Thermal Sand for Underground Cables
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.
Importance of Thermally Stable Backfill All 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, 째C-cm/W), which can vary from 30 to 500째C-cm/W.
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%.
The use of a soil thermal rho of 90°C-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 90°C-cm/W.
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 90째Ccm/W. Sands when moist may even have a rho of less than 50째C-cm/W.
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 150째C-cm/W, and possibly approach 300째C-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.
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.
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.
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.
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.
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.
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.
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.
If a higher strength is required, the cement content can be increased and the water adjusted without degrading the thermal performance. Backfills ‌ The Right Way The 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.
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.
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 highquality cable backfill.
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