Ground movement (characteristic surface movement, ys)

Ground movement, technically the characteristic surface movement (ys), is the expected vertical movement of the soil surface due to seasonal moisture changes. Reactive clay soils swell when they wet up and shrink when they dry out; the magnitude of this movement is what AS 2870 uses to classify a site and design the slab and footings.

AS 2870 site classes mapped to ground movement bands:

Site class	Ys range	Soil behaviour
Class A	0 mm (negligible)	Stable sand, rock, or non-reactive material.
Class S	0 to 20 mm	Slightly reactive. Common urban soils.
Class M	20 to 40 mm	Moderately reactive. The volume residential default for most metropolitan reactive clay sites.
Class H1	40 to 60 mm	Highly reactive.
Class H2	60 to 75 mm	Highly reactive (upper band).
Class E	Above 75 mm	Extremely reactive. Engineered design only.
Class P	Variable	Problem site (uncontrolled fill, soft soil, deep reactive, mine subsidence, etc.). Engineer-designed regardless of ys.

How ys is determined. The geotechnical engineer performs a soil investigation (typically test pits or boreholes) and tests samples for shrink-swell index (Iss) in a lab. Combined with the depth of suction change (Hs) for the climate zone, the ys is calculated. The geotech’s soil report states the ys value and the resulting site class.

What ground movement does to a building:

• Edge heave: soil under the slab edge swells (rainy weather, garden watering) and lifts the edge. Internal walls can crack at corners; doors bind at the top edge.
• Centre heave: soil under the slab centre swells. Less common; produces dome-shaped distortion.
• Edge shrinkage: soil under the slab edge dries (drought, large trees) and the edge drops. Cracks open at corners.
• Differential settlement: parts of the slab move differently. The worst case for cosmetic and structural damage.

Slab design under AS 2870 sizes the footing stiffness and edge beam depth to absorb the expected ys without transmitting it to the superstructure as cracking damage.

Influences on ys:

• Tree influence: large trees within 1.5× tree-height of the slab perimeter increase ys substantially (root suction in dry weather, then re-wetting after removal). Geotech reports map “tree-influence zones”.
• Garden watering and irrigation: localised wetting under slab edges drives edge heave.
• Plumbing leaks: a leaking waste under slab can drive ys to extreme values within months.
• Climate zone: longer dry seasons increase the dry-cycle shrinkage; tropical wet-dry cycles can drive H2 sites that would be H1 in a temperate climate.

For builders.

1. Read the ys value, not just the site class. A Class M site at the upper end (38 mm) behaves more like H1 than like a low-end M (22 mm). Engineering response should reflect that.
2. Plan for moisture stability. Compliance with AS 2870 design only works if the post-construction site is maintained: no large trees within tree-influence zones, no plumbing leaks, no extreme garden watering at slab edges.
3. Communicate to the client. A Class H1 site behaves correctly only if the homeowner respects the maintenance regime. Hand over the soil report and the maintenance summary at completion.

Also known as: ys, characteristic surface movement, soil heave (informal), reactive movement.

Category: Site preparation / engineering / soils.

Related

• AS 2870
• Site classification
• Reactive soil
• Class P site

See also

• Cut and fill

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Last updated: 2026-05-14. Verified: 2026-05-14.