Merton which stipulates that all new developments must generate at least 10% of their energy from renewable sources.

Ideal geothermal energy piling

Mechanical and electrical consultant on the project Gifford recommended that the client opt for the relatively new geothermal piling option to contribute to the 10% renewable criteria. With a confined site, geothermal energy piling was ideal because it would not demand extra room unlike boreholes that would require a large rig onsite.

Concrete geothermal piles incorporate plastic pipework to transport and store geothermal energy from the surrounding ground.

“My major concern was that it would slow down my already tight piling schedule and affect the overall programme.”

Craig Herbert, BAM

Geothermal piles can also be installed to shallower depths than boreholes which use traditional open and closed loop energy circulation systems. These often have to be bored to depth of up to 120m and take four to five days per borehole to complete.

“It is the first project I have worked on using geothermal piles,” says BAM senior site manager Craig Herbert. “My major concern was that it would slow down my already tight piling schedule and affect the overall programme for the project.”

BAM called in specialist geothermal consultant Geothermal International to ensure the job ran smoothly. “We work with piling contractors to make sure the pipework is inserted in the most effective way, with no damage and as quickly as possible,” says Geothermal International marketing and sales director Patrick Sheriff.

“Installing a geothermal pile should not take substantially longer than a normal pile.”

Logistics is the overriding factor

Geothermal piles are designed with load bearing capacity in mind. The depths and widths of the piles are determined by their structural requirements, with the geothermal elements built around them.

To support the three storey education facility 170, 450mm diameter and 23, 600mm diameter piles had to be installed at depths of between 10m and 13.5m. Using this specification and information on the level of heating and cooling required by the client, Geothermal International calculated that 96 of the smaller diameter piles would need to be installed with plastic pipework to achieve the 40kW peak load needed to contribute to the 10% renewable criteria.

“The integrity of the plastic pipe is essential as any damage means the geothermal aspect will not work.”

Neal Willis, May Gurney

Logistics is the overriding factor when deciding which piles are to have the geothermal elements within them. Options that are easy to install and do not affect sequencing are selected. To install the geothermal element a regular pile is constructed.

Continuous flight auger (CFA) piles were used on this site as the granular ground conditions to depths of 8m suited this method. Rotary drilling would have required bentonite, therefore, CFA was the preferred option.

Once the concrete was pumped and the pile formed, the 10m long U-shaped pipe, which will circulate liquid that transfers the ground heat inside the building, is manually inserted into the wet concrete to depths of 10m.

The concrete mix used for the pile was tailor-made by piling contractor May Gurney to give a slightly longer period to insert the pipe before the concrete sets.

Risks of damage

Although the 32mm diameter plastic pipe − similar to the piping used in gas distribution − is quite rigid, May Gurney found that attaching the pipe loop to a sacrificial 40mm diameter steel T-bar helped while manually plunging the pipe into the wet concrete mix. A reinforced cage is installed to the full length of the pile after the pipe is inserted into the concrete. This is done after pipe installation to avoid tangling the pipe in the steel wires of the cage.

“The integrity of the plastic pipe is essential as any damage means the geothermal aspect will not work,” says May Gurney contracts manager Neal Willis. “Insertion must be smooth. While using CFA piles we’ve found attaching [the pipe] to the T-bar helps prevent damage.”

“Our experience on geothermal sites means we have refined our installation process. Adding geothermal elements only adds about 10% extra time to a piling schedule.”

Neal Willis, May Gurney

The risk of damaging the pipework does not end after its insertion. Workers excavating pile heads create the greatest risk to the integrity of the pipes, according to Geothermal International. May Gurney added reinforcement to the pile heads in the form of debonding foam to ensure that when the pile is trimmed down there is no risk of damage.

Piling was completed at the beginning of June, three days ahead of schedule. “Our experience on geothermal sites means we have refined our installation process. Adding geothermal elements only adds about 10% extra time to a piling schedule,” says Willis.

Geothermal International conducts extensive pressure tests on the pipes before and after piling is complete to ensure there is no damage.

A contingency plan is built into its geothermal pile designs, with a certain percentage able to fail without affecting the heating and cooling capacity of the system.

After pressure testing, the area is concreted, with the pipework connected to manifolds that feed into the plant room of the building.

“Using geothermal piles has killed two birds with one stone − structural stability and heating and cooling,” says Herbert. “We are making holes in the ground anyway so we might as well make use of it.”

Readers' comments (1)

  • It's an interesting idea but the article doesn't mention the COP of the system. Given the additional carbon costs of pump supply (typically 200%+ over fossil fuels for the UK energy supply mix) and the usual COP of perhaps 3-4 for these systems, are they really environmentally friendly? Some would accuse these systems of being just green-wash if their COP is not greater than 4 (a typical system).

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