Soil thermal conductivity has an important role in geo-energy applications and heat transfer modelling. The performances and efficiencies of such applications are strongly affected by the saturation conditions, especially at the lower saturation, which can negatively affect the soil thermal conductivity. Therefore, it is essential to improve the soil thermal conductivity at lower saturation levels. Here, we investigate the effect of the fillers, soil gradation and the water saturation on the thermal conductivity of sand as an innovative improvement method. The experimental results show a significant improvement in the thermal conductivity at dry and lower saturation degrees and a considerable improvement in the case of full saturation. The improvement in the thermal conductivity is comparatively high in dry state and is decreasing with increase in the saturation. The existing soil thermal conductivity models, which are limited to a specific boundary conditions and soil types, fail to consider the effect of soil filler composite behaviour. Therefore, a mesoscale numerical method is developed to model the change in effective thermal conductivity. The developed model shows a good agreement with the measured thermal conductivity values.