August 11, 2014
Customer Requirements
Thermal Backfill for Underground
Power Cable Installations C-UG-2050
Application
Flowable Thermal Backfill (FTB) is placed around power cable conduit runs to more
effectively dissipate heat generated by the cables into the surrounding
environment. FTB is a uniform and efficient heat conducting medium that also
provides 100% compaction, structural support, and mechanical protection for the
conduit systems.
Why use FTB?
• Multiple cables in conduit installed in the same trench will experience mutual
heating and run hotter.
• Multiple conduits in a common trench add air insulation which holds in heat.
• Communication duct banks parallel to the power trench insulate heat and
cause the power cables to run hotter.
• Other heat sources that are either parallel or cross the power trench will
reduce heat dissipation and cause the power cables to run hotter and/or
create hot spots.
• All of the above conditions may require de-rating of the current carrying
capacity of the power cables unless the heat can be dissipated.
• Using FTB around the power ducts helps disperse the heat from the trench
line to the surrounding air.
When to Install FTB
Use FTB when a power trench has any of the following:
• 3 or more feeder conduits (include spare feeder conduits in the count).
• 4 or more feeder and distribution conduits combined.
• When a communication duct line is within 12 inches and parallels the feeder
trench line (for trenches containing 2 or more feeders).
• When the power conduits are within 2 feet of a secondary heat source, such
as a steam line or another underground feeder crossing. FTB use is needed
only in conflict areas.
Performance vs. Cost
Industry consultants report a common 5% to 10% increase in cable ampacity with
the use of thermal backfills. See standard E-GR-1500, “Data for Underground
Primary Cables” for cable ampacities.
Similar to Controlled Density Fill (CDF) backfill, FTB does not need to be compacted,
hardens quickly so that backfill can be completed the following day, and readily fills
into all trench cavities without vibration.
Expect an approximate 10 % increase in cost for FTB over CDF backfill.
Transmission & Distribution Standards Page 1 of 4
August 11, 2014
Customer Requirements
Thermal Backfill for Underground
Power Cable Installations C-UG-2050
FTB Selection
The approved FTB local mix designs are available for the following compressive
strength ratings:
Non-Traffic Mix Traffic Mix
Supplier
(300 psi FTB) (1000 psi FTB)
Corliss J5201FTB J5201FTSB
Glacier 0307 3253
Other suppliers may be available if approved by the Tacoma Power Engineer. See
the following “FTB Components” section for more information on FTB components.
FTB Components
Components The proportions of all FTB components are balanced such that when the specified
amount of water is added a uniform mix will be obtained that will not segregate when
installed by pouring. No substitution of materials is permitted.
Component Description
Normal Portland cement conforming to ASTM
Cement Designation C150. The quality of the cement determines
the compressive strength.
Class "C" or "F" fly ash. The amount of fly ash
Fluidizer
influences the flow.
Concrete sand with a particle size distribution meeting
Fine Aggregate ASTM C33 limits for fine aggregates. This governs
thermal properties.
The maximum aggregate gravel size shall be 3/8" minus.
Medium Aggregate
This also governs thermal properties.
Additives No other additives shall be added. Any deviations may lead to problems with flow
and water demand which in turn would affect soil thermal resistivity and strength.
Under no circumstances shall air entrainment additives be added. The only
remedy available to the Contractor if an air entrainment additive is included in the
mix is to physically remove all of the FTB from the cable trench and start over.
Installation
Water It is recommended to add slightly less than the required amount of water at the
batching plant. Should there be a need to add more water to achieve a good
homogeneous flow; this can always be done at the job site prior to the pour.
Batching Batching shall be done at a central plant and the blended FTB supplied by ready mix
concrete trucks (ASTM C94). The FTB shall be supplied and transported in such a
way as to minimize segregation and facilitate installation.
Transmission & Distribution Standards Page 2 of 4
August 11, 2014
Customer Requirements
Thermal Backfill for Underground
Power Cable Installations C-UG-2050
Installation (continued)
Pouring The ends of the trench shall be secured by bulkhead or earth fill.
The FTB is to be installed by pouring into the trench and completely filling all the
voids without causing excessive segregation. No vibration or compaction is
allowed.
At the discretion of the Contractor, the flow may be adjusted slightly by changing the
amount of water slightly (as long as no segregation occurs, and the FTB fills all the
voids completely when poured).
Pumping For pumping applications the flow may have to be modified. Changes to the mixture
to facilitate pumping shall be submitted to Tacoma Power 30 days prior to
installation.
Bleed Water The FTB is in a slurry form so it can flow and fill all voids. The slurry phase will
quickly consolidate, resulting in excess bleed water. The Contractor shall make
provisions to allow this bleed water to be contained, drained, or be pumped away in
an environmentally acceptable manner.
Temperature FTB shall not be placed on frozen ground. At the time of placement, FTB must have
Restrictions a minimum temperature of 40˚F. Mixing and placing shall stop when air temperature
is 38˚F or less and falling.
Conduit Conduits to be encased in FTB shall be installed with spacers and hold downs (see
Buoyancy C-UG-1100) to be adequately anchored so that they do not float during FTB
placement. Tie-Downs and/or FTB piles should all be installed at regular intervals
depending on quantity and size of conduits, i.e., the overall buoyancy of the duct
bank. Other methods may be used if approved by the Tacoma Power Engineer.
Quality Control
With thermal backfills, the quality is in the mix. If it is properly formulated and
blended, then the installation and final product performance will meet the
specifications. Information for various tests and reports may be required should the
Tacoma Power Engineer deem them necessary.
Thermal The FTB should have a thermal resistivity Rho (ρ) of less than
Resistivity 40°C-cm/Watt when moist and less than 100°C-cm/Watt when in place and totally
Test & Values dry. If testing should be required, samples shall be sent to Geotherm, Inc., or an
alternate testing laboratory approved by the Tacoma Power Engineer.
Some typical values for RHO (ρ) that are commonly used:
Thermal Rho (ρ)
Backfill Material
°C-cm/Watt
Moist native soil 90
Concrete 55
FTB 40
Transmission & Distribution Standards Page 3 of 4
August 11, 2014
Customer Requirements
Thermal Backfill for Underground
Power Cable Installations C-UG-2050
Other Factors
Other factors can change the soil thermal resistivity:
• Moisture is critical to soil thermal resistivity. Sand, for example, dries out
easily resulting in high Rho values.
• Burial depth also has a small impact on cable ampacity. Tests have shown
reducing depth from 36” to 30” (to top of enclosure) will increase ampacity by
1-3%.
• Soil thermal resistivity decreases with compaction.
Sources
Standard Title
Standard Practice for Making and Curing Concrete Test
ASTM C31
Specimens in the Field
ASTM C33 Standard Specification for Concrete Aggregates
ASTM C94 Standard Specification for Ready-Mixed Concrete
ASTM C172 Standard Practice for Sampling Freshly Mixed Concrete
IEEE 442 IEEE Guide For Soil Thermal Resistivity Measurements
Geotherm Inc.
561 E Elliot Road # 155
Chandler, AZ 85225-1119
Thermal Resistivity
480-892-9723
Consultant
www.geotherm.net
TM
Flowable Thermal Backfill (FTB ) is a Geotherm Trade Mark
Transmission & Distribution Standards Page 4 of 4
