Site Classification Reports in Queensland: What Builders and Designers Need to Know
A site classification report Queensland builders receive before design can have a direct effect on slab selection, footing costs, earthworks decisions and programme certainty. It is not a paperwork exercise to tick off before a building approval. It is the starting point for designing foundations that suit the ground actually on site.
On a straightforward residential lot, that may mean confirming a conventional slab can be designed to AS 2870-2011. On a cut-and-fill site, a reactive clay block or a lot with established trees, the findings may change the design approach, construction sequence or budget allowances. Getting the investigation done early gives the project team time to respond before drawings are locked in and machinery is on site.
What a site classification report is for
A site classification report assesses the expected ground movement beneath a residential building and assigns a site class in accordance with AS 2870-2011, Residential slabs and footings. The classification helps the building designer or structural engineer select an appropriate slab and footing system for the soil conditions.
The focus is the zone of seasonal moisture change — the soil affected by climate, vegetation, drainage and construction activity. AS 2870 defines this as the design depth of suction change (Hs), which is taken from the climatic zone map in the Standard. In coastal south-east Queensland this depth is typically shallower than in the drier inland areas of the state, where suction change extends further below the surface.
In much of Queensland, clay soils shrink when dry and swell when wet. That movement is not always visible before construction, but it can lead to cracking, sticking doors, uneven floors and drainage issues if the footing system is not designed for it.
A clear report translates field observations and laboratory test results into practical information: the soil profile, the measured reactivity, the calculated characteristic surface movement, site constraints and the classification the designer needs. It should be written for use, not left as a set of unexplained borehole logs.
Site classes under AS 2870
AS 2870 site classes describe the reactivity, or movement potential, of the founding material. They are not a simple good-to-bad ranking. A more reactive class can still be built on successfully, provided the footing design, drainage and site preparation are right.
Classification is based on the characteristic surface movement (ys) calculated from soil reactivity testing:
| Site class | Description | Characteristic surface movement (ys) |
|---|---|---|
| A | Most sand and rock sites, with little or no ground movement from moisture changes | Negligible |
| S | Slightly reactive clay sites | 0 – 20 mm |
| M | Moderately reactive clay or silt sites | 20 – 40 mm |
| H1 | Highly reactive clay sites | 40 – 60 mm |
| H2 | Highly reactive clay sites with greater movement potential | 60 – 75 mm |
| E | Extremely reactive sites | More than 75 mm |
| P | Problem sites | Not applicable — classified by condition, not movement |
Classes M, H1, H2 and E may also carry a “-D” suffix (for example M-D or H2-D) where deep-seated moisture changes are expected, such as sites with deep reactive profiles or a history of abnormal moisture conditions.
Class P is different. It identifies a problem site where one or more abnormal conditions may affect performance. These can include uncontrolled or non-engineered fill, soft or compressible soils, collapsing soils, significant slope, mine subsidence, erosion, landslip potential, trees with a notable influence zone, abnormal moisture conditions, or any site that cannot otherwise be classified within the assumptions of AS 2870.
A Class P result does not mean a site is unbuildable. It means the site needs project-specific engineering rather than a deemed-to-comply footing solution from the Standard. That distinction matters when planning cost and time. The right response is to define the condition properly and design for it, not to make assumptions based on a neighbouring lot.
What is investigated on site
A dependable classification starts with work in the field. The investigation typically includes boreholes or test pits at locations suited to the proposed building area, together with observations of surface conditions, slope, drainage, fill, vegetation and any signs of previous disturbance.
Soils are logged and described in accordance with AS 1726, Geotechnical site investigations, which sets the terminology used for soil type, consistency, moisture condition and structure. Samples are recovered from the relevant layers for laboratory testing.
The consultant also considers available information about the site history, nearby development, and any proposed cuts, fills or drainage changes.
Laboratory testing to Australian Standard methods
Field logging alone cannot quantify reactivity. Where a reactive profile is present, testing is carried out in accordance with the AS 1289 series, Methods of testing soils for engineering purposes. The tests most relevant to a residential site classification are:
- Moisture content — AS 1289.2.1.1, determination of the moisture content of a soil by oven drying. This establishes whether the profile was sampled in a wet, dry or intermediate condition, which affects how the shrink-swell result is interpreted and how much further movement is available in the soil.
- Shrink-swell index (Iss) — AS 1289.7.1.1, determination of the shrinkage index of a soil by the shrink-swell test. This is the primary reactivity test used for AS 2870 classification in Queensland. Undisturbed core samples are subjected to a swell test and a shrinkage test, and the resulting index is used to calculate the characteristic surface movement.
- Loaded shrinkage index — AS 1289.7.1.2, an alternative reactivity measure where the shrink-swell test is not suitable.
- Core shrinkage index — AS 1289.7.1.3, used in combination with other methods where appropriate.
- Atterberg limits and linear shrinkage — AS 1289.3.1.1 (liquid limit), AS 1289.3.2.1 (plastic limit), AS 1289.3.3.1 (plasticity index) and AS 1289.3.4.1 (linear shrinkage). These characterise soil plasticity and support the correlations permitted by AS 2870 Appendix D where direct reactivity testing is not carried out.
- Particle size distribution — AS 1289.3.6.1, used to confirm soil type where the profile is mixed or where sand or gravel layers influence the assessment.
The shrink-swell index and the moisture condition of the profile are then combined with the design depth of suction change for the relevant climatic zone to calculate ys using the method in AS 2870 Appendix D. That calculated movement determines the site class.
Testing should be carried out by a laboratory competent in the relevant AS 1289 methods, and the report should state which tests were performed, where the samples were taken from and what the results were. A classification presented without supporting test data or a clear basis for the assessment is difficult for a designer to rely on.
Trees and vegetation
Trees deserve particular attention. A mature tree can draw substantial moisture from reactive soil, while removing it can allow the soil to re-wet and heave — sometimes over many years. The location, species, mature height and planned retention or removal of trees may therefore influence the classification and the footing advice, and can be enough to move a site to Class P under the abnormal moisture conditions provisions of AS 2870.
A report based on the current site layout can become less reliable if the landscape plan changes later.
Why timing affects the result
The best time to commission a site classification is before finalising footing design, and preferably before major earthworks. Once fill has been placed, existing ground levels have been altered or vegetation has been removed, establishing the original site condition becomes more difficult.
Early investigation also gives the team room to make commercial decisions. If shallow fill is identified, it may be more economical to remove and replace it as controlled fill — placed and compacted to the requirements of AS 2870 Section 6 and AS 3798 — than to carry an uncontrolled fill condition through to a more complex foundation solution. If a highly reactive layer is present, the slab designer can allow for it from the outset instead of redesigning after approval.
Construction does not wait, but rushing the wrong information creates delays later. A rapid field response and clear reporting are useful only when the investigation scope is adequate for the proposed works.
A site classification is not the same as a full geotechnical investigation
For a typical house on a conventional lot, an AS 2870 site classification may be the right starting point. It does not automatically cover every ground-related risk on a project.
Retaining walls, steep sites, deep cuts, commercial buildings, warehouses, pavement areas, large fills and civil infrastructure often need a broader geotechnical investigation completed in accordance with AS 1726. That work may consider bearing capacity, settlement, slope stability, excavation conditions, groundwater and earthworks requirements.
Likewise, a site classification does not replace wastewater land application investigations under AS/NZS 1547, hydraulic conductivity or permeability testing, acid sulfate soil assessment, or construction materials testing and compaction control under AS 1289 and AS 3798. Each has a different purpose and a different design decision behind it. Combining scopes where appropriate can reduce repeat site visits, but the report must still address the actual project risk.
Using the report on a live project
The value of a report comes from how it is applied. The building designer uses the classification to nominate a compliant footing and slab system. The builder uses it to plan excavation, manage tree and drainage controls, and confirm whether the site conditions match the assumptions made at design stage.
Before construction, check that the proposed building footprint, finished levels and earthworks concept align with the investigated conditions. If the design moves across the block, substantial cut or fill is introduced, or trees are removed or retained differently from the original plan, seek advice before proceeding. These changes can alter the foundation environment and, in some cases, the classification itself.
During construction, good drainage remains essential. Surface water should be directed away from the building, roof water managed properly, and plumbing leaks rectified promptly. On reactive clay sites especially, the site maintenance guidance in AS 2870 Appendix B is part of the footing system’s long-term performance, not an afterthought for handover.
What to provide when commissioning a report
A consultant can mobilise faster and target the investigation better when the project information is ready. At a minimum, provide:
- site address, and lot and plan details
- proposed building footprint
- available architectural or survey plans
- existing and proposed levels
- details of known fill, demolition or vegetation changes
For more complex sites, include retaining concepts, civil drawings, service layouts and any previous geotechnical information. If the programme is tight, say so early. The scope can then be matched to the decision required, whether that is a residential slab classification, a preliminary development constraint assessment or a staged geotechnical investigation.
Coastal Geo Consulting approaches this work from the ground up: field investigation, AS 1289 laboratory testing, Queensland-specific interpretation and RPEQ-backed reporting that designers and builders can put into action.
When the ground changes, ask the question
Ground conditions are not fixed simply because a report has been issued. Rain, excavation, filling, drainage changes and revised building layouts can all expose conditions that were not apparent at the investigation stage. If site works uncover soft material, uncontrolled fill, seepage, rock, buried debris or soil that does not resemble the report description, stop and have it reviewed.
The most useful site classification report in Queensland is one that gives the project team a clear footing design path, identifies the conditions worth managing, and remains connected to what is happening on site. That is how foundations are built for construction rather than designed in isolation.