Thermo-mechanical field-scale testing of a partially activated energy pile in Vienna

Abstract

The City of Vienna and TU Wien have a long history of using and testing energy geostructures as part of ground source heat pump systems. In the framework of a research project, in which numerous foundation elements were tested for their serviceability and load-bearing capacity, two dedicated field-scale energy piles were investigated for the first time in Vienna. The large-bored pile presented here was equipped with a double steel pipe in the upper part for shaft-friction to be limited to the Miocene sediments soil layer consisting of silty fine sands and sandy silt. Furthermore, the heat transfer pipe loops were installed in the lower areas only while being fed through the upper parts of the pile, allowing a concentrated activation of both the pile skin friction and the thermal load in the homogeneous soil layer. The test pile was fully instrumented with strain gauges, extensometers, temperature sensors and optical fibers and tested for nearly three months. The mechanical load, applied as dead load, remained constant during the heating and cooling cycles to isolate the pile responses. In addition to the geomechanical investigation, the pile setup was also used as part of unconventional thermal response tests to establish the thermal parameters of the soil layers and concrete via numerical back-calculations. Finally, the pile was subjected to a conventional static load pile test to determine the ultimate load, enabling comparisons between the energy pile and other conventional large-bored piles close by.