Building a new home in the Riverina is a significant investment. The site you build on is the foundation, literally, of everything you put on top of it. And in Wagga Wagga and the surrounding region, where reactive clay soils are prevalent and soil conditions vary across surprisingly short distances, understanding what is beneath your lot before you commit to a footing design could save you tens of thousands of dollars in future repairs. This guide explains what soil testing before building involves, when it is required, and what the results mean. For help interpreting the resulting report, see our guide on how to read a soil classification report.
Quick answer (BLUF)
A geotechnical investigation before building in the Riverina classifies the site’s soil reactivity class (AS 2870), identifies groundwater depth and soil chemistry, and guides the structural engineer’s footing design. It costs $500 to $2,000 for a residential investigation. In Wagga Wagga’s Class H2 and E environment, this is not optional paperwork, it is essential engineering data.
When soil testing is required
New residential construction: Under AS 2870 (the Australian Standard for residential slabs and footings), a site classification is required before a new home is designed. The footing design must be appropriate for the site class. A builder or certifier will request the soil report as part of the documentation for a complying development certificate or development application.
New extensions or additions: Significant extensions, particularly those involving new concrete slabs or new stump footings, typically require a site assessment. The relevant building certifier will advise whether a formal soil classification is required or whether the existing report (if any) is sufficient.
Buying vacant land: Before purchasing land in a Riverina subdivision, it is prudent to commission or review a soil classification. Land marketed as suitable for residential construction is not always on stable, well-classified soils. Some fringe lots in Wagga involve filled areas or low-lying positions that attract a Class P (problem site) classification, requiring more expensive specialised footing designs.
What a geotechnical investigation involves
Field investigation
The investigation starts on site with hand auger boring or machine-assisted boring to extract soil samples from defined depths, typically to 1.5 m or 2.5 m for residential investigations. Borings are spaced across the building footprint to detect any variation in soil conditions across the site.
In some cases, a dynamic cone penetrometer (DCP) test is also conducted, this measures the resistance of the soil to penetration at depth, giving information about soil strength and consistency.
The investigator records the soil description at each depth: colour, texture, structure, consistency, and moisture state. Groundwater depth is recorded if encountered.
Laboratory testing
Soil samples are sent to a geotechnical laboratory for testing. The critical measurements for reactive clay investigations are:
- Atterberg limits (liquid limit and plastic limit): These measurements characterise the clay’s plasticity and moisture sensitivity.
- Shrink-swell index (Iss): The most directly relevant measure for AS 2870 classification. Determined by comparing the volume of soil at different moisture states.
- Moisture content: The current moisture state of the sample, used to calibrate the shrink-swell calculations.
- Soluble sulfates (in some cases): To assess the aggressiveness of the soil to concrete, relevant where groundwater sulfate levels could attack footing concrete.
Report
The laboratory results are combined with the field data to calculate the characteristic surface movement (ys) and assign the AS 2870 site class. The report then specifies the minimum footing system for the site, slab thickness, edge beam dimensions, reinforcement grade and spacing, moisture barrier requirements, and any special provisions.
Cost of soil testing in the Riverina
Indicative costs for a Wagga Wagga residential soil investigation:
- Standard residential (2-3 borings, lab testing, report): $600 to $1,200
- More detailed investigation (4-6 borings, broader testing, complex site): $1,200 to $2,500
- Specialist investigations (acid sulfate soils, contamination screening): $2,000 to $5,000+
These costs are a small fraction of the total building cost and a negligible fraction of the cost of rectifying a footing that was inadequately designed for the actual site conditions.
Interpreting results for common Wagga Wagga scenarios
Class M result: Less reactive soil, possibly on a ridge or elevated position with good drainage. Standard AS 2870 M-class footing design is appropriate and cost-effective.
Class H1 or H2 result: The most common outcome in Wagga’s established and developing suburbs. The footing design must reflect the higher reactivity, deeper edge beams, more reinforcement, specified moisture barriers. A well-designed H2 footing is entirely reliable, but the design must be correct.
Class E result: Extremely reactive. The footing design will be more robust and more expensive. For new construction, this is the time to factor in additional edge beam depth, potential for stiffened raft design, and enhanced drainage provisions. The incremental cost of getting the footing right at the building stage is far less than the cost of rectification later.
Class P result: Problem site. A bespoke engineering design is required. The extra engineering cost is the price of building on difficult land.
FAQs
My builder says I do not need a soil test because the neighbouring block is already built, is this right?
This is not an acceptable substitute. Soil conditions can vary significantly within a single block, let alone between adjacent lots. Differences in fill, drainage paths, vegetation history, and subsoil geology can all produce different site classifications within a few metres. Always commission your own investigation for your own lot.
Can I use a soil test from a previous development on the same land?
It depends on how recent it is and whether site conditions have changed. If a soil test was done within the last 5-10 years and there have been no significant changes to drainage, vegetation, or earthworks, it may be usable. However, your structural engineer and building certifier will make this determination. A fresh test is always more defensible.
What if the soil test reveals Class P or Class E conditions on land I have already bought?
A Class P or Class E result does not mean the land is unbuildable, it means the footing design must be appropriate for the conditions. The cost of a compliant footing on Class E land is higher than on Class M land, but homes are built successfully on Class E sites throughout the Riverina. Get an engineering design, price the footing cost into your build budget, and proceed. Do not try to use a less expensive footing design than the site class requires, the long-term consequences are far more costly.