Unconfined compressive strength (Qu) is a means of determining the ability of a soil to withstand loading pressures. It is an important consideration in the design of structures that will be supported by the soil, as it is used in calculating how much stress, or weight, the soil will initially support. Saturated soils are more prone to failure than unsaturated soils, and building on a soil can proceed quickly enough that the soil can't immediately expell its pore water - effectively rendering the soil saturated, as the water is being 'squeezed out.' The strength of the soil in this condition is refered to as Undrained shear strength, Su. Once the pore water has been expelled, the soil gains strength, but its initial strength must be sufficient to support construction as it is being done, as well as conditions in which the soil may subsequently become saturated. The greater the Qu (and therefore the Su), the more weight the soil will support.
To determine Qu, pressure is applied to a cylinder of the soil until the soil fails, the data gathered is charted, the moisture content of the sample is determined, and the unconfined compressive strength is derived. (For an example of the procedure and calculations, see this lab writeup by Prof. Krishna Reddy at UIC.) Qu = P / A where P is compressive force and A is the cross-sectional area. Qu is measured in tons/sq ft or lb/sq ft and can be used as a measure of the consistency of the soil. Some sample ranges in lb/sq ft are: weak (0-500), soft (500-1000), medium (1000-2000), stiff (2000-4000), very stiff (4000-8000). The full procedure is also listed under ASTM test D2166. From this value, the undrained shear strength is obtained (one-half of the value of the unconfined compressive strength value).
In the field, less formal methods, such as a thumb penetration test, are sometimes used to assess the safety of an excavation for individuals on site, but these methods are not sufficient for use in structure design. (See OSHA manual for discussion of onsite excavation concerns.)
-Chris and Eric