Reading a soil report: what residential builders need to know

TL;DR

A residential geotech report classifies your site under AS 2870:2011 from Class A (stable) to Class P (problem), and that class drives every structural dollar: slab thickness, edge beam depth, concrete grade, and whether your engineer can use the ABCB Housing Provisions 2022 DTS tables at all. Class A and S get prescriptive DTS footing designs. Class M moves slightly (Ys 20-40 mm) and still has DTS options; Class H1 (40-60 mm) and H2 (60-75 mm) need full AS 2870 engineering. Class E (>75 mm) and P go straight to custom design. Cost: a standard residential geotech runs $500 to $1,500 ex-GST (verified 2026-05-10); a Class P site that needs a custom foundation adds $15,000 to $40,000+ before a slab is poured. The report is usually three to ten pages. Read the site classification, the borelog, the water table note, and the fill section before the engineer does anything else. Those four items determine your exposure.

When you do this

You read the soil report at two points:

1. Before quoting the slab and footings: know whether the prescriptive DTS tables apply or whether you need an engineered design. Quoting on an assumed Class M when the site is H2 means you absorb the cost delta.
2. When the engineer returns the report: before a structural design is commissioned, check the four key sections yourself. A five-minute read surfaces most of the commercial surprises before they get priced into a structural fee or missed entirely.

Most councils and certifiers require the geotech report before issuing a Construction Certificate. Get it at the DA stage wherever possible.

Who’s involved

• Geotechnical engineer: conducts the site investigation, writes the report, determines the site class under AS 2870:2011. Must comply with AS 1726:2017 for investigation methodology (verified 2026-05-10).
• Structural engineer: uses the site class to design the footing and slab system. On Class P sites, both engineers often collaborate to produce the foundation design.
• Builder: reads the report to understand commercial exposure before quoting. You do not need to interpret every line. Read the four sections below and flag anything that needs engineering clarification.

Steps

1. Find the site classification

The site classification is the summary verdict. It appears on the first or second page of any competent report, usually in a prominent box or table. Note both the class and any suffix:

Class	Characteristic surface movement (Ys)	What it means for footing design
A	0 mm	Stable sand or rock. DTS prescriptive tables in ABCB Housing Provisions 2022 apply.
S	0-20 mm	Slightly reactive clay. DTS tables still apply with light clay provisions.
M	20-40 mm	Moderately reactive. DTS tables may apply; check the Housing Provisions for your specific element.
M-D	20-40 mm, deep suction	As M but suction change zone extends to 3 m or deeper (dry climate). Requires AS 2870 engineering.
H1	40-60 mm	Highly reactive. DTS tables do not apply; engineer designs to AS 2870 directly.
H2	60-75 mm	Very highly reactive. Full AS 2870 engineering required; typically a stiffened raft with heavy beams.
E	>75 mm	Extremely reactive. Custom engineering mandatory; rare in residential.
P	Problematic	Problem site: uncontrolled fill, soft soils, abnormal moisture, mine subsidence. Custom engineering always required.

(Ys values per AS 2870:2011 via Standards Australia; verified 2026-05-10.)

The “-D” suffix (M-D, H1-D, H2-D, E-D) flags deep-seated moisture change: the design depth of suction change (Hs) is 3 m or greater. This is common in dry inland climates and increases the slab reinforcement and beam depth requirements.

What to do: If the class is H1 or above, your structural engineer cannot use the Housing Provisions DTS tables. Budget for a full AS 2870 design fee and a slab quote based on an engineered drawing, not a prescriptive table. If the class is P, read the problem-site section carefully and expect a custom foundation package.

2. Read the borelog

The borelog (also called a borehole log or test pit log) is a column chart showing the soil profile from the surface downward. The geotech drills or digs to a depth appropriate for the site class (typically 3 to 5 m for a Class M or H site) and records what they find at each depth interval. The log format follows AS 1726:2017 (verified 2026-05-10).

What each column tells you:

• Depth (m): how far below natural surface each observation was made.
• Material description: soil type (clay, sand, gravel, silt, rock), colour, moisture condition, consistency (soft, firm, stiff, very stiff), and plasticity (low, medium, high). High plasticity clays are candidates for high-reactivity classifications.
• Sample type: whether the material was disturbed (auger sample) or undisturbed (thin-wall tube). Undisturbed samples are used for laboratory shrink-swell testing.
• Test results: DCP (Dynamic Cone Penetrometer) blows/300 mm or pocket penetrometer readings in kPa. Very low readings (soft ground) at shallow depth are a red flag.
• Groundwater: depth at which water was encountered during drilling, noted as GW @ Xm or similar.

What builders need from the borelog:

• Is there a consistent stiff material at founding depth? Soft or variable material at the designed pad or beam level means the engineer needs to know before finalising the design.
• Is the fill annotated as controlled or uncontrolled? (See Step 3.)
• Do multiple boreholes show consistent soil conditions, or does the profile vary between holes? Significant variation (e.g., rock on one side, soft clay on the other) often upgrades the classification or triggers a Class P designation.

A competent residential geotech report includes at least two boreholes or test pits. Three is better on sites over 600 m2 or where the surface topography suggests variable conditions.

3. Check the fill notation

Fill is former topsoil, excavated material, demolition debris, or imported material placed on the site before construction. The distinction that matters is whether the fill is controlled or uncontrolled.

Controlled fill has been placed and compacted in layers, tested for relative density per AS 3798:2007, and documented with a compaction report from a geotechnical engineer (verified 2026-05-10). The documentation chain must be continuous: no gaps, no missing test results. If controlled fill documentation exists, the engineer can treat it as a known bearing material and design accordingly.

Uncontrolled fill (also called undocumented fill) has no compaction testing or has been placed without geotechnical supervision. It cannot be relied upon for footing bearing without investigation, because the density, composition, and moisture are unknown. Common sources: old garden fill, demolition rubble pushed into a low spot, material dozed across a site during subdivision. A site with uncontrolled fill under the building footprint is almost always Class P.

What to look for in the report:

• A note like “fill to X m depth, controlled per AS 3798” and a reference to a compaction report is good.
• A note like “fill encountered, origin unknown” or “variable fill material including brick fragments, organic matter” is a red flag.
• If fill is noted but no AS 3798 compaction documentation is referenced, ask your engineer whether existing fill needs to be removed, over-excavated, or replaced before the foundation is designed.

Flag to the engineer: If the borelog shows fill and the report is silent on whether it is controlled, do not assume controlled. Ask for written clarification before quoting.

4. Note the water table depth

The report records the depth at which groundwater was encountered during drilling. This is stated as the standing water level (SWL) or groundwater level (GWL). A typical notation: GWL @ 2.4 m bgl (below ground level).

Water table depth affects:

• Edge beam depth: if the water table is above the underside of the edge beam, the design may need to account for buoyancy or moisture-driven reactive movement.
• Subfloor drainage: on Class H and E sites, persistent high water can amplify reactive movement significantly.
• Fill assessment: high water tables combined with fill are a compounding risk.

The water table reading is a snapshot at the time of drilling. The report should note seasonal variation expectations; if it does not, ask. In reactive clay areas, the wet-season water table can be substantially higher than the dry-season drilling depth.

What to flag: Water table within 1 m of natural surface in a reactive clay area (Class M or above) is worth raising with the structural engineer before the footing design is finalised. It can change beam dimensions and concrete specification.

5. Read the engineer’s recommendations section

Every competent geotech report includes a recommendations section. At minimum it will state:

• The recommended footing system (stiffened raft, waffle pod, pier and beam, etc.)
• Any site preparation required (remove fill, import controlled fill, treat reactive clay)
• Whether tree influence zone (TPZ) investigations are needed (mature trees within 1.5 to 2x canopy radius of the building footprint can cause localised moisture change that upgrades classification)
• Whether further investigation is recommended

If the recommendations section says “further investigation required”, that is not optional language. It means the engineer cannot complete the classification reliably on current data. More boreholes, deeper bores, or laboratory tests are needed before a structural design can proceed.

Common red flags: when to send it back to the engineer

These findings in a report warrant a written query to the engineer before any structural design is commissioned:

Finding	Why it matters
Fill noted, origin or compaction status not documented	Site may be Class P; foundation design cannot proceed without fill investigation
”Further investigation recommended” in the recommendations	Classification is incomplete; current data is insufficient
Soft or very soft clay at founding depth (DCP very low blows)	Bearing capacity may be inadequate for prescriptive footing designs
GWL within 1 m of surface on Class M or above	Reactive movement may be amplified; structural design needs to account for moisture variation
Single borehole on a large or topographically variable site	Profile may not represent the full building footprint
Significantly different profiles between boreholes	Variable conditions; may trigger Class P or require deeper investigation
Acid sulfate soil noted or flagged	Requires treatment plan; acid sulfate soils affect concrete durability and excavation spoil disposal
No wind classification stated	Report incomplete for full structural design
Site class is E	Rare in residential; confirm the engineer has designed to AS 2870 Part 4 directly

Documents needed

• Geotechnical soil report (by a geotechnical engineer, per AS 2870:2011 and AS 1726:2017)
• Compaction report for any fill (per AS 3798:2007, if controlled fill is present)
• Tree report if large trees are within the influence zone

What the report does NOT tell you

A residential geotech report classifies the site and recommends a footing system. It does not:

• Design the slab or footing (that is the structural engineer’s job)
• Tell you the slab cost (that comes from the structural engineer’s drawings)
• Guarantee conditions between boreholes (it samples; it does not scan the whole footprint)
• Address contamination (a separate Phase 1/2 environmental assessment is needed if contamination is suspected)

References

• Standards Australia, AS 2870:2011 Residential slabs and footings, Standards Australia Limited (verified 2026-05-10 via store.standards.org.au)
• Standards Australia, AS 1726:2017 Geotechnical site investigations, Standards Australia Limited (verified 2026-05-10 via ansi.org)
• ABCB, Housing Provisions Standard 2022 Part 4.2, Australian Building Codes Board (verified 2026-05-10 via ncc.abcb.gov.au)
• Standards Australia, AS 3798:2007 Guidelines on earthworks for commercial and residential developments (verified 2026-05-10)
• SkyCiv, AS 2870 Residential Slab Design Calculator, AS 2870:2011 site classification Ys values (verified 2026-05-10 via skyciv.com)

Related

• AS 2870:2011 residential slabs and footings
• Site classification
• Reactive soil
• Soil report (geotech)
• Class P site
• Controlled fill
• Slab on ground construction
• Pad footings

See also

• Engineer’s details
• Stiffened raft slab
• Waffle pod slab
• Footing
• Concrete grade
• Borelog
• Earthworks: cut and fill
• Strip footings

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Last updated: 2026-05-10. Verified: 2026-05-10. Quarterly review for currency.