The dynamic foundations behind a renewable energy source

Several meters beneath our feet lies a formidable heat reservoir, a renewable energy source that can be used to meet a building’s daily heating and cooling needs and even to store summer heat for the colder winter months. To tap into this reservoir, researchers at EPFL have contributed to the development of a technology that equips concrete piles that anchor a building’s foundations to the ground with heat exchangers. In two recent papers, they present research into the impact of daily and seasonal temperature fluctuations of up to 50 degrees Celsius on the geothermal piles and the surrounding soil.

Buildings are anchored to the ground using piles, concrete pillars that are sunk into the earth during construction. In the case of so-called floating piles, these pillars are held in place by the friction between the rough concrete and the surrounding substrate – the rougher the substrate, the firmer the grip. The long-term fate of these standard piles, a well-established construction technique, is well understood.

Things become more complex when the concrete piles are equipped with heat exchangers that transfer heat to and from the ground. “Like most materials, concrete expands at high temperatures and contracts when it cools down. In some cases, these temperature cycles can raise and lower buildings by several millimeters,” says Lyesse Laloui, director of the Laboratory of Soil Mechanics.

Furthermore, as the piles heave and sink periodically over heating cycles, they repeatedly rub against the surrounding substrate. Over time, the roughness holding the floating geothermal piles in place could be lost. In an unlikely worst-case scenario, this could cause the entire building to sink.

Evaluating the long-term impact of geothermal piles on actual buildings is difficult, not least because the technology only recently found its way into full-scale applications. Reliable experimental tools, such as reduced-scale laboratory experiments and computer simulations, are essential to predict assess the longevity of these structures in different types of soils. In two publications, researchers working with Lyesse Laloui from EPFL’s Laboratory of soil mechanics have investigated the dynamic behavior of geothermal piles and their surroundings in two different settings.

In a research paper published in the journal Engineering Geology, Alice Di Donna, a former PhD student in Laloui’s group, investigated the response of silty clay soils to cyclical temperature fluctuations, such as those imposed by geothermal piles. Using reduced-scale models, she showed that during the first heating cycle, these clay soils undergo a large irreversible deformation. Subsequent cycles lead to smaller and smaller irreversible deformations, until finally deformations become elastic. By developing a numerical model that accounts for this phenomenon, Di Donna’s work provides insight into the long-term behavior of the soil surrounding geothermal piles.

Working with peers from the Hong Kong University of Science and Technology, researchers from ENAC’s Laboratory of Soil Mechanics focused on the dynamics of the geothermal piles in saturated sand. Using centrifuge tests, they were able to accelerate the physical time of the processes. Publishing in the journal Geomechanics for Energy and the Environment, they reported that temperature variations of up to 50 degrees Celsius could cause piles to heave by almost 15 millimeters. Their results, allowed the development of a numerical model that accounts for the thermal, hydrological, and mechanical processes that occur in and around the piles. The model predictions compared well with more laborious laboratory experiments.

This research paves the way for numerical tools to design geothermal piles and predict their long-term behavior. And the studies indicate that they are structurally sound as well. “Piles designed as geothermal energy sources are stronger than conventional piles from a structural point of view. The cyclical temperature variations increase their safety by hardening the surrounding soils. As a consequence, energy piles could be designed following the same standards as conventional ones, avoiding any additional costs,” says Laloui.

References:

A. F. Rotta Loria, A. Gunawan, C. Shi, L. Laloui and C. W. W. Ng. Numerical modelling of energy piles in saturated sand subjected to thermo-mechanical loads, accepted in Geomechanics for energy and the environment, 2015.

A. Di Donna and L. Laloui. Response of soil subjected to thermal cyclic loading: experimental and constitutive study, in Engineering Geology -Amsterdam-, vol. 190, p. 65-76, 2015.