Cable Thermal Backfill Sand and Thermal Resistivity Guide
Why does thermal backfill sand matter for underground power cables? We explain thermal resistivity, the suitability of silica/quartz, the critical role of moisture, dry-out risk and the IEEE 442 test.
The safe and efficient operation of underground power cables depends on how well the surrounding backfill material conducts heat. Cables heat up as they carry current; if this heat cannot be removed quickly, the cable overheats, its capacity (ampacity) drops and its service life may even be shortened. This is where thermal backfill sand — and its most important property, thermal resistivity — comes in. In this article we explain what thermal resistivity is, why silica/quartz sand is suitable for the job, and the critical role of moisture.
What Is Thermal Resistivity?
Thermal resistivity is a measure of a material's resistance to heat flow, expressed in K·m/W (or °C·m/W). The lower the value, the better the material conducts heat — in other words, the faster it carries heat away from the cable. That is why low thermal resistivity is required in cable backfill.
Thermal resistivity is the reciprocal of thermal conductivity (λ, W/m·K). For example, a resistivity of 0.5 K·m/W corresponds to a conductivity of 2.0 W/m·K. For reference, still water has a thermal resistivity of about 1.65 K·m/W, while air is around 40 K·m/W; this huge difference explains why the moisture issue we discuss below matters so much.
Why Is Silica/Quartz Sand Suitable?
Among natural materials, the quartz mineral has a high thermal conductivity; it conducts heat far better than feldspar or clay. High-SiO₂ silica and quartz sand is therefore physically well suited to cable thermal backfill. In addition, well-graded sand (with a broad grain size distribution) packs more tightly when compacted, reducing the voids between grains and improving heat transfer.
There is, however, a critical detail: the thermal performance of sand emerges in its moist, compacted state — not when it is dry or loose.
The Critical Factor: Moisture and Compaction
The performance of dry sand is misleading. In dry sand, the voids between the grains are filled with air, and air has a very high thermal resistivity. Dry sand therefore conducts heat poorly and its thermal resistivity rises. When the sand is moist, water fills the voids between the grains, forming heat bridges, and the resistivity drops significantly. Compaction reinforces the same effect by reducing the voids; indeed, resistivity falls as the sand is compacted.
There is an important risk here known as thermal dry-out: as the cable heats up, it can drive the moisture out of the surrounding sand. As the sand dries, its resistivity rises, which leads to more heating and further drying; left unchecked, this can become a vicious circle known as thermal runaway. Serious projects therefore evaluate the backfill's resistivity in both the dry and the moist state, together with a reference density.
How Is It Tested?
To be able to guarantee that a sand achieves a specific thermal resistivity value (for example 0.5 K·m/W), measurement is required. The standard method is the thermal needle probe test under IEEE 442 and ASTM D5334. The test is performed at controlled moisture and density, and a "thermal dry-out curve" (the moisture–resistivity relationship) is usually produced. In other words, before a sand can be said to "achieve 0.5", the moisture and compaction conditions under which that value was obtained must be stated.
When a Dry-State Guarantee Is Needed: Stabilised Backfill
In critical projects where the value must be guaranteed regardless of moisture — even in the dry state — stabilised (cement-bound) thermal backfill comes into play. Known as fluidised thermal backfill (FTB), this solution is a pourable, concrete-like mixture of aggregate, sand, a small amount of cement, water and a plasticiser. It settles into place without compaction and provides a low, stable thermal resistivity. The base material in such a backfill is, again, silica/quartz sand.
You can read about the general properties of silica and quartz sand in our silica sand and quartz sand articles.
Conclusion
In summary, the purpose of thermal backfill sand is to carry heat away from the cable quickly, and the measure of this is low thermal resistivity. High-SiO₂, well-graded silica/quartz sand is a suitable material for the job; however, the target resistivity can only be guaranteed under the right moisture and compaction conditions, backed by measurement. To determine the right thermal backfill solution for your project, please get in touch with us.