Thermal characterization of a geothermal precast pile in Valencia (Spain)

Abstract

The geothermal use of pile foundations represents a useful, efficient and cost effective method of installing ground heat exchangers for cooling and heating buildings, which may potentially favour a decrease in installation cost, considering that it involves a minimal impact on the piling program.
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 Polytechnic University of Valencia Campus. The pile was subjected to two types of loading: mechanical and thermal. The mechanical load was 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 was provided by a reversible heat pump, with a data logger to record the outflow and return temperatures (thermal response test installation).
This paper presents the thermal characterization of the thermal precast pile in terms of an average thermal conductivity value for the ground around the pile (ground) and a thermal resistivity value (Rb) for the precast pile by means of two different approaches. Firstly, an in situ thermal characterization of the system pile-ground was done by the thermal response test installation. Secondly, 550 thermal conductivity measurements were performed by the “thermal needle method” in samples extracted from the cores obtained from a geothermal drilling conducted 2 m away from the thermal pile. Controlled variation of factors such as the degree of saturation – moisture and dry density allowed to assign a thermal conductivity range to each geological level. Once the stratigraphic profile is known, the average thermal conductivity of the ground around the pile can be determined by weighting the thermal conductivity with the thickness of each geological strata.