By Tony Suckling and Robert Cannon, Cementation Foundations Skanska.
This paper presents a case study of a piling contract undertaken in Chichester in 2003. Unusually, the piles contained a network of plastic pipes, which were subsequently connected to the heating and cooling system for the new building.
The plastic pipes cast in the piles facilitate the transfer and storage of geothermal energy between the structure and the underlying ground mass, allowing the building to be both heated in winter and cooled in summer economically and in an environmentally friendly way.
The philosophy of the geothermal pile technology is presented, together with the modified pile construction techniques used on the site.
In 2002 Cementation Foundations Skanska received a subcontract tender enquiry from Haymills, which was tendering for the construction of an extension to Pallant House Art Gallery, Chichester, UK.
The new two-storey structure will be used to display fine works of art to the public.
The foundations comprised about 60 bearing piles to provide structural support for the extension.The engineer, Arup, had also specified the incorporation of a network of plastic pipes in the piles, to give them the dual function of being energy piles as well as structural piles.
Energy piles form part of an environmentally advantageous building design, and facilitate the transfer of geothermal energy into or out of the underlying ground mass. The technology has been used for many years on mainland Europe and in the US but it is believed that this project represents only the second or third such use in the UK.
Skanska was responsible for the first time use of the Enercret system in the UK, at Keble College, Oxford.
This paper describes the general design philosophy of the energy piles and how it benefits both the client and the environment. The successfully modified piling techniques employed on site are also presented.
The soils profile can be summarised as: 0 to 2m Made ground 2 to 5m Alluvium 5 to 9m Medium dense River Deposits 9m and below Stiff to very stiff
The perched groundwater table was encountered in the boreholes at about 5m depth, and water inflows were observed within the London Clay.
Oil price rises during the 1970s, and more recently issues such as global warming and the depletion of the ozone layer, have resulted in increasing public awareness of environmental issues.
Austrian engineering company Naegelebau first patented the environmentally-friendly way of heating and cooling buildings using concrete structural elements to obtain energy from both the ground and from the groundwater. This technology is marketed as Enercret by Naegelebau and was introduced in 1980.
With a growing number of initiatives, grants and favourable legislation becoming available from bodies such as local councils, government departments and the European Union, it is clear that in the near future forms of energy management for heating and cooling buildings, which reduce pollution and global warming, will become increasingly desirable.
In the UK and central Europe, buildings have to be heated in winter and cooled in summer.
Modern improvements in thermal insulation have been successful in reducing heating energy requirements, but there has been much less success in reducing cooling energy requirements.
Below about 10m depth in these areas, the ground temperature averages about 10degreesC to 15degreesC, and is quite consistent throughout the year. Above this depth, the ground temperature varies with weather conditions.
The philosophy behind the Enercret system is to extract this heat from the ground and to use it for heating a building. Conversely, the principle can be used for cooling a building by dissipating excess heat into the ground. It is possible in suitable soil and groundwater conditions to store heating and cooling energy for use from season to season throughout the year.
New structures which have deep foundations to transfer structural loadings down to a more suitable founding medium, such as concrete bearing piles, barrettes, pile or diaphragm walls, can make an effective use of the available geothermal energy as they offer a large underground contact area.
Concrete has two properties which make it an ideal medium as an energy absorber. These are good thermal conductivity and good thermal storage.
To use these properties, high density polyethylene plastic pipes of either 20mm or 25mm diameter, with 2mm or 2.3mm wall thickness respectively, have to be cast within the concrete. These are placed to form several individual closed circuits, which circulate a heat transfer medium of either water, water with antifreeze or a saline solution.
The pipe circuits are attached to the reinforcement within the foundation elements before concreting. This can be undertaken in the factory or on site. Once cast, the pipe circuits within the foundation are individually joined to a manifold and header block. They are joined by connecting pipes which are normally laid within the blinding beneath the floor slab.
The manifold and header block are connected to a heat exchanger, which is in turn connected to the fluid-based building heating or cooling network. For the heating network this connection is via a heat pump.
The heat pump uses the same principles as a refrigerator, but in reverse, ie the heat is absorbed in the evaporator and emitted at a higher temperature in the condenser.The heat pump can increase the temperature of the transfer medium from 10infinityC to 15infinityC to between 25infinityC and 35infinityC.
If ground direct cooling is unavailable, a reversible heat pump can be connected to the cooling network as well. In this case the heat exchanger acts just like a refrigerator to cool the transfer medium.
For clients, the installation of the Enercret system has a higher initial cost outlay than more conventional modern heating and cooling methods. Using cost benefit analyses carried out by Naegelebau, dependent on the energy prices and the climatic conditions in the country concerned, and the particular building and its underlying ground conditions, it will take between two and 10 years for the energy saving to balance the higher initial cost outlay.
However, the installation of the pipe circuits within the piles can mean that the most cost efficient piling technique for the ground conditions may not be possible, and advice must be sought from an experienced piling contractor at the beginning of the design process.
Geotechnical pile design
The geotechnical analysis of the bearing piles was undertaken by Cementation Foundations Skanska and will not be discussed in this paper. For structural loads of 750kN to 1,100kN the piles were designed with 600mm diameter and between 18m to 25m length.
An important point to note is that once the pile diameter and length had been designed for the applied loads, then the quantity of pipework within the energy piles was designed to fit within the structural requirements. In other words, no pile diameter or length was increased to suit the geothermal requirements.
Energy pile design
Design of the pipework within the piled foundations was undertaken by Naegelebau. To determine the geothermal characteristics of the ground mass, the following ground properties are required:
water flow direction and velocity.
In practice it is necessary to supplement the normal type of site investigation with empirically established geothermal values.
The result of the design work was that all of the bearing piles needed one continuous full depth 25mm diameter pipework circuit, comprising eight vertical runs equally spaced around the perimeter of the pile reinforcement.
The piles were installed using the rotary bored technique, as shown in Figure 1.
After Cementation's experiences in Oxford, even though the water bearing London Clay indicated that continuous flight auger (CFA) piling was the most appropriate for the site and the ground conditions, the technique was not used.
This was because of concerns over the ability to guarantee the pipework installation integrity in each and every pile.This increased both the cost and construction programme of the piling by a factor of almost two.
Skanska will be undertaking some CFA piling trials in 2004 to investigate if this piling technique can be used economically to construct energy piles in the UK, and to discover the associated limitations of pipework materials and depths.
The piles were bored to their design depth and a temporary steel casing was used to support the unstable soils overlying the London Clay, see Figure 2.
The pile reinforcement cages had to be fabricated earlier than for normal piling projects, to enable them to be handed over to a pipework fixer employed by Naegelebau.The 25mm plastic piping was delivered to site on a reel and could be easily fixed to the pile reinforcement cages, see Figures 3 and 4.
At the start and return ends of the pipework, a locking valve and a manometer were fixed, see Figure 5. This allowed the pipe circuit to be pressurised to 8bar, which acted as a check of the integrity of the circuit. Additionally, this pressure is required to allow the piping to resist the head of the wet pile concrete without collapsing. This pressure was maintained until after the pile concrete was a few days old.
Upon completion of the pipework fixing, a visual check was made on the final location of the pipes. This was to ensure that the flow of the wet pile concrete through the reinforcement cage would not be impaired. Figure 6 shows completed pile reinforcement cages ready for placing in their pile bores.
The pile reinforcement cages were lifted at both ends to prevent damage to the pressurised pipe circuits (Figure 7).
A full depth tremie pipe was used to place concrete in all of the piles. This is not normal practice for dry rotary bored piles, where the self compacting concrete is placed via a short tremie pipe from the ground surface.
However, this was necessary to prevent concrete impact damage on the plastic pipes. The temporary steel casing was then withdrawn.
The rotary bored piling technique was not the most cost efficient for this project. This method was used, however, as being best able to guarantee the pipe circuit integrity in each and every pile. This increased both the cost and construction programme of the piling by a factor of almost two.
Also reported are the modified pile construction techniques employed on the Chichester site. All bearing piles were constructed with the pipe circuits intact, proved by the manometer readings.
If energy piles are to be used on any project, advice must be sought from an experienced piling contractor at the beginning of the design process.
The authors wish to thank Pallant House Art Gallery for its support in publication of this paper. Both the authors and Cementation are proud to be associated with environmentally friendly construction, such as that described in this paper, and are actively pursuing more projects with energy piles.
More information on the systems used on this project can be found at: www. naegelebau. at www. enercret. com
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Brandl H (1998). Energy piles and diaphragm walls for heat transfer from and into the ground, Deep Foundations on Bored and Auger Piles Conference, Ghent, Belgium.
Ennigkeit A and Katzenbach R (2001). The double use of piles as foundation and heat exchanging elements, 15th International Conference on Soil Mechanics and Geotechnical Engineering, Istanbul, Turkey.
Suckling TP and Smith P (2002). Environmentally friendly geothermal piles at Keble College, Oxford, UK, Deep Foundations Institute Conference, Nice, France.