| Abstract |
The geothermal use of pile foundations represents an useful, efficient and cost effective method of installing ground heat exchangers for cooling and heating buildings, which may potentially favor a decrease in installation cost, considering that it involves a minimal impact on the piling program. Throughout the last years (since 2006), however, a number of studies have identified that temperature changes in piles lead to variations in shaft resistance and axial loads. This entails that heating-cooling load alternation on the building foundations can induce significant modifications in the soil-structure interaction, leading to additional stresses in the piles and decrease of the lateral friction. In this sense, specific research is still needed to better understand the totality of the loads affecting the pile: mechanical actions, from both external loads and stress concentrations in concrete; geotechnical actions originated in soil consolidation or uplift movements and shaft friction variations and finally thermal actions giving rise to small expansion/contraction phenomena in the pile. A new research and development project has been recently launched in Spain to undertake some studies on this complex matter (PITERM PROJECT). The experiment, consists of a specifically designed, constructed and fully monitored geothermal precast pile driven at Valencia University Campus. The pile will be subject to two types of loading: mechanical and thermal. The mechanical load will be applied by means of a mechanical frame anchored to the ground, as element of reaction, the three anchors being used to induce an active compressive force. The thermal load will be provided by a reversible heat pump, with a data logger to record the outflow and return temperatures. A borehole located at 0.5 m of the test pile will allow measuring the temperature depthdistribution in the adjacent soil mass. The testing set is fully instrumented in order to register the response of the pile to mechanical and thermal load changes: extensometers and fiber optic sensors to measure vertical strain and temperature distribution through the test pile and the variations in the shaft resistance mobilized as a result of cooling and heating. The most reliable way to demonstrate that the repetitive heating and cooling cycles do not affect the bearing capacity of the pile is to perform both static load tests before and after the thermal actions. With these results, the response of the pile and surrounding soil during the test will be analyzed, and modeled through numerical simulation, being the aim to propose an adequate method of design that takes into account the thermal and mechanical effects in thermo-activated piles. |