If you are buying, building, renovating, or investigating foundation problems on a Wagga Wagga property, you may encounter a soil classification report. These documents, produced by geotechnical engineers, look technical and unfamiliar, but once you understand the framework, they are genuinely useful. This guide walks you through what the classifications mean, how to find the critical numbers, and what they imply for your foundation. For a deeper look at how reactive soils behave in the Riverina, see our reactive clay soils guide.
Quick answer (BLUF)
Australian Standard AS 2870 classifies residential building sites from Class A (stable) to Class E (extremely reactive). Most Wagga Wagga properties fall in Class H1, H2, or E. The higher the class, the more your soil moves with moisture, and the more robust your footings must be. A soil report tells you the class, the expected ground movement, and the minimum footing design required.
The AS 2870 classification system
AS 2870, Residential Slabs and Footings, is the Australian Standard that governs how homes are designed for reactive soil conditions. The standard defines six site classes:
| Class | Name | Expected surface movement | Typical soil type |
|---|---|---|---|
| A | Stable | Negligible | Sand, rock, stable gravel |
| S | Slightly reactive | Up to 20 mm | Sandy loam, weakly reactive clay |
| M | Moderately reactive | 20-40 mm | Moderately reactive clay |
| H1 | Highly reactive | 40-60 mm | Reactive clay |
| H2 | Highly reactive | 60-75 mm | Reactive clay |
| E | Extremely reactive | 75 mm+ | Highly plastic clay, organic fill |
| P | Problem site | Varies | Filled land, soft soil, aggressive soils |
In Wagga Wagga and the broader Riverina, H2 and E classifications are common, particularly in established suburbs developed over Murrumbidgee alluvial clay. Class P can appear in newer fringe subdivisions where fill has been imported or compacted over variable natural soils.
What the report actually contains
A typical residential soil classification report will include the following sections:
Site description. The address, a description of topography, existing vegetation, and observable surface conditions at the time of investigation.
Investigation method. How the soil was tested, commonly through hand auger borings (typically to 1.5-2 m depth for residential investigations), dynamic cone penetrometer tests, or laboratory analysis of disturbed samples. Better reports will also record the depth to groundwater if encountered.
Soil profile log. A table showing what was found at each depth increment, the soil colour, texture, consistency, and moisture state. Look for references to “medium to high plasticity clay” or “fat clay”, these indicate reactive material.
Shrink-swell index (Iss). This is the key measurement. It is determined in the laboratory from the soil samples and represents the percentage volume change per unit change in moisture. Higher Iss values mean more reactive soil.
Characteristic surface movement (ys). This is calculated from the Iss values across the soil profile and represents the expected vertical movement at the surface in millimetres. It is the single most important number for foundation design.
Site classification. The formal AS 2870 class (A, S, M, H1, H2, E, or P) derived from the ys value.
Footing recommendations. The minimum footing design required for the site class, typically minimum slab thickness, edge beam depth and width, reinforcement specification, and any special requirements such as moisture barriers, drainage, or fill compaction.
How to find the critical information
Open the report to the conclusions or recommendations section first. This will state the site class directly and give the characteristic surface movement. Note both figures.
Then look at the soil profile logs. Check the depth range over which reactive clay was found. If reactive clay extends only to 0.5 m depth, the risk profile is different from a site where it extends to 2 m or more. Deep reactive profiles require deeper footings to reach stable material.
Check whether groundwater was recorded. If the report notes groundwater at 1 m depth, your site is also subject to moisture variation from below as well as from surface conditions.
Finally, read the footing recommendations carefully before any extension or renovation. If your plans involve new footings, an extension, a garage slab, a new verandah, those footings must comply with the soil report recommendations or a new geotechnical investigation must be commissioned.
What to do if you cannot find a report for your property
Many existing Wagga homes, particularly those built before the 1990s, have no soil classification report. Options include:
- Commission a new investigation. A geotechnical firm can produce a residential soil classification report for a few hundred dollars. This is well worth it before a renovation or if you are investigating foundation problems.
- Use council records. Some councils retain soil investigation records from the original development application. Contact Wagga Wagga City Council to ask.
- Use neighbouring reports. If a similar property on the same street has a recent report, the classification is likely to be similar, though not identical.
FAQs
Does a Class E rating mean my house is about to fall down?
No. Class E simply means the soil is highly reactive and requires a well-engineered footing system. Many thousands of homes across Australia sit on Class E soils and perform perfectly well because they were designed appropriately. Problems arise when older homes were built without adequate footing depth or when drainage conditions deteriorate. A foundation inspection can assess whether your existing footings are coping with the soil conditions.
I am building an extension on an existing Class H2 site, do I need a new soil report?
Not necessarily for the extension itself if the original report is relatively recent (within 10 years) and the site conditions have not changed. However, if the original report only covered the original building footprint, you may need an investigation in the extension area, particularly if it is a significant distance from the original borings. Confirm with your structural engineer or building certifier.
What is a Class P site and should I be worried?
Class P (Problem site) is the classification for sites that do not fit neatly into the reactive-soil framework, typically filled land, sites with soft or compressible soils, sites with aggressive ground conditions (acid sulfate soils, saline groundwater), or sites with variable fill quality. New estates such as Estella may include Class P lots where fill has been used in low-lying areas. Class P requires site-specific footing design rather than the standard prescriptive solutions. If your report shows Class P, insist on a structural engineer’s bespoke footing design.
Can a soil report expire?
AS 2870 reports do not have a formal expiry date, but they reflect conditions at the time of investigation. Significant changes, nearby earthworks, removal of large trees, changes to drainage, or major flood events, can alter the site conditions enough to warrant a fresh assessment.