Characteristic surface movement (ys)
Characteristic surface movement (ys) is the millimetres of seasonal soil movement on a reactive site. Drives AS 2870 site class A through P and footing design.
Ask Chalkline about this →The characteristic surface movement (ys) is the millimetre measure of how much a residential building site is expected to move vertically at its surface under seasonal moisture changes, before the building goes on it. It is the engineering input that classifies a site under AS 2870:2011 (Residential slabs and footings) and that drives the footing design (verified 2026-05-16).
The AS 2870 site classification by ys:
| Class | ys (mm) | Description |
|---|---|---|
| Class A | ys ≤ 0 | Stable, non-reactive (sand, rock) |
| Class S | 0 < ys ≤ 20 | Slightly reactive (some clay; minor movement) |
| Class M | 20 < ys ≤ 40 | Moderately reactive |
| Class H1 | 40 < ys ≤ 60 | Highly reactive |
| Class H2 | 60 < ys ≤ 75 | Very highly reactive |
| Class E | 75 < ys | Extremely reactive |
| Class P | (any ys, plus problem conditions) | Problem site (controlled fill, abnormal conditions); site-specific engineering required |
The “D” suffix (e.g. M-D, H1-D) denotes deep movement: where the reactive zone extends deeper than standard and the footing design must address that. AS 2870 deemed-to-comply tables vary between standard and D-suffix sites.
How ys is determined. Under AS 2870, ys is calculated from a soil report prepared by a geotechnical engineer or qualified soil tester. Common methods:
- Block-sample shrink-swell test in a lab, mapping moisture content vs volume change.
- Field investigations (boreholes, test pits) characterising soil profile depth and clay composition.
- Climate adjustment using the Thornthwaite Moisture Index map of Australia, which captures regional climate effects on soil moisture cycling.
A site classification report (sometimes called a “soil report” or “geotechnical report”) arrives early in the design process, usually before slab design. The engineer marks the site class on the structural drawings.
Practical builder implications:
- Footing depth and reinforcement scale with ys. A Class A site can use shallow strip footings or thin raft slab; a Class H2 site needs deep beams, heavy reinforcement, and articulation joints in the masonry above.
- The cost difference is substantial. A residential slab on Class A may cost $25-30K; the same dwelling footprint on Class H1 may cost $45-60K, on Class H2 over $65K. ys is the single biggest cost driver in residential footings.
- Confirm ys before contracting. A fixed-price contract signed without knowing the site class is a major risk to the builder. Stipulate “subject to site classification not exceeding Class M”.
- D-suffix sites need re-checking. A site that classifies as M might be M-D after a deep moisture investigation. The deep-movement requirement adds further cost and depth.
Also known as: ys; characteristic ground movement; soil reactivity number; expansive soil rating.
Category: Structure.
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Last updated: 2026-05-16. Verified: 2026-05-16. Quarterly review for currency.