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3.2. LABORATORY TESTS 29
3.2.4 Unconfined compression
Figure 3.2: Plasticity diagram to DIN 18196:2006-06
The unconfined compression test imitates the load-deformation behaviour of a soil. It is the
most important parameter and supplies the modulus of compressibility ES or the coefficient
of compressibility Cc of a soil. In the unconfined compression test, the undisturbed or prepared
soil sample is placed in a ring (normally 70 mm dia.) which prevents radial deformation of the
sample. The sample is subsequently compressed in the axial direction in several loading steps
and the axial deformation measured. In doing so, at least the primary consolidation should be
waited for per loading step. The values measured are plotted on a stress–compression graph
(see Fig. 3.3) or stress–void ratio graph. As the stress–compression ratio of a soil, in contrast
to steel, is non-linear, it must be ensured that the modulus of compressibility is determined for
a stress range to which the soil is actually subjected in the field.
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