Boreholes in areas that permit the use of natural materials for backfill must be sealed at the surface in accordance with governing codes. If holes are to be grouted from bottom to top, high solid bentonites should not be used. The commonly used “high solids” bentonite grouts have around 20% solids- and a thermal conductivity value of approximately .40. This is a very poor heat transfer capability and impedes heat exchanger to ground heat transfer. This is not a problem when the bentonite is used as the sealing material in the top 10-20 fee or so of the borehole. However, if it used to grout the entire borehole, it has a very negative impact on the U-bend’s heat exchange capacity.
To overcome this, thermally enhanced grouts were developed. These grouts use small granule sands in the bentonite to thermally enhance the grout (increase the solids percentage to 65% or more) while still maintaining the permeability ratings necessary to protect the ground water and environment.
As the percentage of solids is increased, the thermal conductivity value of the grout incrases0 starting at about .85 for 65% solids and increasing as solids content increases, up to a thermal conductivity of 1.2 or more.
The high capacity group pump is capable of mixing and pumping 2 batches of group ( a typical batch of thermally enhanced grout is 50 pounds of bentonite grout, around 18 gallons of water, and 200 pounds of the small granule sand) through a that may be 400-500 feet long. Grout is pumped from the bottom of the borehole up to ensure the entire borehole is filled with the thermally enhanced grout.