How the soil and ground score works

A ground-condition screen, not a soil test

The soil and ground score uses SGU’s mapped soil classification, an estimated depth and supported evidence modifiers. It rates the ground condition on 0–100, not excavation affordability. Rock can be competent to build on while expensive to excavate; weak clay can be easy to dig while requiring difficult foundation work.

Resolve soil family and depth first

Bilby maps the SGU code to an engineering family, with some code-specific overrides. It uses a combined soil-depth estimate first, then the midpoint of a usable combined range, then the SGU model estimate. Depth bands are ≤0.5 m, >0.5–1.5 m, >1.5–3 m, >3–5 m and >5 m. An unmapped code, conflicting soil name, ambiguous/no point match or source error can require manual review instead of a score.

Apply only available evidence

Base ratings correspond to Excellent 90, Good 75, Average 55 and Poor 30. Add supported modifiers, round and limit to 0–100. The final label is Excellent at ≥85, Good at ≥65, Average at ≥45, otherwise Poor.

• Block-rich soil subtracts 5; very block-rich subtracts 10.
• A conflicting explicit firm-ground classification subtracts 3.
• A weak underlying layer beneath the supported coarse sand families subtracts 7; compatible coarse material below postglacial sand can add 2.
• Minor groundwater evidence subtracts 3; moderate/medium subtracts 5; high/very high/significant subtracts 10.

Missing evidence does not invent a modifier. An explicitly poor override and organic soil deeper than 0.5 m cannot exceed 30. Water is not applicable. Some mapped special classes and missing depth require manual review rather than a zero.

Fictional mapped moraine

Take standard moraine with 2 m estimated depth: the moderate-depth base is Excellent, 90. Explicit block-rich evidence gives −5 and moderate groundwater gives −5. Final score = 90 − 5 − 5 = 80, labelled Good. This does not mean an 80% probability of acceptable bearing capacity. Separately, organic soil at 2 m begins Poor and remains capped at 30 even if another modifier would improve it.

Confidence and investigation are separate

Confidence begins from depth evidence and is reduced for missing depth, inconsistent wells, classification problems, boundary ambiguity, high mapped variability or an unusually wide range. Missing critical evidence makes confidence low; some lesser uncertainties reduce high to medium. Strong well evidence can lift medium to high only without those uncertainties.

Low confidence or inconsistent wells can strengthen the investigation recommendation without simply subtracting another score penalty. Deep fine/organic/fill soils also prompt investigation. Changing a budget price or the visual house design does not change the mapped soil score. A real geotechnical investigation can reveal conditions that mapped data cannot resolve.

Method and review

Method bilby-ground-conditions-v1 and its listed-plot/coordinate displays were reviewed 18 October 2026. The family/depth matrix and modifiers are Bilby’s screening method. SGU supports the geological inputs, not these point values or contractor rates. SGU soil mapping