Geothermal technology could find wider appeal after it was adapted for use in a basement diaphragm wall on a hotel development in Knightsbridge. Gemma Goldfingle visits the site.
Prime Knightsbridge, built a stone’s throw from Harrods, will have all the accessories one would expect from London’s newest luxury hotel: penthouse apartments, grand ballroom, luxurious spa, private viewing rooms and, of course, ground source heat pumps.
It is testament to the growth of this technology that Prime Developments, the client behind the five-star development, approached piling contractor Cementation Skanska with geothermal already in mind.
“Geothermal energy was always in the client’s specification,” says Cementation project manager Neil Abbott.
“Originally they wanted to place pipework in both the diaphragm wall and the basement slab.”
Cementation is a pioneer in geothermal engineering in the UK.
The firm developed its innovative energy pile technology, which draws heat and cooling from the ground surrounding the structural pile, in 2001 on Keble College in Oxford.
As piles are constructed, flexible plastic pipes are incorporated within the pile reinforcement cages.
Upon completion of the foundation works, the sections of pipework embedded in the pile are plumbed into the building’s heating and cooling system.
The concrete forming the piles provides an ideal energy transfer medium, allowing the energy derived from the ground to be used to heat and cool the structure.
The 10-storey Prime Knightsbridge development has a six-level basement requiring a 30m-deep perimeter wall.
“It doesn’t add much time to construction. It’s just a matter of tying the plastic pipes to the cage whilst it is being plunged down.”
Despite Cementation being a geothermal pioneer, it had never before inserted pipework into a diaphragm wall. “This is a first in the UK,” says Cementation Skanska geothermal manager Peter Smith.
“It has been done overseas, which is why the client had heard about it, but never in the UK.”
Harnessing heating and cooling from diaphragm walls will open up a wealth of opportunities for ground source energy.
For example, ground source heat pumps could be incorporated into civils works as well as building projects.
In fact, representatives from Crossrail and London Underground have visited the Knightsbridge site to see the installation.
“I believe Crossrail is considering installing heat pumps into some of the station boxes. The Tottenham Court Road tender documents express an interest in geothermal,” says Smith.
At Knightsbridge Cementation has adopted techniques used at the NEO Bankside development on London’s South Bank to install the geothermal elements in the diaphragm wall.
At Bankside, a project both Smith and Abbott worked on, 52m energy piles - the UK’s deepest - were installed on the outside of the reinforcement cage, under bentonite.
On the Knightsbridge site, the 155m long, 800mm wide wall is being built in panel lengths of between 3.2m and 7m.
Prefabricated plastic loops, which arrive on site in huge reels, are attached to the outside of the 625mm-wide reinforcement cage with cable ties.
Each pipe is tied in a long vertical U-shape - to allow the liquid that transfers the ground heat to travel up and down - as the cage is lifted into the excavation.
A total of 54 U-tubes are being attached across the length of the diaphragm wall.
“It is a relatively simple process,” says Abbott. “It doesn’t add much time to construction. It’s just a matter of tying the plastic pipes to the cage whilst it is being plunged down.”
Small modifications are needed to the diaphragm wall design to accommodate these pipes: the position of the reinforcement cage has to be adjusted slightly to give more space between the cage and the end of the panel.
Pipework can also impede the concrete flow inside the bentonite-supported panel, especially when surrounded by steel reinforcement bars, so the design was rejigged to reduce congestion within the panel, so that no more than two steel bars are in vertical alignment with the pipework, which ensures the concrete can flow freely.
Before the concrete is pumped into the excavation, the integrity of the pipework is tested.
The pipes are flushed with water and valves are placed on the tip of the pipes and pressure is applied.
The same pressure is recorded after concreting to ensure the pipework is intact.
“This is a first in the UK. It has been done overseas, which is why the client had heard about it, but never in the UK.”
It is vital to ensure that no damage is sustained to the pipework throughout the operation so the liquid which transfers the ground heat can pass through the entire circuit.
A rolling programme of testing is being carried out on the pipework throughout construction.
“The plastic is pretty robust. There are anecdotes about these pipes still working post earthquake in San Francisco.
Problems arise with other contractors working on site, post piling.
If they are not fully aware of how important this pipework is, it can be damaged during groundworks,” says Abbott.
Only three sides of the diaphragm wall are being fitted with the geothermal pipework.
The central London site is surrounded by buildings, and on the western side the perimeter wall is only 1m away from the basement of 199 Knightsbridge.
Pipes are not being installed in this stretch of wall as the proximity of the adjacent basement means a lack of available ground to heat the water, and the ground temperature is too low.
Rather than inserting a slinky - a flattened, overlapped circular coiled pipe - in the basement slab to supplement the diaphragm wall system, Cementation and Geothermal International, a specialist geothermal consultant advising the client, persuaded Prime Developments to opt for energy piles instead.
“We have a lot of experience of working alongside Geothermal International and we both felt more comfortable using energy piles,” says Smith.
“Perhaps we were staying in our comfort zone, but they [slinkys] are quite fiddly to install, with more joints, which means more chance of losing integrity in the pipework. Plus, by using energy piles we increase the level of heating and cooling harnessed from 85kW to 140kW.”
In total there are 35 low cut-off bearing piles and 15 tension piles around the perimeter, all 1.5m in diameter and bored under bentonite to depths of up to 62m.
All bearing piles are plunged with steel columns with the cut-off level 23m below ground level.
Only one out of the 50, a 12.3MN pile, is not an energy pile.
Abbott says that, at the time the geothermal aspect was designed, the exact load of that pile was not determined.
Also, installing the reinforcement cage in a pile that deep would need monstrously large cranes, which would not fit on the congested site.
Pipework comes prefabricated inside the reinforcement cages of the energy piles.
Up to three cages were needed to stretch the length of the fully reinforced piles.
The pipes, attached to the top of the cage, hang loose while a crane carries the cage to the pile location.
The pipe is fixed to the inside of the cage insitu, using U-bolts at 1m intervals.
The cage is then installed and tied to the connecting cage before being lifted out and the pipework fixed.
“The beauty of using foundations to generate heat is that it is a relatively simple operation, no specially trained people are needed.”
Once again, testing is carried out to verify integrity before tremmies pipe in the concrete.
On site, the process looks seamless and the time added to the construction process is minimal.
“The beauty of using foundations to generate heat is that it is a relatively simple operation, no specially trained people are needed. Piling contractors are basically doing what we do, just incorporating one small additional element into it,” says Abbott.
Once building work begins the pipework is plumbed into heat pumps in the building’s plant room, which converts the low-grade heat or cooling from the ground and upgrades it to a higher, more useful temperature.
With Smith insisting he cannot think of any foundation element that geothermal pipework could not be inserted into, and delays to construction minimal, is there any reason why a client would not adopt the technology on their scheme?
“Money,” says Abbott. “The system does require more capital. Although there is only five to seven years’ pay back, in times of recession clients can’t justify any extra up-front costs.”
Cementation Skanska began working on site in September 2009 on the £4M contract, and piling is due to finish at the end of March.