Thermal Sand for Underground Cables

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Thermal Sand for Underground Cables
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• A significant source of problems with underground
  cables is poor selection and installation of
  thermal backfill materials. To prevent premature
  failures, you must ensure you place cable systems
  in a hospitable environment.

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• Importance of Thermally Stable BackfillAll the
  heat generated by an underground power cable must
  be dissipated through the soil. This is
  quantified by the soil thermal resistivity (or
  thermal rho, C-cm/W), which can vary from 30 to
  500C-cm/W.

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• The ability of the surrounding soil to transfer
  the heat determines whether an operating cable
  remains cool or overheats. Improving the external
  thermal environment and accurately defining the
  soil and backfill thermal rho commonly results in
  a 10 to 15 increase in cable capacity and
  sometimes up to 30.

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• The use of a soil thermal rho of 90C-cm/W has
  become cable engineering practices. Soil studies
  performed in the 1950s found this was a safe
  value for most moist soils. Howver for
  transmission cables, it is assumed that the
  thermal backfill placed around the cables will
  have a thermal rho of less than 90C-cm/W.

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Thermal Backfills

• Most moist soils (with the exception of organic
  clays and silts, volcanic soils, peat and fills
  with ash and slag) have a rho of less than
  90C-cm/W. Sands when moist may even have a rho
  of less than 50C-cm/W.

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• However many soils, especially uniform sands, can
  dry substantially when subjected to heat from the
  cables. The thermal rho of a dry soil would
  exceed 150C-cm/W, and possibly approach
  300C-cm/W for a dry uniform sand. Most
  contractors would use readily available fine sand
  or concrete sand as the backfill as this sand
  makes an inexpensive backfill material, but
  thermally, it is very poor because it dries out
  easily under high cable loads.

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• Poorly compacted trench backfill is another major
  problem. Not only is the thermal rho of
  uncompacted soil significantly higher, but the
  loose soil will dry more easily.

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Corrective Thermal Backfills

• Native soils usually do not make good thermal
  backfills because their thermal rho values are
  poor. The operational reliability gained by
  placing a properly constituted thermal backfill
  around the cable has advantages over the
  variability of re-compacted native soil.

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• Yuleba Minerals (www.yulebaminerals.com.au) has a
  graded and tested thermal backfill that has been
  used in Roma, Miles and Surat basin projects.
  There is a need for quality assurance during
  installation. If the gradation of the backfill is
  not the correct size moisture or not enough
  compaction effort is applied then the maximum
  density will not be achieved and the thermal
  capability degraded.

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• Cement stabilized sand frequently has been used
  as a cable trench backfill. A typical mix design
  consists of 15 parts sand to 1 part cement, mixed
  with about 10 parts water. However, this backfill
  is quite strong and thus would be difficult to
  excavate. 

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• Achieving soil density is needed in the
  restricted trench areas near cables or around
  cable pipe groups where proper compaction is
  difficult. Yet, it is precisely in these zones
  adjacent to the cables, where the heat flux is
  highest.

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Fluidized Thermal Backfills 

• Fluidized thermal backfills (FTB) is a slurry
  backfill consisting of medium aggregate, sand, a
  small amount of cement, water and a fluidizing
  agent. FTBs can be made with locally available
  sand and aggregates. The component proportions
  are chosen by laboratory testing of trial mixes
  to minimize thermal resistivity and maximize flow
  without segregating the components.

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• Fluidized thermal backfills do not have to be
  compacted they flow in a fashion similar to
  concrete. In fact, FTB is typically supplied from
  concrete trucks, and may be poured or pumped. It
  solidifies to a uniform density by consolidation,
  with excess water seeping to the top. It hardens
  quickly so that the ground surface may be
  reinstated the next day, but the low strength
  (100 to 250 psi 0.7 to 1.8 MPa) allows it to be
  broken up with a backhoe if required.

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• If a higher strength is required, the cement
  content can be increased and the water adjusted
  without degrading the thermal performance.Backfil
  ls The Right WayThe use of a well-designed
  thermal backfill can enhance the heat dissipation
  and increase the allowable increased capacity of
  an underground power cable, as well as
  alleviating thermal instability concerns.

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• The corrective backfill will reduce the heat flux
  experienced by the native soil so that it will
  not dry out therefore, the stability of the
  native soil is no longer a concern. A good
  backfill should be better able to resist total
  drying and also have a low dry thermal rho if it
  is completely dried. It should be available at a
  reasonable cost, and be easy to install and easy
  to remove if required.

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• The thermal backfill must be laboratory evaluated
  and include specifications for mineral quality,
  gradation (sieve analysis), thermal dry out curve
  and optimum density. Typically, the entire trench
  width is filled with thermal backfill to a
  minimum height of 300 mm (12 inches) above the
  cables. For poor native soil conditions or
  heavily loaded cables, the thickness of the
  backfill can be increased to maintain a low
  composite thermal rho. A fluidised thermal
  backfill is the ideal way of providing a
  high-quality cable backfill.

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• For further information please visit

http//www.yulebaminerals.com.au/