ASTM D2435 oedometer test: from the curve to Cc, mv and Eoed

Updated: 2026-08-04 · ISKRIT

A one-dimensional consolidation test produces a single curve of void ratio against applied stress. Everything a designer takes from the test is a slope of that curve, read either on a logarithmic or on a linear pressure axis, and the choice of axis is what separates the reporting traditions.

The logarithmic reading

ASTM D2435 and EN ISO 17892-5 report the compression index:

Cc = (e1 − e2) / lg(p2 / p1)

It is dimensionless and roughly constant along the normally consolidated part of the curve, which is what makes it useful for settlement prediction over a wide stress range. It is also undefined when p1 is zero, since the logarithm of the ratio is not defined, so the first loading step from an unloaded state never yields a Cc.

The linear reading

Over a narrow stress interval the curve is close to straight and three linear quantities are used:

m0 = (e1 − e2) / (p2 − p1), the coefficient of compressibility;
mv = m0 / (1 + e0), the coefficient of volume compressibility;
Eoed = (1 + e0) / m0, the oedometer modulus, which is simply 1/mv.

These depend on the interval chosen, so an Eoed quoted without its stress range is meaningless. Reports that give a modulus of 6 MPa for a soft clay without saying over which two pressure steps it was read cannot be checked or reused.

Worked example

Initial void ratio e0 = 0.72. On the interval from 0.1 to 0.2 MPa the curve gives e1 = 0.700 and e2 = 0.672.

The Cc looks small because the interval sits in the recompression range, below the preconsolidation stress. Read across a decade of stress above that point, the same soil would give a Cc several times larger. Mixing the two regions in one average is the most common source of underestimated settlement.

Questions

Is Eoed the same as Young's modulus? No. Lateral strain is prevented in an oedometer, so Eoed is stiffer than E and needs a Poisson correction to be converted.

How many steps should the curve have? Enough that the interval of interest is bracketed by measured points rather than interpolated across a wide gap; loading is normally doubled each step.

Enter the curve step by step and get m0, mv, Eoed and Cc in the consolidation and shear tool.