What is Fluidized Thermal BackfillTM (FTB)?
Fluidized Thermal Backfill (a trademark of Geotherm, Inc.) is a controlled low-strength material composed of certain types of stone, sand, fluidizing agents and cement specifically designed to dissipate the heat generated by underground electric transmission cables within city streets or on rural wind farms.
Characteristics of FTB:
“FTB” is designed to be low strength (<250 psi) like a stiff soil allowing for future access to the conduits below if necessary. We refer to this as hand excavate-able.
During installation, FTB must be able to flow adequately to fill all voids and crevices, and to bond with the trench side-walls. We refer to this as flow-ability.
Once hardened, FTB must be able to support load from the traffic above, and must be designed to eliminate the possibility of future settlement. Our FTB is designed to achieve maximum density at initial set.
Why is FTB Tricky?
FTB must be dense and strong enough to support vehicular traffic without settling. Yet, it cannot be so dense that it disrupts natural sub-surface water flow leading to heaving and cracking of the asphalt surface above. And, FTB needs to be low strength enough to dig up if access to the cable is needed in the future. Balancing these three requirements with the density properties required to meet the typically stringent thermal specification is a challenging task that if not designed, manufactured and delivered properly can work against each other.
Why inTerra?
inTerra’s geotechnical engineering background and vast experience with the manufacture and delivery of thermal materials allows for us to meticulously balance these three critical ‘DOT’ requirements (load bearing; hand excavate-ability; permeability) with the requirement that is most critical to the electric utility: thermal conductivity.
Typical FTB use scenarios:
For more about the need for thermal protection around electric power cables, see “Thermal 101”.
Thermal Concrete
Thermal Concrete is a form of duct bank concrete used to both protect the conduits and encased electric transmission cables from future excavations and other disturbances, as well as to dissipate the heat generated from the electric transmission cables. Thermal Concrete is usually designed with a compressive strength >3,000psi at 28-days. The Thermal concrete must have a small enough aggregate size and consistency to flow easily through the duct bank conduit and spacer system and bond with the soil of the trench walls.
For more about the need for thermal protection around electric power cables, see “Thermal 101”.
Thermal Grout
When an underground electric transmission line transitions into a Horizontal Directional Drill or a Jack and Bore (under a river or highway, etc.), the need for heat dissipation does not go away.
Opposed to open trench heat transfer solutions (Thermal Concrete, Fluidized Thermal Backfill) which are simply backfilled into the trench, Thermal Grouting is a complex process that must be engineered from start (mix design engineering) to finish (delivery within the casing). inTerra engineers superior thermal grouts and delivery systems eliminating the Achilles heal (hot spot) within the transmission line route.
Dense, thermally conductive grout is required to fill the voids within the HDD or jack & bore casing and around the conduits (which house the cables) or within the conduit around the cable. Thermal grouts can be cementitious or non-cementitious depending upon the specific use. The diagrams below, depict typical HDD, jack & bore or conduit grouting arrangements.
What is Thermal Grout?
Thermal grout is a combination of fine aggregates with heat transfer properties, water, additives, and cement or bentonite designed to stay in suspension during and after installation to maintain the integrity of the original mix design and thus heat transfer properties. The specific combination of ingredients is, among other factors, tailored for:
- Pumping over long-distances (1,000′-5,000′) within tight spaces
- Filling the annulus of a casing with a crowded conduit system
- Providing resistance to the conduit system during cable pulling
- Most importantly: optimal heat transfer to protect costly power cables
For more about the need for thermal protection around electric power cables, see “Thermal 101”.
Why is Thermal Grout Tricky?
Harmoniously balancing density, fluidity and the ability of the grout to stay in suspension while traveling through tight voids and at long distances is a difficult yet critical task. Pumping a thick, viscous, heat transfer material in these conditions can easily create internal pressures high enough to deform the conduits inside an HDD or jack & bore. The result can easily be that the conduit bundle is rendered useless, and a complete extract and rebuild is the only solution.
Thermal grouting is a bid item that is often overlooked and addressed when it is too late, and when the wrong conduit, spacers, strapping, welding and other items have already been specified, purchased and assembled into a conduit bundle. This is a costly mistake you can ill afford to make.
Thermal grout installation must be meticulously engineered prior to construction and in strict coordination with the drilling and/or electrical contractor to best mitigate the inherent risks associated with delivering a high density — low thermal grout within a closed environment housing the expensive high voltage electric transmission cables. Trying to “bull” your way through this sort of grouting project is a recipe for a very expensive disaster.
Why inTerra?
inTerra has the experience to work seamlessly with all parties involved from the design phase through construction to help mitigate potential risk of an inherently risky task, by providing recommendations on the conduit bundle system and material selection to ensure the system can withstand the grout delivery process.
inTerra is expert at sourcing thermal aggregates and conducting pre-design assessments in remote locations throughout the world, ensuring that the materials selected are functional for the project from a thermal and flow-ability/pump-ability standpoint, yet are not cost-prohibitive. To complete the design and ensure the thermal properties adhere to the specification, we retain the third party services of Geotherm, Inc., the world-wide leader in thermal design. Combining our lab and field expertise with Geotherm’s prowess ensures an optimized thermal grout that is able to be pumped at the long distances and in the tight spaces often required.
inTerra also devises comprehensive grout manufacture and delivery systems to ensure that all voids within the casing are filled with grout so that the cable can perform at its’ design ampacity. We work seamlessly with drilling contractors and electrical contractors to help mitigate the inherent risks associated with thermal grout delivery.
With virtually no margin of error, inTerra is dedicated to ensuring the success of thermal grout installation in the design phase as opposed to reacting to the many mistakes often made due to the industry’s lack of knowledge.
inTerra is a national specialty contractor and construction materials provider focused on innovative solutions for the design, manufacture, delivery and installation of products within the utility, renewable energy, geo-technical, heavy-civil and infrastructure industries.
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