Field investigations on the thermo-mechanical behavior of a partially activated energy pile in Miocene sediments

The City of Vienna, as one of the largest public clients in Austria, initiated a research project to determine the load-bearing behavior of various foundation elements in typical Viennese soils. Numerous large-scale tests were carried out on bored piles, micro piles, anchors, and jet grouted columns. In addition, two energy piles were installed in different soil layers to investigate their behavior under mechanical and thermal loading in each soil layer separately. This paper discusses the energy pile in the Miocene sediments, which consist mainly of silty fine sand and some sandy silt. The energy pile – fully instrumented with strain gauges, extensometers, heat gauges and optical fiber sensors – was loaded for two and a half months. The mechanical load was kept constant throughout the thermal cycles to determine the response of the pile to thermal loads. The measured temperature data were used in numerical back-calculations to determine the thermal parameters of the soil layers and the concrete. Based on the measured strain and deformation data, the deformation behavior of the energy pile due to the thermal load was investigated. Finally, a static pile load test was carried out on the energy pile and the results are compared with those of the conventional reference piles installed in the vicinity, which have not been subjected to thermal loading. The test demonstrated that, after several weeks of cyclic thermal loading, the energy pile exhibited more favorable load-deformation behavior compared to conventional reference piles.