Silty

You are correct that the high installation cost is what is inhibiting widespread GSHP use. I am looking for a way to reduce it. For the borehole architecture, I want to better match the piping to the rock thermal conductivity, but bedrock also has a wide TC, apparently ranging from about 0.5 W/mK for sedimentary to more than 5 W/mK for metamorphic and igneous.

Sorry for my delayed response. I didn't get alerted to your post. My application is ground-loop design for a ground-source heat pump. I just wanted to get a top-level understanding of how moisture content increased soil thermal conductivity at different depths. I will check out your references. Thanks.

Thanks for your input. I want to clarify that I am interested in the soil's thermal conductivity rather than its electrical conductivity.

Thanks! Yes, I am referring to the region above the capillary zone where there are varying degrees of saturation. I am assuming that the thermal conductivity will be maximized below the capillary zone. However, above the capillary zone, my thought is that, even if the material composition of the soil is the same through a vertical column, because of reduces aeration the effect of moisture content on thermal conductivity will decrease with depth. In other words, with greater depth, the thermal conductivity becomes closer to that of the soil material and less on the mix of that material and water or air. Near the surface, where the soil will likely contain more air, substituting water for air greatly increases the thermal conductivity, but if the capillary zone is below the root line, then the soil is not going to be broken up by the roots, and the soil will be compacted by the weight of the column of soil above it so there will be less air volume to displace with water volume as the moisture content changes so the soil will never be as insulating as the same soil was at the top. In summary, I am thinking the thermal conductivity will be its maximum at the capillary zone. Moving up to the ground surface from there it will become increasing sensitive to moisture content with the largest sensitivity at the surface.

I understand that the thermal conductivity of a soil sample that is mostly clay will increase about three fold when transitioning from a dry state to a saturated state while one that is 100% sand may increase as much as ten fold. In the latter case, e.g., in a sand dune or gravel bed, these properties may not depend on depth. For most soil mixtures, however, I am hypothesizing that the sensitivity of the thermal conductivity of the soil to moisture changes will decrease with depth due to the reduction and then elimination of root aeration, and the general compaction of the soil which will reduce the voids where water can displace air. If my logic is correct, has anyone measured this effect versus depth for different soil types?