Laboratory Measurements of Gravel Thermal Conductivity : An Update Methodological Approach

Direct measurements of gravel thermal properties are quite challenging mainly due to technical issues related with the very coarse sediment size, which impedes an appropriate physical contact between the material and the traditional measuring sensors. In addition, the variability of the mineralogical composition of the polygenic samples in many cases requires a quite large volume of geological material to be involved in the measurement procedure, in order to obtain representative thermal properties values. As a consequence, up to now the reference thermal values provided by literature for gravels are quite limited and dispersed. With the purpose to improve the gravel’s thermal properties knowledge, a new measurement methodology has been proposed and tested on several gravel samples, by means of a single guarded hot plate Taurus Instruments TLP 800 usually used for buildings materials, modified on purpose. The instrument adjustments were needed in order to measure unconsolidated materials and to allow the testing of even with interstitial fluid. The measurements have been performed at the FISTEC laboratory (Environmental Technical Physics Laboratory) of the IUAV University in Venice (Italy). A preliminary calibration has been performed comparing the outcomes obtained with the single guarded hot plate with a needle probe of a portable thermal conductivity meter (ISOMET) by using Standard sand (ISO 67:2009) as reference material. This new measuring procedure has been developed within the Cheap – GSHPs Horizon 2020 project, which focuses on the development of more efficient and safe shallow geothermal systems and on the reduction of their installation costs, in order to improve the evaluation of the ground heat exchange capacity of the site where the Ground Source Heat Pump (GSHP) has to be installed, therefore optimizing the probes total length design phase.