The primary purpose of this test is to determine the unconfined compressive strength, which is then used to calculate the unconsolidated undrained shear strength of the clay under unconfined conditions. According to the ASTM standard, the unconfined compressive strength (qu) is defined as the compressive stress at which an unconfined cylindrical specimen of soil will fail in a simple compression test. In addition, in this test method, the unconfined compressive strength is taken as the maximum load attained per unit area, or the load per unit area at 15% axial strain, whichever occurs first during the performance of a test.

Advantages:

• Simple and fast especially for sands.
• Failure occurs along a single surface, which approximates observed slips or shear type failure in natural soils.

Disadvantages:

• Difficult or impossible to control drainage especially for fine-grained soils.
• Failure plane is forced. It may not be the weakest in the field.