Frequently Asked Questions

Most councils and certifiers require a current AS 2870 site classification for any new footing or slab, including extensions, decks with footings, and granny flats — check with your certifier before starting design.

Septic soil testing measures how your soil absorbs effluent (the percolation rate); effluent disposal design uses that data, plus site conditions, to design the actual land application or disposal area.

Fill sites often trigger a “Class P” (problem) classification and may need additional testing or a specific engineering design — we'll flag this early so it doesn't hold up your build.

Before subdividing or building on a sloping site, after any sign of ground movement or cracking, or when council requires it as a condition of development approval.

Thermal backfills are engineered materials (often sand-cement blends or specially graded aggregates) designed to provide consistent, low thermal resistivity around underground power cables. They're used in cable trenches to improve heat dissipation where native soils are too dry, variable or resistive.

Soil thermal conductivity measures how well soil transfers heat, impacting underground cable ampacity. Soil electrical resistivity measures how strongly soil resists electrical current flow, affecting earthing and grounding system performance.

Moist soil typically has lower thermal resistivity, dissipating cable heat more effectively. Dry soil significantly increases thermal resistivity, reducing cable capacity and increasing overheating risk.

Field testing is commonly performed using a thermal needle probe inserted into undisturbed soil samples or in-situ trench locations, applying controlled heat and measuring temperature rise over time.

Solar and renewable sites rely on extensive underground cabling where thermal performance affects system efficiency, and electrical resistivity testing ensures compliant earthing for inverter stations and switchyards.

Consistent material grading, moisture conditioning, controlled compaction, and avoiding contamination with native spoil, installed evenly to eliminate air voids and aligned with project specifications.

Engineers should review dry-out resistivity values, in-situ moisture content, soil classification and test conditions, paying attention to worst-case thermal resistivity values for conservative cable rating and design.

Clay soils can shrink and crack when dry, increasing resistivity. Sandy soils drain quickly and become highly resistive when dry. Rock and uncontrolled fill create inconsistent thermal paths requiring engineered backfill.

Typically for substations, large cable routes, solar farms, wind farms and major civil works, especially with unknown ground conditions, high loads, or deep excavations.

Testing only in wet conditions, assuming uniform soil along long cable routes, using disturbed samples, or failing to test worst-case dry density can all lead to undersized cables or overheating risks.