Selection of backfill grout for shallow geothermal systems: Materials investigation and thermo-physical analysis

A main aspect of the geothermal well design is the selection of grouts backfilling the space between a geothermal probe and the surrounding ground, taking into consideration the mineralogical, thermo–physical, mechanical and flowability properties. The EU GEO4CIVHIC project aimed at developing efficient and cost-effective geothermal systems suitable for the air-conditioning refurbishment of urban buildings. A selection of different grouts was performed to ensure the whole efficiency of shallow geothermal heat exchangers designed within the project. Experimental tests were executed to identify flowable grouts with the best possible combination of different material properties, while also fulfilling a competitive cost target set forth in the preliminary project cost analysis. Grout acceptance criteria were defined: a working time of at least 4 h, a water bleeding target of 2% and a volumetric shrinkage upper bound of 4%, a thermal conductivity in water saturated conditions of at least 1.4 to 1.5 Wm−1K−1, a compressive strength range of 1 to 2.1 MPa, a flow time range of 60 ± 15 s. Two out of six commercial grouts were selected for installations onsite.