Energy Geostructures and Thermal Piles

Suitable developments of sustainable energy sources are drawn by analysing the Energy sector, considering environmental, economic and social aspects. The particular example of Switzerland is detailed with a focus on the energy consumption in the buildings. Based on the statistics from the Swiss Federal Office of Energy, the majority of the energy consumed by the Swiss households and in office and retail spaces is used for space conditioning as well as hot water production. In this scope, energy geostructures represent the next generation of ground heat exchangers for ground source heat pump systems. These are more cost effective than conventional ground heat exchangers, e.g. geothermal boreholes, because they save the specific drilling operations and take advantage of the ground structures required anyway.

First, the geotechnical operational design of energy piles is discussed using Thermo-Pile software, a tool developed at the Laboratory of Soil Mechanics of the Swiss Federal Institute of Technology Lausanne (Switzerland) and based on the load-transfer method.

Next, the effects of pore water pressure build under temperature variations on the bearing capacities of energy piles are investigated using thermo-hydro-mechanical finite element analyses. This phenomenon, not accounted for in the load-transfer method, could have a significant impact by reducing cyclically the effective contact stress at the pile-soil interface.

Then, heat production is investigated on shallow urban tunnels. These geostructures may represent a significant heat source because of their extension. Several solutions were experimentally tested mainly in Austria, considering thermoactive tunnel linings, geotextiles, slabs, walls and prototype self-drilling bolts. The present thesis proposes to deepen the knowledge about heat exchanger anchors as they are the least investigated. Indeed, thermoactive slabs, linings and walls were extensively used on real structures while heat exchanger anchors have only been tested in an embankment in Vienna (Austria).

Finally, full-scale experimental investigations of the thermomechanical response of energy piles are carried out. Four 28 m long test piles were built below a water retention tank that is also supported by conventional piles. The test piles are gathered in a tank corner to allow studying group effects. Three thermomechanical response tests are analysed. The first consisted in heating the piles when no structure was built on top of them. Next, each test pile was individually tested once the tank was built. Finally, the entire group was heated in order to observe the relief of pile to pile interactions as a result of a global group expansion. Comparisons of the different thermomechanical response test are achieved in term of pile tip compression, pile top strains and degree of freedom profiles. It is found that the tank construction influences the thermomechanical response of the piles down to the stiff soil layers while their respective position below the raft impacts their responses down to 10 m. The pile to pile interactions are clearly visible on the first level of neighbouring piles (i.e. directly adjacent) and down to the pile tips. Group effects observed during the heating of the entire group doubled the degree of freedom of each test pile, inducing greater pile heaves but reducing differential settlements, therefore reducing internal thermal efforts.