AS 2870 Site Classification Calculator

Calculate characteristic surface movement (y_s) and determine the AS 2870‑2011 reactive soil site class from shrink‑swell I_ss values and soil layer data.

Site & Soil Inputs

Step 1 - Locate the site

Sets your climate zone from AS 2870 Table 2.4
Shows the mapped geology and lithology at the site
Tells you how many registered bores sit near the site
Puts the site and its geology on your PDF

Climate Zone (AS 2870 Table 2.4)

South-east Queensland councils outside Table 2.4 (Gold Coast, Sunshine Coast, Moreton Bay, Logan, Redlands) adopt the Brisbane/Ipswich parameters.

Design H_s - Active Zone Depth

Cracked Zone Depth h_c

Use your own design values (optional)

Design H_s (depth of suction change) m Cracked zone depth h_c m Surface suction change Δu pF

Leave blank to use the climate zone above. For a site outside the Table 2.4 locations, enter the H_s from the Thornthwaite Moisture Index (Table 2.5) or local practice. Whatever you enter is stated on the PDF as entered by the classifier.

Soil Layers

I_ss values are from laboratory shrink-swell testing (AS 1289.7.1.1)

Top (m)	Bot (m)	I_ss (%str/pF)	USCS

Leave I_ss blank for layers you have not tested. A blank layer is treated as non-reactive in the calculation but is not recorded as a measured zero.

Site Conditions (optional)

Reactive clay in controlled fill placed less than 5 years before construction (no cracked zone) Cut made less than 2 years before construction - depth m Permanent water table depth, if shallower than H_s m

Layer depths are from the finished surface. Fill or a cut reduces the cracked zone (Cl 2.3.2); a water table above H_s ends the suction change there (Figure 2.1).

Trees present nearby? (AS 2870 Appendix H)

Dist (m)	Height (m)	Status	Grouping

Screening limits per Appendix H: single tree > 25 m or group > 50 m from footing are excluded. Distance is from the nearest footing edge to the tree base. For a group, enter the tallest tree's height (H2.2). Leave the height blank if the mature height is unknown - full influence is then assumed (H4(b)).

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AS 2870-2011 Classification Result

Your site, as enteredupdates as you type

◎ Enter your soil layers above and click Calculate to see the site classification.

AS 2870 Site Classes - A to E

AS 2870-2011 assigns a site class based on the calculated characteristic surface movement (y_s), which quantifies how much reactive clay soil will heave or shrink in response to seasonal moisture changes.

Class	ys range	Description
A	≤ 0 mm	Stable - non-reactive soils (sand, rock)
S	0–20 mm	Slightly reactive
M	20–40 mm	Moderately reactive
H1	40–60 mm	Highly reactive
H2	60–75 mm	Highly reactive (upper range)
E	> 75 mm	Extremely reactive
P	–	Problem site - individual engineering required

The site class determines the slab and footing design required under AS 2870 for residential construction. H1 and H2 sites with expansive clay require reinforced waffle raft or stiffened raft slabs.

How ys Is Calculated

The characteristic surface movement is computed by summing contributions from each reactive soil layer within the active zone depth H_s:

y_s = (1/100) × Σ α_n × I_ss,n × Δu_n × Δh_n

Key parameters

I_ss - Instability Index (Shrink-Swell Index), measured in the laboratory in %strain/pF
αn - Lateral restraint factor: 1.0 in the cracked zone (surface to h_c), then 2.0 − z/5 below, with z the depth to the centroid of the suction change over the layer (Clause 2.3.2)
Δu - Suction change, decreasing linearly from Δu_surface at the surface to 0 at H_s
Δh - Layer thickness in mm
H_s - Active zone depth from AS 2870 Table 2.4 (varies by climate zone)

Tree influence (Appendix H)

Where trees are present, the tree-induced movement (y_t) follows AS 2870-2011 Appendix H. The design drying depth H_t and the extra suction change Δu_base come from Table H1 for the site's H_s (single tree or group). The movement is integrated over the suction change in addition to the normal design profile, with the depth of cracking taken as H_t, and reduced with distance up to the influence distance: 1.0 times the tree height for a single tree, 1.5 times the group height for a group, and 2.0 times for a row of four or more (group height = 0.9 times the tallest tree). Single trees beyond 25 m and groups beyond 50 m are ignored. The design mound height is y_m = 0.7y_s + y_t. Trees are excluded from y_s and do not alter the S/M/H1/H2/E class directly; a reactive site with a tree inside its influence distance is classified Class P (Clauses 1.3.3(c) and 2.1.3(e)).

Frequently Asked Questions

What is AS 2870 site classification?

AS 2870 is the Australian Standard for Residential Slabs and Footings. Site classification rates how reactive a site is to moisture changes, using the characteristic surface movement (ys) to assign a class from A (stable) through S, M, H1, H2 to E (extremely reactive) or P (problem site). The class determines the slab and footing design required.

What is ys (characteristic surface movement) in AS 2870?

Characteristic surface movement (ys) is the predicted vertical movement of the ground surface due to seasonal moisture changes. It is calculated by summing contributions from each reactive soil layer using the formula ys = (1/100) × Σ(αn × Issn × Δun × Δhn), where α is the lateral restraint factor, Iss is the shrink-swell index, Δu is the suction change, and Δh is the layer thickness.

What is the Iss (Instability Index or Shrink-Swell Index)?

The Instability Index (Iss), also called the Shrink-Swell Index, is a laboratory-measured soil property expressed in %strain/pF. It quantifies how much a soil sample expands or shrinks per unit change in soil suction. Higher Iss values indicate more reactive clay soils.

What Iss value is considered reactive?

Generally, Iss > 1.5 %str/pF indicates reactive behaviour. Highly plastic clays (CH) in south-east Queensland typically have Iss values of 2.5–5.0 %str/pF. Low-plasticity soils (CL, ML) are usually below 1.5.

What are the AS 2870 site classes?

AS 2870-2011 defines six site classes based on ys: Class A (ys ≤ 0 mm, stable/non-reactive), Class S (0–20 mm, slightly reactive), Class M (20–40 mm, moderately reactive), Class H1 (40–60 mm, highly reactive), Class H2 (60–75 mm, highly reactive) and Class E (ys > 75 mm, extremely reactive). In addition, sites with abnormal conditions are designated Class P (problem sites requiring individual engineering).

How do I use the AS 2870 calculator?

Select your climate zone (which determines Hs and Δu from AS 2870 Table 2.4), choose the Hs and hc bounds, then enter each soil layer with its top depth, bottom depth, Iss value from laboratory testing, and USCS classification. Click Calculate to get the ys and site class. Tree influence (Appendix H) can also be included.

Which Australian locations are covered?

The calculator covers the AS 2870 Table 2.4 climate zones including Brisbane, Ipswich, Toowoomba, Sydney, Melbourne, Adelaide, Perth, Hobart, Newcastle and the Hunter Valley, plus south-east Queensland councils (Gold Coast, Sunshine Coast, Moreton Bay, Logan, Redlands) which adopt the Brisbane parameters.

Can this calculator replace a geotechnical investigation?

No. AS 2870 site classification requires laboratory testing (Iss from shrink-swell tests) on soil samples from a proper geotechnical investigation. This calculator performs the AS 2870 formula calculations once you have the Iss values from test results.

Generate Full Geotechnical Desktop Studies

LayeredGeo compiles a geotechnical desktop study for QLD, NSW, VIC and SA addresses - the context to scope a site investigation, not a replacement for one:

Indicative soil reactivity range to expect (for example Class S to M) - the site investigation decides the class
Mapped geological unit, soil type and nearby bore logs
Groundwater depth from the national bore database
Topography, flood risk, and planning overlays
Professional PDF report delivered in minutes

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The *ys* formula, site classes A–E, and when to get a classification - explained for builders and developers.

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How reactive clay causes structural defects, what rectification involves, and why early ground information matters.