Before a builder finalises a footing design, the soil under the block is tested and given a site classification under AS 2870, the Australian Standard for residential slabs and footings. The class tells the engineer how much the ground is likely to move as its moisture rises and falls through the year. On a flat block with stable soil that can be close to a formality. On a sloping block with clay in it, the soil report becomes one of the most important documents in the build.
This guide is for anyone holding a soil report, or about to order one, for a block that slopes. It covers what the classes mean, the numbers behind each letter, what Class P covers and how a slope changes the picture.
What is site classification?
Site classification is a standardised assessment of how much a block of land is expected to move due to changes in soil moisture. It applies to almost every new home and major extension in the country, and a soil report is normally part of the building permit paperwork. The City of Monash, for example, lists a soil report as a requirement of a building permit when you build a house, an extension or an attached garage.
A geotechnical engineer or qualified soil tester drills boreholes or digs test pits across the site, takes samples at different depths and assesses how much those soils swell when wet and shrink when dry. Sand and rock barely react to moisture. Clay can react a great deal, and the more active the clay, the bigger the movement.
The result is a class letter that feeds straight into the footing design. Each class has an established set of footing responses in the standard, which is why the classification comes first and the footing design follows.
The site classes and the movement behind each one
Each reactive class corresponds to a range of characteristic surface movement, written as ys. It is the amount the ground surface is expected to rise and fall with seasonal moisture change, and it is the movement the footing has to be designed for.
| Class | Typical site | Characteristic surface movement (ys) |
|---|---|---|
| A | Most sand and rock sites | Little or none |
| S | Slightly reactive clay | 0 to 20 mm |
| M | Moderately reactive clay or silt | 20 to 40 mm |
| H1 | Highly reactive clay | 40 to 60 mm |
| H2 | Highly reactive clay, very high movement | 60 to 75 mm |
| E | Extremely reactive clay | More than 75 mm |
| P | Problem sites (see below) | Not rated by movement |
Movement ranges from AS 2870, as published by the City of Monash.
Two details on a report are worth knowing. A class can carry a -D suffix, such as H1-D, which flags deep-seated moisture change where the zone of seasonal change reaches three metres or more below the surface. And a site with fill on it can be classified anywhere from A to P, depending on how that fill was placed.
What Class P covers, and what a slope has to do with it
Class P is the problem site category. The standard applies it to sites with soft soils such as soft clay, silt or loose sand. It also covers landslip, mine subsidence, collapsing soils, soils subject to erosion, reactive sites subject to abnormal moisture conditions and any site that can't be classified otherwise.
A slope on its own isn't on that list. What a slope can do is bring those conditions onto a site. Steep or unstable ground raises the question of landslip, and water running across disturbed soil can cause erosion. Levelling a building pad adds a third issue if the fill isn't placed and compacted under control. The City of Monash singles out sloping land as a case where a geotechnical report matters, and notes that a full report can assess the stability of the slope.
A Class P rating doesn't mean a site can't be built on. It means the footings need specific engineering attention instead of a standard response from the tables.
Clay soils in Melbourne's south-east
Clay sites are common across this part of Melbourne. The City of Monash notes that soil in its municipality tends to have a strong clay component and commonly classifies anywhere from Class S up to H1 and H2. In clay country, a Class M or H result is a normal outcome and says nothing bad about the land. It tells the engineer to design the footings for that movement.
What the soil reports on our own jobs show
On our jobs around Berwick, Pakenham and Officer, soil reports come back Class P often, and more often still on knockdown rebuilds. The letter on its own doesn't tell us how hard or how costly the build will be. For that we need the detail inside the report and the reasons the site ended up in that class.
So that's where we look first. How deep does the clay go, and how far down did the engineer find suitable founding material? How reactive is the soil? Are nearby trees, existing fill or unusual moisture conditions affecting the site? Fill and abnormal moisture conditions are both on the Class P list above. A knockdown rebuild adds one more catch: with the old house still standing, the geotechnical engineer can only drill where there's open ground, so further testing may be needed once demolition is done.
On a sloping block, the depth of the cut matters as much as the class. These sites often need a deeper cut than a flat block, so the ground at that depth shapes the excavation, the retaining wall design, the construction materials and how much engineering goes into the footings. Around Pakenham, Officer and Berwick we often dig into mudstone, which in our experience is manageable depending on how hard it is and what condition it's in. Harder rock close to the surface is a tougher job and can call for specialised equipment and more expensive excavation methods.
A Class P result doesn't automatically mean significant extra cost. We read the full report alongside the site levels and the proposed design, then work through the conditions with the engineer to find the most practical way to deal with them. Once that's done, we explain it to you in plain English. You hear what the findings mean for your build and any additional costs they bring, along with any restrictions, further investigations or construction requirements we need to allow for.
How a slope compounds reactive clay
On a flat site, moisture in the ground tends to rise and fall reasonably evenly across the footprint. On a sloping site, water moves sideways as well as up and down. Rain draining down the slope can saturate the lower part of a block while the upper part dries out faster, so different parts of the same footing can be swelling and shrinking at the same time. That differential movement is harder on a structure than uniform movement, even when the soil classification matches a flat block nearby.
On a sloping site, water moves sideways as well as up and down. Rain draining down the slope can saturate the lower part of a block while the upper part dries out faster, so different parts of the same footing can be swelling and shrinking at the same time.
Schematic drawing, not to scale
Drainage becomes a bigger factor too. Water that isn't directed away from the building will find the low points on a sloping site and concentrate there, which can push the moisture content of the clay well beyond what the original soil test measured. Retaining walls, subsoil drains and surface falls all help manage this, and they're usually specified alongside the footing design rather than left as a landscaping decision.
Cut-and-fill can change your classification
Getting a level building platform on a sloping block usually involves cutting into the high side, filling the low side or both, and that can change how the site is classified.
Cutting into a slope can expose a different clay profile from what sat at the original surface, sometimes more reactive and sometimes less. Fill changes the picture again. Uncontrolled or loosely compacted fill behaves very differently under a footing from the natural ground below it, and enough of it can push a site into Class P regardless of how the natural soil would have classified.
Tell your soil tester about any planned cuts and fills. Geotechnical consultants factor proposed earthworks into the movement calculation, and a report that assumed the natural ground surface may not hold once the site has been reshaped. Our guide to building on a sloping block explains how cut-and-fill, split-level and raised floors interact with the site, and it's worth reading alongside this one if you're weighing up how much earthworks a block needs.
What a site classification report contains
A typical site classification report sets out:
- where the boreholes or test pits went and a log of the soil profile at each one
- results from testing samples for how the soil shrinks and swells with moisture
- the characteristic surface movement (ys) calculated for the site
- the site class, including any -D suffix
- notes on anything that affects the class, such as existing fill, trees or abnormal moisture
Where a full geotechnical report is needed, the City of Monash notes it can also specify the slab design, the type and number of stumps or underpinning, any soil contamination, the stability of the slope and an assessment of existing cracking.
Footings that answer the soil: rafts, waffle pods, stumps, piers and screw piles
Once the classification is known, the footing system is chosen to suit it.
A stiffened raft slab is a common response on Class M and Class H sites. It is a slab reinforced with a grid of stiffening beams underneath, designed so the whole slab moves as one rigid unit and no single section shifts on its own.
A waffle pod system is a widely used version of a stiffened raft. Void-forming polystyrene pods sit between the ribs of the slab before the concrete is poured, which reduces the concrete needed while keeping the stiffened-raft structure reactive sites call for.
A deep or drop-edge beam extends the perimeter of the footing further into the ground than a standard edge beam. Moisture near the surface changes the most with the seasons, so taking the edge beam down past that zone reduces how much the perimeter of the slab moves. The detail shows up often on reactive clay and on sloping sites where the ground level changes across the footprint.
Stumps and piers hold up a raised floor on individual footings instead of a slab on the ground. Piers are drilled or bored down to a firmer bearing layer. Both come up often on steeper sites, where the slope and the earthworks it would take feed into the choice as much as the soil does. On Project Solstice, the earthworks a slab would have taken on that fall were the deciding factor, and the house went onto split-level stumps.
Screw pilesare steel piles wound into the ground rather than poured or dug. They are a common choice on sloping or filled sites where access is difficult or where the structure shouldn't bear on reworked fill, and they found the house in stable ground below the fill or below the zone of reactive movement.
None of these are interchangeable by preference. The soil classification and the site's slope and fill history determine which footing type the engineer specifies, and the geotechnical report and the structural engineer's design decide it, not the builder's default.
Why the soil report has to come first
It's tempting to lock in a design early so the build can move, but the soil report has to come before the footing design is finalised.
The classification determines the footing type, and the footing type affects the slab or floor level, which in turn affects floor heights, step-downs between levels and how the design meets a sloping block. Locking in a design before the soil is tested risks rework later, and that costs more time than testing first would have. On a sloping block, where the cut-and-fill decisions and the footing response are tied together, an early report lets the footing system and the architectural design develop together.
The same logic applies earlier still, if you haven't bought the block yet. The soil report is the one investigation we'd prioritise before you buy a sloping block. It won't answer every question, but understanding the ground early puts us in a much better position to assess the likely costs, and gives you what you need to make an informed decision before you're committed.
How NE Homes builds the soil result into the design
We treat the soil report as a design input from day one, not a compliance step to tick off at the end. Once we have the classification for a block, whether that's a straightforward Class M result or a more involved Class P site with fill and slope to manage, we bring the structural engineer's footing response into the design process alongside the architectural plan.
On sloping blocks, that means working out the cut-and-fill balance, the drainage strategy and the footing type together, so the final design reflects what the ground needs. It's also why we recommend soil testing early for any client looking at a block in south-east Melbourne's clay country, including the corridor covered in our knockdown rebuild guide for south-east Melbourne, where the same reactive clay questions apply to a rebuild on an existing block.
Getting that sequence right is one of the more overlooked parts of building on a sloping or reactive site. Old Berwick is where we most often see a fall and reactive clay on the same block, which is the work our sloping block builder in Old Berwick page covers.
Frequently Asked Questions
What is site classification?
Site classification is the assessment, set out in AS 2870, of how much a block of land is expected to move as its soil moisture changes. A soil test produces a class rating that tells the structural engineer how to design the footings for that ground. A soil report is normally part of the building permit application for a new house.
What do the site classes mean?
Each class reflects how far the ground surface is expected to move with moisture change. Class A sites show little or no movement. Under AS 2870, Class S covers 0 to 20 mm of characteristic surface movement, Class M 20 to 40 mm, Class H1 40 to 60 mm, Class H2 60 to 75 mm and Class E more than 75 mm. Class P covers problem sites that need specific engineering.
What does a Class P site mean?
Class P is the problem site category. It covers soft soils and loose sands, landslip, mine subsidence, collapsing soils, soils subject to erosion, reactive sites with abnormal moisture conditions and sites that can't be classified otherwise. A slope on its own doesn't make a site Class P, although landslip risk, erosion or uncontrolled fill on a sloping block can. The footings need specific engineering, but the site can usually still be built on.
Does a Class P result mean the build will cost a lot more?
Not automatically. Class P says the footings need specific engineering, but the letter on its own doesn't tell you how difficult or costly that will be. The detail in the report matters more: how deep the clay and suitable founding material sit, how reactive the soil is and whether trees, existing fill or unusual moisture are affecting the site. Read alongside the site levels and the design, that detail shows what the site actually needs.
Can the soil be tested before the old house is demolished?
It can, but the existing house limits where the geotechnical engineer can drill. On a knockdown rebuild, further testing may be needed once the house has been demolished.
How does reactive clay affect building on a slope?
On a slope, moisture doesn't move through the ground as evenly as it does on a flat site. Water draining down the grade can saturate the lower part of a block while the upper part dries faster, so different parts of the same footing swell and shrink at different rates. That differential movement is harder on a structure than the more even movement typical of a flat, reactive site.
What is a drop-edge beam?
A drop-edge beam, or deep edge beam, is a footing detail where the perimeter beam of a stiffened raft slab extends further into the ground than a standard edge beam. It takes the edge of the footing below the zone near the surface where seasonal moisture change has the most effect, so the edge of the slab moves less as the clay swells and shrinks.
Does a sloping block always need special footings?
Not always, but it's common. The footing type depends on the soil classification, the degree of slope and how much cut-and-fill the site needs, not on the slope alone. Some sloping blocks with mild grades and favourable soil use a standard stiffened raft. Steeper sites, filled sites and sites with more reactive clay often need stumps, piers, screw piles or a raft with deeper edge beams.