What the Letter on Your Soil Report Means
The letter is the site classification under AS 2870, the Australian Standard for residential slabs and footings. It describes how much the ground surface is expected to move as clay soaks up water and dries out again. The more reactive the clay, the more the slab has to cope with.
| Class | What it means | Expected surface movement |
|---|---|---|
| A | Mostly sand or rock | Little or none |
| S | Slightly reactive clay | Up to about 20 mm |
| M | Moderately reactive clay | About 20–40 mm |
| H1 | Highly reactive clay | About 40–60 mm |
| H2 | Highly reactive clay, more than H1 | About 60–75 mm |
| E | Extremely reactive clay | More than about 75 mm |
| P | Problem site: fill, soft ground, trees, abnormal moisture or similar | Assessed by the engineer |
Most lots we cut in Melton, Rockbank and Aintree come back somewhere between M and H2, and E turns up in Melbourne's west too. P is the one that frightens people. It usually means there's a specific issue the engineer has to design for, not that you can't build.
Who the Letter Really Matters To
The class is mostly the engineer's problem. A higher class means a stiffer slab: deeper edge beams, more steel, sometimes piers down to firmer ground. That's concrete and reinforcement, and it shows up in your builder's site costs as a slab upgrade. It isn't earthworks.
For the site cut, the letter on its own changes very little. What changes the cut is the detail behind the letter.
What It Changes for the Site Cut
P because of fill
Plenty of estate lots are built up with fill during subdivision. If that fill was placed and tested properly, the developer often has a compaction report for it and the engineer can design on it. If there's no report, or the fill is poor, the engineer may want it dug out to natural ground or bypassed with piers. Digging out fill means a deeper cut and more loads, so ask the developer for the fill report before you sign a build contract.
Highly reactive clay and moisture
H and E clays are the ones that misbehave when wet. A pad cut in H2 clay and left open through a wet month can soften, swell and need re-trimming before the slab goes down. Keep the gap between the cut and the slab short, and make sure water drains away from the pad instead of sitting on it. More in our wet-weather clay guide.
Rock in the bore logs
Basalt is never far below the surface in parts of Melton and Rockbank. If the bore logs show refusal on rock or floaters at shallow depth, expect a rock allowance in the quote. Our rock guide explains how it's priced.
Trees
Trees, or trees recently removed, can push a site to P because roots dry the clay unevenly. On the cut, that means roots to grub out and haul away. If a tree on or near the lot needs to go, check with council before anyone touches it.
We cut to the engineer's levels whether the block is M or E. It's what sits under the letter that changes the job.
What to Send Us
- The full soil report, including the bore logs at the back
- The engineer's cut-and-fill plan or slab design with the pad level
- Any fill compaction report from the developer
- A few photos of the lot and the street frontage
With those we can give you a site cut quote that already allows for what's under the grass.
Frequently Asked Questions
Is a class H2 block bad in Melton?
No. H2 is common in Melbourne's west. It means the engineer designs a stiffer slab. For the earthworks it mostly means timing: cut the pad and get the slab on before rain softens it.
What does P mean on a soil report?
P means problem site: the reactivity class alone doesn't describe the risk. Common reasons are uncontrolled fill, soft ground, trees or abnormal moisture. The engineer specifies the fix, and sometimes that means digging fill out to natural ground, which changes the site cut quote.
Does the soil class change the site cut price?
Indirectly. The letter itself doesn't, but the reasons behind it can: fill that has to come out, rock in the bore logs, or wet reactive clay that slows the job down.