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 instability 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 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), Class E (ys > 75 mm, extremely reactive), and 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.

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

Climate Zone (AS 2870 Table 2.4)

Design H_s - Active Zone Depth

Cracked Zone Depth h_c

Soil Layers

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

Trees present nearby? (Appendix H)

Distance (m)

Height (m)

Vigor

Grouping

AS 2870-2011 Classification Result

◎ Enter your soil layers above and click Calculate y_s 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 - Instability factor: 1.0 in the cracked zone (surface to h_c), then 2.0 − z/5 below
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, tree-induced suction changes (y_t) are calculated using the k-factor envelope method. The governing movement is max(y_s, y_t).

Frequently Asked Questions

What I_ss value is considered reactive?

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

Which Australian locations are covered?

The calculator covers all zones in AS 2870 Table 2.4 including Brisbane, Gold Coast, Sunshine Coast, Ipswich, Moreton Bay, Toowoomba, Sydney, Melbourne, Adelaide, Perth, Hobart, Newcastle, Hunter Valley, and more.

Can this calculator replace a geotechnical investigation?

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