Site investigation · Laboratory: soil classification
Shrink–swell index & AS 2870 site class calculator
Iss from swell and shrink strains, characteristic surface movement ys for a triangular suction profile, and site class.
Standard: AS 1289.7.1.1 · AS 2870 (simplified ys)
Open the live calculator → All site investigation calculators
Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Swell strain | 2.4 | % |
| Shrinkage strain | 4.6 | % |
| Surface suction change Δu | 1.2 | pF |
| Depth of suction change Hs | 1.8 | m |
| Lateral restraint factor α | 1 |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Iss = (εsh + εsw/2)/1.8 | 3.22 | %/pF |
| ys = α·Iss·(Δu/2)·Hs | 34.80 | mm |
| Site class (AS 2870 ranges) | M |
- ✓ Not class E (ys ≤ 75 mm)
Method, formulas and assumptions
- Iss = (εsh + εsw/2)/1.8 = (4.6+2.4/2)/1.8 = 3.2222 %/pF
- ys = α·Iss·(Δu/2)·Hs = 1*3.2222/100*1.2/2*1.8*1000 = 34.8 mm
- Site class (AS 2870 ranges) = IF(34.8<=20,"S",IF(34.8<=40,"M",IF(34.8<=60,"H1",IF(34.8<=75,"H2","E")))) = M
- Simplified: uniform Iss over Hs with a triangular suction profile. AS 2870 uses layered summation with α varying with depth and cracked-zone depth.
Related site investigation calculators
- Specific gravity (pycnometer) — Two-trial lab sheet for soil particle density with temperature correction to 20 °C and repeatability check.
- Particle size distribution — Sieve masses to % passing, grading curve, D10/D30/D60, Cu, Cc and a USCS group symbol.
- Hydrometer analysis — Fine-fraction grading from hydrometer readings: particle diameter by Stokes' law and percent finer.
- Atterberg limits & plasticity chart — Liquid limit from cone penetrometer or Casagrande points, PI, liquidity index and position on the plasticity…
- Moisture, density & phase relations — Bulk and dry density, void ratio, porosity, saturation and field density ratio, with a three-phase diagram.
- Moisture content (oven drying) — Moisture content of several specimens from tin masses, with mean, range and repeatability check.
- Fines content by washing — Percentage finer than 75 µm from the dry mass before and after washing plus dry sieving of the residue.
- Liquid limit: cone penetrometer — Linear fit of moisture content against cone penetration; the liquid limit is w at 20 mm penetration.
- Liquid limit: Casagrande cup — Flow curve (w against log N) by least squares; LL at 25 blows and the flow index.
- Liquid limit: one-point method — LL from a single Casagrande reading using LL = wN (N/25)^0.121, valid for 20–30 blows.
For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.