Engineers need to know how soil actually behaves in place before they can size an underground cable with confidence. A lab test on a disturbed sample can tell you a lot, but it can’t fully capture the compaction, layering and moisture profile of the real trench. That’s what field testing is for — precise, depth-specific thermal resistivity data collected under the conditions the cable will actually see.
Why Field Testing, Not Just Lab Testing
Laboratory thermal resistivity testing is useful for specifying backfill materials before they’re ordered, but it can’t account for how native soil varies along a real cable route — a mix of clay, sand, fill and rock that no single lab sample represents. Field testing closes that gap by measuring resistivity directly in the trench, at the depth and moisture content the cable will operate in.
The Field Testing Process
- Route assessment and test point selection. The cable route or trench alignment is reviewed and test points are marked at intervals that reflect changes in visible soil type, fill depth or drainage.
- Probe installation. A thermal needle probe is inserted into undisturbed soil at cable-laying depth, either in an open trench or via a purpose-drilled hole, ensuring full contact between the probe and surrounding soil.
- Dry-out curve measurement. A controlled heat input is applied and the temperature rise is logged over time. As the soil immediately around the probe dries out under sustained heating, resistivity increases — this dry-out curve is the critical design input, not just a single reading.
- In-situ moisture sampling. Samples are taken alongside each probe location to record the as-tested moisture content, so lab and field results can be compared on a like-for-like basis.
- Repeat testing along the route. Multiple test points are run across the full route or site, not just one, since soil conditions can shift significantly over even a short cable run.
- Reporting against recognised standards. Results are compiled with test conditions, dry-out values and worst-case resistivity, referenced against the methods set out in IEEE 442 and ASTM D5334, which underpin most Australian utility specifications.
What the Results Tell Engineers
The worst-case (highest) resistivity value from the dry-out curve is what designers use for cable ampacity calculations, because it reflects the conditions under the greatest thermal stress. Field data collected this way gives a defensible, site-specific basis for cable sizing and backfill specification — rather than relying on generic soil assumptions that may under- or overstate a cable’s real capacity.
If you’re scoping a cable route, substation or trench project and need field data collected and reported to a recognised standard, our soil thermal & electrical resistivity testing service covers the full process above, Australia-wide.