Basaltic clay in Melbourne's west: a desktop worked example

A residential block on the basalt plains of Melbourne's western suburbs will usually sit on reactive basaltic clay weathered from the Newer Volcanics Formation, and a desktop review will most often point to a Class H1, H2 or E site under AS 2870 before anyone drills a hole. This article follows one hypothetical block through a desktop geotechnical review from first principles to the point where a field site classification takes over. The site, settings, soil values and class are illustrative, chosen to show the order of the work and what each finding changes. They are not advice for any real lot.

The block

Take a flat 450 square metre infill lot in a newer estate west of Melbourne, somewhere on the basalt plain between Werribee and Melton. The owner plans a single-storey slab-on-ground house. The question the desktop review answers is not "what is the class" (only a field test gives that) but "what class should we budget for, and what does the footing design likely have to deal with".

Step 1: Geology from the published mapping

The first layer is the geological map. The northern and western suburbs of Melbourne and much of western Victoria are covered by the Newer Volcanics Formation, and the basaltic clays over it come from the full weathering of the basalt rock (Srithar 2014). On a block in this setting the desktop review expects basaltic clay at or near the surface, typically extending a few metres, though the thickness can change markedly over a short distance (Srithar 2014).

That single finding sets the tone for everything after it. Basaltic clay here is dominated by montmorillonite, with kaolin and mica less present, and that montmorillonite content is why the soil swells and shrinks so much with moisture change (Srithar 2014). Sites on it are commonly classified Class H or Class E (Srithar 2014).

Step 2: What the shrink-swell numbers suggest

The reactivity of a clay is captured by its shrink-swell index (Iss), measured in percent strain per pF. For Newer Volcanics basaltic clay the published laboratory values for Iss run from about 3 to 8 percent strain per pF (Srithar 2014). A Melbourne-wide database found the Newer Volcanics basaltic samples, mostly from the western suburbs, sat consistently between roughly 4 and 6.5 percent strain per pF, higher than the older volcanic basalts (Li et al. 2016).

That same study is a useful reminder not to lean on a single blanket figure: highly reactive soils cluster in the western and northern suburbs, and many of those areas returned an Iss above 6 percent strain per pF, but there was real scatter even within one soil type (Li et al. 2016). For context, a block at Braybrook on this formation returned an Iss of about 4.5 percent strain per pF at 0.75 metres, while a nearby Plumpton sample on the same formation came back near 2.2 percent strain per pF (Li et al. 2016). The desktop review carries that uncertainty forward rather than pretending the number is settled.

Step 3: Translating that into a likely AS 2870 class

The class is driven by the characteristic surface movement (ys), the estimated vertical ground movement from seasonal moisture change. AS 2870 bands it like this:

Class	Reactivity	Characteristic surface movement (ys)
A	Not reactive	Negligible
S	Slightly reactive	0 to 20 mm
M	Moderately reactive	20 to 40 mm
H1	Highly reactive	40 to 60 mm
H2	Highly reactive	60 to 75 mm
E	Extremely reactive	Greater than 75 mm

(Srithar 2014)

The ys depends on three things together: the shrink-swell property, the depth of seasonal moisture change (Hs), and the suction change over that depth. For Victorian conditions, soil with a shrink-swell index above 3 percent strain per pF reaching deeper than about 2.3 metres is enough to push ys past 40 mm, which is the H1 threshold (Srithar 2014). Our illustrative block, sitting on basaltic clay with an Iss in the 4 to 6.5 range extending a few metres, lands squarely in that territory. The desktop expectation is H1 or H2, with E possible where the clay is deep and the moisture swing is large.

Historic field heave measurements on this formation back the range up. Recorded seasonal surface heave on Newer Volcanics clay reached about 65 mm at one Sunshine site and about 49 mm at Whittlesea, while a Keilor site with better drainage sat near 24 mm (Srithar 2014). Same formation, very different movement. Drainage and the local moisture regime matter as much as the clay itself, which is why building on reactive clay is decided before the footing design, not after.

Step 4: Checking the Vicmap layers for things that change the answer

With geology and reactivity scoped, the desktop review turns to the planning and hazard layers. On a western-suburbs basalt-plain block the ones worth checking are a Land Subsidence Overlay or similar around old volcanic features, any fill history in a recently developed estate, and the drainage lines. Uncontrolled fill over the natural clay is common on estate lots and changes the investigation scope entirely, so the review flags where earthworks records exist and where they do not. The Thornthwaite Moisture Index for Victoria also sets the climate zone and the design Hs, which feeds straight back into the ys estimate from step 3.

Step 5: What the field test confirms, and what it overturns

Here is the boundary. The desktop review has predicted H1 to H2, flagged E as possible, and noted that fill and drainage could move the answer either way. None of that is a classification. A field site classification drills or digs, logs the actual profile, samples the clay, measures Iss on recovered samples, and calculates ys for the real depth of reactive soil and the real moisture conditions. It can confirm the H class, or it can find shallow rock that reduces the reactive depth, or uncontrolled fill that drives it to Class P regardless of the clay below.

For the illustrative block the most useful thing the desktop review did was price the job honestly. The owner budgeting for a simple Class S slab on this formation would have been wrong. Budgeting for H1 to H2, and for a field test that might return E or P, is the realistic position the desktop sets up.

When you need that scoping done quickly for a Victorian address, generate a desktop report for the address or see a sample report to see how the geology, soils and site layers come together before the drilling rig arrives.

LayeredGeo compiles geology, soils, groundwater and site data into an automated geotechnical desktop report for any address in Queensland, New South Wales, Victoria and South Australia.

Sources

• S.T. Srithar (2014). Engineering design and earthworks aspects related to basaltic clays in Victoria. Australian Geomechanics, Volume 49, Number 2 (Jun 2014). geomechanics.org.au
• Jie Li, Jian Zou, Peter Bayetto and Nick Barker (2016). Shrink-swell index database for Melbourne. Australian Geomechanics, Volume 51, Number 3 (Sep 2016). geomechanics.org.au

About this article. Published by LayeredGeo and written from the published research cited in the Sources section above. It is general information about how property and ground conditions are assessed in Australia, not engineering, planning, legal or financial advice, and it is not specific to any property. Check anything that matters against the source dataset or a suitably qualified professional before you rely on it. If you spot something wrong, tell us at hello@layeredgeo.com.au and we will fix it.

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