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TB 714 long term performance of soil and backfill systems
The loading of aging underground cable systems is both increasing and becoming more stochastic. A major impact to conductor temperature rise is due to the installed environment. Soil and backfill conditions are thus critical to the integrity of a cable system. WG B1.41 has investigated the long-term degradation mechanisms and their mitigation. A Users Guide provides straightforward assessment methods to determine the suitability of soil and backfill systems for cables of all voltages.
The backfill or material surrounding the cable is a critical engineering component of the cable system, which in conjunction with the surrounding material is the means of conduction of the heat, produced by the operation of the cable, to the ground surface. The thermal behavior of the cable system is dependent on the backfill characteristics. . The technical brochure covers the experience from 19 countries covering failures and success in these areas. It also covers the following:
- Terminology and types of backfill soils
- Moisture migration including physics and modelling with some experimental results
- Installation of backfills including direct buried cables, cables in ducts, steel pipes and troughs. Submarine cables are also covered. Backfill compaction and tools are also discussed.
- Test methods for backfill properties are covered including standards, measurement methods,
- The long-term degradation and behavior of the soils, the consequences thereof and mitigating techniques are also covered.
- A user's guide is also provided with flow charts for rating and backfill assessment.
There are several well-known incidents in which the performance and /or condition of the backfill has contributed to the failure of a cable system. One of the most notable was in Auckland in 1998, which suffered failure of all 4 of the 110kV cable circuits feeding the Central Business District. The initial cable breakdown was attributed to either gas loss (from the gas pressure cable) or thermo-mechanical problems; the subsequent 2 circuit failures were due to thermo-mechanical problems (gas pressure and fluid filled circuits) whilst the final (fluid filled) circuit failed due to thermal runaway. One of the main factors contributing to thermo-mechanical and thermal runaway problems was cited as the high thermal resistivity of the backfill material when it was dry, the original system design being based on thermal resistivity values that were not achieved in the actual installation. This brochure will assist designers and operators to understand the criteria related to cable systems to avoid such occurrences.
The document is applicable to MV and HV cable systems
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