What it does
Two everyday soil lab tests in one place.
Atterberg limits describe how a fine soil changes as it gets wetter: from solid, to plastic (it can be rolled into threads), to liquid. The page works out:
- the liquid limit LL from several Casagrande cup trials (a straight flow curve of water content against the log of the blow count, read at 25 blows), or from a single trial by the one-point formula LL = wN (N/25)^0.121;
- the plasticity index PI = LL − PL, the flow index and toughness index, where the soil plots on the plasticity chart (CL, ML, CH or MH);
- with the natural water content, the liquidity index and consistency index; with the clay fraction, the activity.
The compaction (Proctor) curve shows how dense a soil gets when compacted at different water contents. The page turns each bulk density into a dry density, fits a smooth curve, and reads the maximum dry density and the optimum water content at its peak. It draws the zero-air-voids line and the 5 % and 10 % air-voids lines from Gs, and gives the relative compaction of a field test.
How to use it
- Paste or open the cup trials: blows, then water content in %. Use the one-point method only for a single trial between 20 and 30 blows.
- Type the plastic limit, and if you have them, the natural water content and the percent finer than 2 µm.
- Paste or open the compaction points: water content in %, then bulk density. Choose the unit (Mg/m³, kg/m³ or kN/m³), Gs and the curve shape. A parabola suits four to six points around the peak; a cubic can follow a lopsided curve when you have more points.
- Optionally type a field dry density to get the relative compaction.
Worked example
All numbers are made up. Four cup trials at 34, 27, 21 and 15 blows with water contents of 41.2, 43.0, 44.9 and 47.3 % give LL = 43.5 %, a flow index of 17.15, and with PL = 22.4 %, PI = 21.1 %, which plots as CL. At a natural water content of 35 % the liquidity index is 0.60 (plastic), and with 28 % clay the activity is 0.75 (normal).
Five Proctor points from 10.2 to 19.1 % water, with Gs = 2.70, give Maximum dry density 1.816 Mg/m³ at 14.5 % water. At the optimum the soil is 81 % saturated with 6.4 % air voids. A field dry density of 1.74 Mg/m³ is a relative compaction of 95.8 %.
Good to know
- The flow curve is only straight over a modest range: keep trials between about 15 and 35 blows.
- A compaction point above the zero-air-voids line is impossible, so it means a wrong Gs or a mistake in the data. The page counts such points.
- If the fitted curve has no peak between your points, you need points on both sides of the optimum.
- For study and planning. Test methods and the compaction specified differ between standards; check your local code.
FAQ
Why is LL read at 25 blows? By definition: the liquid limit is the water content at which the groove closes after 25 blows of the cup.
Parabola or cubic? A parabola is symmetric about the peak and is the usual choice. A cubic can follow a curve that falls faster on the wet side, but it can also wiggle with few points.
What is activity? PI divided by the percent of clay-size particles. It says how much the clay minerals themselves swell and shrink: high activity hints at minerals such as smectite.
Where the numbers come from
- Casagrande liquid limit flow curve (water content against log blows, read at 25 blows), flow and toughness indices, liquidity and consistency indices, and Skempton's activity PI / clay fraction: general soil mechanics.
- One-point liquid limit LL = wN (N/25)^0.121: the commonly used exponent from the general literature; the page lets you change it.
- Compaction: ρd = ρ/(1 + w), zero-air-voids and air-voids lines ρd = Gs ρw (1 − Av)/(1 + w Gs), degree of saturation S = w Gs / e: soil phase relations.
- Example data are made up by fibub for illustration.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.