Experimental Investigation on the Thermal Conductivity of Alternative Backfill Materials for District Heating Networks

This study investigates the thermal conductivity of backfill materials for buried bonded pipe systems, with a focus on comparing conventional natural aggregates and recycled alternatives. Three natural and three recycled materials were tested under compacted conditions using a thermal needle probe in accordance with ASTM D5334-22a. Thermal conductivity was continuously recorded and correlated with volumetric water content. A classification methodology based on EN 13,941–1 was developed to assign thermal conductivity values to defined moisture levels (dry, medium, wet). The results show that the investigated natural materials exhibit higher thermal conductivity across all moisture states, while the investigated recycled materials demonstrate significantly lower values. Through this two-fold approach—measurement and classification—representative parameters were derived for future application in heat loss calculations. In addition, the findings could improve the understanding of the thermal interaction between soil and pipe systems, supporting the potential integration of alternative materials into sustainable infrastructure design.