The primary purpose of the Unconfined Compression Test is to quickly determine a measure of the unconfined compressive strength of rocks or fine-grained soils that possess sufficient cohesion to permit testing in the unconfined state. This measure is then used to calculate the unconsolidated undrained shear strength of the clay under unconfined conditions. In general, The UCT can be conducted on rock samples or on undisturbed, reconstituted or compacted cohesive soil sample.

In the unconfined compression test, the sample si placed in the loading machine between the lower and upper plates. Before starting the loading, the upper plate is adjucted to be in contact with the sample and the deformation is set as zero. The test then starts by appling a constant axial strain of about 0.5 to 2% per minute. The load and deformation values are recorded as needed for obtaining a reasonably complete load-deformation curve. The loading is continued until the load values decrease or remain constant with increasing strain, or until reaching 20% (sometimes 15%) axial strain. At this state, the samples is considred to be at failure. The sample is then removed for measurement of the water content.

As for the results, the axial stress is usually plotted versus the axial strain. The maximum axial stress, or the axial stress at 20% (sometimes 15%) axial strain if it occurs earleir, is reproted as the unconfined compressive strength σc. The undrained shear strength then reads

S_u= σ_c / 2

S_u : ndrained shear strength

σ_c : unconfined compressive strength