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What is Fluidized Thermal BackfillTM(FTBTM)?

Fluidized Thermal Backfill (a trademark of Geotherm, Inc.) is a form of controlled low-strength material specifically designed to dissipate the heat generated by underground electric transmission cables within city streets or on rural wind farms.  Unlike many native soils, FTB provides consistent heat dissipation along the entire transmission line. 

  Grout Products:

    Thermal Grout

    SlipLine Grout

    Fill Grout

    Compaction Grout

  Thermal Products:

    Fluidized Thermal Backfill (FTB)

    Thermal Concrete

    Thermal Grout

  Flowable Fill & Concrete:

    Flowable Fill

    Structural Concrete

    Rapid-Set Concrete

    Duct-Bank Concrete

    Insulating Concrete

    Roller Compacted Concrete

Characteristics of FTB

Superior Thermal Properties:                              

FTB is aggressively designed to have low thermal resistivity, typically less than 100 degrees C-cm/W in the total dry condition (0% moisture).  

Hand Excavate-able:

FTB is designed to be low strength (50-150 psi) like a stiff soil allowing for future access to the conduits below if necessary.  


During installation, FTB must be able to flow fluently enough to fill all voids and crevices in the trench, between conduits, and to bond with the trench sidewalls.

Maximum Density

Once hardened, FTB must be able to support load from the traffic above, and must eliminate the possibility of future settlement.  inTerra FTB is designed to achieve maximum density at initial set. 

Why is FTB Tricky?

FTB must balance various characteristics critical to many different governing bodies (e.g., DOTs) while maintaining the critical thermal properties most important to the electric utility for longevity of the cables.  Sourcing the minerals that transmit heat, and then figuring out the design for combining these ingredients into a thermally conductive mix is something that your local ready-mix supplier typically will not have familiarity.

DOT Requirements:

  1. FTB must be dense and strong enough to support vehicular traffic without settling.

  1. FTB cannot be so dense that it disrupts natural sub-surface water flow leading to heaving and cracking of the asphalt surface above. 

  1. Despite its high density, FTB must flow fluently within the trench filling all voids eliminating future settlement.  

Other Existing Utility Requirements:

  1. FTB needs to be low strength enough to dig through if access to the electric cable or other utilities within the same right-of-way is needed in the future. 

Electric Utility Requirement:

  1. FTB must be thermally engineered and delivered in accordance with mix design parameters to achieve the thermal resistivity specified by the owner of the project, the electric utility.

Why inTerra?

Our staff’s background in geotechnical engineering and operational  has led to the successful design, manufacture and delivery of over 40,000 cubic yards of FTB throughout the United States and Singapore.

inTerra’s design staff has over 10-years experience sourcing optimal thermal minerals, as well as designing thermally-enhanced FTB mixes meeting and exceeding criteria required to avoid the problems inherent to FTB.

inTerra’s field staff is expert at delivering FTB that will flow within the trench, filing all the tight voids, and creating a superior thermal environment, ultimately ensuring that the heat generated by the cables is dissipated for years to come.

For more about the need for thermal protection around electric power cables,

see Thermal 101.


Fluidized Thermal Backfill