Oedometer test

The degree and rate of consolidation is determined by the consolidation properties of the material and the consolidation pressures put upon the sample. One of the most common methods to determine the consolidation properties is through incremental loading using an oedometer. During this test a series of constant stress levels (increments) are applied to a sample. The subsequent deformation of the material is measured versus time during each constant stress increment. Furthermore, un-loading stages and re-loading can be added to the test as well. The addition of un- and re-loading stages are, for instance, used for a better calculation of the ABC-isotachs parameters.
 
The loading schedule for the test with the type of stages and the pressures, the method of trimming and the capacity of the loading frame are based on (an estimate of) the current in-situ effective stress, as well as the pre-consolidation pressure and sampling effective stress.  The lowest stress that prevents swelling of the sample is the first usable stress increment after which the loading schedule can be followed.

Typically an oedometer test takes up to several weeks, during which each stress increment results in a plot of strain versus time over the duration of the increment. These plots must then be interpreted to determine the rate of consolidation and the strain associated with the stress level.  The consolidation coefficient (Cv) is determined using either Taylor’s method, that is, consolidation versus the square root of time, or the Casagrande method, which calculates consolidation versus logarithmic time.