Ground source heating systems help reduce costs while meeting sustainability targets, but adding thermal capacity has implications for pile design, according to Arup geotechnical engineer Paul Bailie.
Demand for energy efficient buildings is growing as developers start to look more closely at the ongoing costs of operating their buildings, while looking to improve their sustainability credentials.
One of the main areas of focus is the use of ground source heat pump systems to reduce heating and cooling costs and, with many structures supported on piled foundations that reach the depths needed, the systems are being combined.
Thermal piles differ from conventional foundation piles because they use pipes cast into reinforced concrete piles, which are then filled with a thermal transfer fluid, connected to a heat pump and used to exchange heat with the ground.
In Europe, thermal piles have been growing in popularity since the 1980s, with installations in the UK gaining momentum since around 2005, encouraged by Brandl’s 2001 Rankine Lecture on energy foundations.
The case promoting the use of thermal piles - piles that are adapted to extract heat energy from the ground or to return heat energy to the ground - is well established due to the economic and environmental benefits of renewable energy.
However, the geotechnical performance of foundation piles that are expanding and contracting during operation is easy to misinterpret, and has led to some confusion about how thermal piles should be designed.
Until recently, programs for analysing pile design focused on the geotechnical capacity and settlement performance of single piles.
But an update of Oasys Pile is currently in progress. The new version will allow the effects of heating and cooling of thermal piles to be included in the design, giving the geotechnical engineer more confidence in the pile performance.
Normal pile capacity and settlement behaviour is controlled by resistance along the shaft of the pile and end bearing at the toe of the pile.
Shaft resistance generally acts in one direction only (except for cases where negative skin friction occurs due to ground movements around the pile), resisting applied compressive or tensile loads.
However, when thermal piles are heated and cooled, this results in thermal strain (expansion and contraction) of the pile.
The resistance of the ground to the thermal strain of a pile limits the amount of movement occurring, but there are changes in shaft resistance and axial stress along the length of thermal piles compared to normal piles.
The typical function of thermal piles is to provide buildings with heating during the winter and cooling during the summer.
When thermal piles are heated and cooled, this results in thermal strain of the pile.
In winter, when extracting heat from the ground, the pile is cooled and it tries to contract, although this movement is resisted by the surrounding soil. The top of the
pile moves downwards, resulting in increased pile head settlement and increased shaft resistance.
At the same time, the base of the pile tries to move upwards, resulting in a decrease or possibly a reversal in the direction of the shaft resistance.
In summer when discharging heat to the ground, the pile is heated and the opposite occurs, with the pile trying to expand and the surrounding soil again resisting movement.
The top of the pile moves upwards, resulting in decreased pile head settlement and decreased or reversed shaft resistance, and the base of the pile tries to move downwards, resulting in an increase in the shaft resistance/end bearing.
The Oasys Pile program works by converting applied stresses into mobilised shaft resistance, axial stress and settlement using a finite difference method of analysis.
The same method can also be used to deal with the thermal stresses within the pile that are a result of the thermal strain caused by heating and cooling. In the user interface of the program, the tabular and graphical layout of results currently available for normal piles will also be available for thermal piles.
The program update that is currently underway - due to be completed later this year - will allow more accurate modelling of thermal piles, including thermal effects on shaft resistance, axial stress and settlement behaviour.
The revised program will allow geotechnical engineers to have more confidence in promoting the use of thermal piles and making use of this sustainable source of energy.
Nonetheless, this is not the end of the story for thermal pile design and there are other considerations that cannot be addressed by design software until more research is carried out.
The effects are not yet fully understood and are the subject of ongoing research, including the effects of heating and cooling on soil properties, the effects of thermal strain of a pile in the radial direction, and the potential ratcheting effect of cyclic heating and cooling thermal strain on pile settlement and shaft resistance.