Design suction change (Hs / Hsm)

Design suction change (denoted Hs, sometimes Hsm) is the AS 2870 reactive-soil design input quantifying the expected change in soil moisture suction from wet to dry over the design life of the building. The geotech derives Hs from soil-investigation data (typically Atterberg limits, shrink-swell index, depth to active zone). Hs feeds into the characteristic surface movement (ys) calculation, which drives site classification (M, H1, H2, E) and therefore the slab and footing design under AS 2870.

Why builders care:

A higher Hs means more reactive soil and a more conservative slab design (stiffer slab, deeper edge beams, more reinforcement, sometimes piers). The cost difference between Class M and Class H2 is significant.

Site class	Typical ys range (mm)	Typical slab cost premium
A	< 10 mm	baseline
S	10-20 mm	+5%
M	20-40 mm	+15-25%
H1	40-60 mm	+25-50%
H2	60-75 mm	+50-100%
E	> 75 mm	piered footing + waffle slab, +100% or more

How Hs is calculated (overview):

The geotech uses AS 2870 Section 2.2 with site data:

1. Identify the active zone depth (Hs depth): the depth to which seasonal moisture cycling penetrates. Climate-dependent (Sydney coastal ~1.5 m, dry inland ~3-4 m).
2. Measure suction change over the active zone: from soil samples, using shrink-swell index and Atterberg limit data.
3. Compute Hs in suction units (typically pF units, e.g. ΔpF = 1.2 over the active zone).
4. Compute characteristic surface movement (ys) in millimetres from Hs and soil properties using AS 2870 methods.
5. Map ys to site class (A / S / M / H1 / H2 / E).

For most residential builds, the geotech delivers a single-page soil classification certificate that gives the site class and the underlying ys / Hs values.

Where Hs matters for builders:

• Reading the geotech report: a builder negotiating a quote should know whether the report quotes Hs / ys directly or just the class.
• Marginal classifications: a site at ys = 40 mm is on the M/H1 boundary; small changes in soil sampling can push it either way. Worth asking for a second opinion if the consequence is a slab redesign.
• Climate change considerations: some recent practice updates Hs upward to allow for drier summers in southern Australia. Worth asking the geotech whether they’ve applied a climate factor.
• Existing site re-investigation: an older soil report (pre-2011) may have used an earlier Hs methodology; a fresh report under AS 2870:2011 can sometimes yield a different class.

Common builder issues:

• Cheaper geotech misses high Hs: skimping on the soil report ($800 vs $1,500 typical 2026 cost) can miss reactive clay below the active zone, leading to slab cracking years later.
• Hs not reflecting tree influence: trees within the building zone draw moisture and increase effective Hs locally. Some geotechs flag this; some don’t. Always ask.
• Climate zone wrongly applied: Hs depends on climate zone (AS 2870 climate zones 1-6, mapped). A geotech from out of region may apply the wrong zone.

For builders:

1. Read the soil report’s ys / Hs values, not just the site class. A site at the top of a class can be optimised; at the bottom, the engineer has margin.
2. Question marginal classifications: a ys 38 (top of M) might be over-engineered; ys 42 (bottom of H1) might be under-engineered.
3. Spend on the geotech report on reactive soil sites: $1,500 spent at the geotech stage prevents $50,000 in slab repairs at year 5.
4. Brief the geotech on local trees and drainage: site-specific factors affect Hs.

Also known as: Hs, Hsm, characteristic surface movement input.

Category: Geotechnical / slab design / AS 2870.

Related

• AS 2870
• Footings (pier)
• Site classification

See also

• Soil report

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Last updated: 2026-05-15. Verified: 2026-05-15.