FAQs

Geothermal Grouts – The Basics

• Thermal Conductivity of most ground is  0.8 -1.8 Btu/ft degree Fahrenheit or 1.4 - 3.1 watts/m degree Celsius
• It is logical then that the Geothermal grout should match the conductivity of the ground or be close to it.
• Less than this level the grout is acting as more of an insulator than a conductor and impairs heat pump efficiency considerably, necessitating much longer pipe lengths and deeper holes.
• Regular CE bentonite which is still widely used in Geothermal grouting can only be mixed to a maximum of 10-15% solids and then only achieves a thermal conductivity of around 0.2 Btu/ft degree Fahrenheit or .35 watts/m degree Celsius so they behave as insulators and should NOT be used for Geothermal work.
• Furthermore at low solids i.e (20%, Bentonite may loose water to formation and so dry out and crack so 30% solids should be considered the minimum for a Geothermal grout
• Proprietary grouting bentonites such as Geoseal, can be mixed and pumped at )30% solids with relative ease and permit long placement times to achieve thermal conductivities around 0.38 Btu/ft degree Fahrenheit or 0.65 watts/m degree Celsius . If these special bentonites are then enhanced with thermal conductivity improvers (eg sand ) up to 70% solids they achieve thermal conductivities up to 1.4 Btu/ft degree Fahrenheit or 2.4 watts/m degree Celsius and still deform and seal better than cement based grouts.
• One study in the magazine ‘Outside the Loop’ indicated a saving of 23% in pipe design length per ton was achieved by moving from a conductivity of .4 up to 0.85 Btu/ft degree Fahrenheit (0.65 up to 1.5 watts/m degree Celsius) and a further 19% saving in length was achieved when increasing conductivity from 0.85 – 1.4 Btu/ft degree Fahrenheit (1.5 -2.5 watts/m degree Celsius) giving a total saving of 47%. However for practical purposes most companies find little benefit in raising conductivity over .85 Btu/ft degree Fahrenheit or 1.5 watts/m degree Celsius
• In nature, clay deposits are typically 65-85% solids and contain or seal off strata containing water or oil and even radioactive material, preventing migration or contamination of the ground above. In the same way, clay grouts deform and maintain a seal irrespective of most ground movement and the sand enhanced bentonite grouts are typically 60-70% solids. Furthermore bentonite/ sand grout will not shear the pipes if the ground moves as it simply deforms.
• By contrast rigid cement based grouts do not deform with ground movement but fracture damaging any pipes buried within the grout and allowing leakage of gas or liquids around the grout mass.
• Cement on its own has relatively poor thermal conductivity ( 0.46 Btu/ft° ) due to micro pores and shrinkage so must be mixed with bentonite and thermal conductivity improvers to achieve higher values. However while cement mixtures form harder grouts which are needed for certain applications, it is felt that for Geothermal work, these are not ideal.
• To sum up, proprietary bentonite based grouts such as Geoseal mixed with sand, or pre-mixed bentonite/sand grouts like Thermoseal, are the best grouting solution available today. They can be tailored to produce efficient heat conducting mediums that protect and seal effectively and are stable long term.'