The One New change project in London in the UK’s largest energy pile contract ever, but it’s the first tie the foundation contractor has been responsible for the plumbing as well.
It is rare for a foundation engineer to see the results of his or her work. Even with basement construction, the foundation contractor is long gone from site before the excavation exposes the retaining wall.
So for Cementation Skanska, its One New Change office and retail project for developer Land Securities issomething of an unusual opportunity.
Some nine months after the big piling rigs left the high-profile commercial development that flanks Cheapside and St Paul’s Cathedral, Cementation Skanska is back at the central London site.
And clearly it is something of a treat for project manager Ian Lovett.
“You don’t often get to see £13.5M of foundation engineering,” he says.
The real purpose of the revisit, of course, is not to admire the finish on the secant pile basement retaining wall, or the perfect alignment of the plunge columns. Instead, Skanska is firmly ensconced in the newly excavated basement to oversee the installation and commissioning of a complex array of pipework, as part of the country’s biggest energy pile project to date.
The energy piles are an integral part of the development’s groundsource heating and cooling system that extracts heat from the ground during winter and exhausts heat back to the ground in the summer (see box overleaf).
In the summer of 2008, Cementation Skanska installed continuous loops of 32mm diameter highdensity polythene pipe within 230 of the 30m deep bored piles. Piles vary from 1,200mm to 2,400mm in diameter, depending on design loads, and the pipe loops extend to the full depth of the pile. Each contains between two and eight connected loops, giving some 800 pile loops in total.
The task now is to plumb all the piles back to what is destined to be the development’s plant room in which the Cementation Skanska geothermal subcontractor, coordinating with the M&E trade contractor, will later install the heat exchangers.
This is the first time Cementation Skanska has been responsible for “headering up” work, and it is working closely with its long-term partner Geothermal International, which on previous energy pile projects has been separately contracted for this element of the work.
Typically, groups of 15 to 20 piles are linked into one of 14, 90mm diameter “closed-loop” ring main circuits.
In the process, the 32mm pipes embedded in the piles are connected to 63mm pipes that link into the 90mm ring main circuits, each of which terminates with inlet and outlet vertical risers in the plant room, situated in the three basement floors.
The pipe network has a 200-year design life and, once constructed, is totally inaccessible, buried below the basement’s structural floor slab. Quality control is crucial, and templates ensure the alignment of the pipes at the electro-fused joints.
Debris in the pipes is the real problem, and breaking down the piles needs extreme care. The secret is to keep testing at each stage, says Lovett. “If anything is leaking in the system, it must be identified straight away, especially before the floor slab is cast.”
The headering up work is further complicated at One New Change by the need for a drainage blanket below the floor slab. The ring mains are laid first below the drainage blanket, while the final connections to the piles are made within the drainage layer.
Once fully connected, each loop circuit is signed off by pressure testing to 12bar using standard water industry tests. Each test is closely scrutinised by the client’s appointed testing team. Once passed, the concrete frame contractor lays a blinding layer over the drainage blanket and the steel fixers move in to start work on the structural base slab.
The Principles of Geothermal Heat Pumps
Geothermal heat pump systems don’t break the laws of physics, but in harnessing solar thermal energy absorbed by the ground, they do provide energy for free. Near-surface ground is at a fairly constant temperature throughout the year, typically 12˚C-14˚C, which, compared with air temperature, is warm in winter and cool in summer. Heat exchangers can “magnify” this temperature differential by compression or expansion to produce highly energy efficient heating in winter and cooling in summer.
Simplistically − and in winterheating mode − water with anti-freeze at 6˚C is circulated through pipes in the ground (in this case embedded in the piles), which heats up to 12˚C. Compressing this through heat exchangers elevates the temperature to about 60˚C, which is used to heat the building.
As it circulates through the building’s heating system, the temperature drops to, say, 40˚C. Finally, back at the plant room, heat exchangers expand and reduce the temperature of the liquid back to 6˚C, ready for recirculation through the ground.
The efficiency or coefficient of performance (COP) of most geothermal heat pump systems is about four (but some are as high as eight). This means that for every one unit of electricity the system uses during compression, pumping and expansion, it provides an average of four units of heating/cooling energy. In other words, the process is 400% efficient, compared to, say, the 90% efficiency of a super-efficient gas condenser boiler.
The most challenging aspect of the project, asserts Lovett, is the logistics and interface between the different organisations managed by Bovis Lend Lease. In particular, coordination between Cementation Skanska, Geothermal International, the groundworker McGee and the concrete frame contractor Byrne Bros, is key. And it is not just about the coordination of the site teams − the interface between the different design teams for each of these elements is equally critical. For example, the designers of the energy pile ring main layout need to be aware of the drainage system.
Technically there could have been substantially fewer than 14 closedloop circuits, but this was determined by the complexity and phasing of the work to ensure continuity for all trades within the basement.
With the pipework, headering up, and additional plant room kit, the overall scheme is more expensive to install than a conventional heating and cooling system. But Lovett points out that the extra costs of installing the pipework within the piles over the normal foundation contract are small and the installation process has minimal impact on the piling programme itself.
Energy Piles Take Off After Slow Start
Cementation Skanska installed the first UK energy pile system on a project at Keble College, Oxford, in 2001 and has remained the sector’s market leader.
Following the initial interest in energy piles, the company didn’t undertake a commercial project for nearly three years. Nevertheless, it secured funding for a number of trials that helped it pioneer new methodologies, including techniques for installing pipe loops in continuous flight auger and driven piles.
Commercial projects reappeared in 2005 when Cementation Skanska installed 150 piles. Since then, the market has grown significantly to about 1,500 piles annually.
Recent landmark projects include Paddington Basin, a new headquarters for North Kent police and colleges for Lambeth and Westminster.
From a construction point of view, it is all about how you install the pipe loops into the piles. Different piling techniques − bored, continuous flight auger or driven − present different challenges.
The relatively shallow rotary bored piles at Oxford were, in hindsight, straightforward, with the pipe loops being simply attached to the inner side of the singlesection reinforcement cage.
Deeper-bored piles, in which the reinforcement cage is made up of a number of sections connected on site, are more complex. In these cases, the pipe loops are bundled up and temporarily attached to the top section of the cage. Once the cage is fully formed, the loops are released and drop down to their full depth.
Some of the piles at One New Change included reinforcing cages made up of four sections and at the time were the most complex installed.