What is the undrained shear strength (Su) equivalent to in a UU test?

In a UU (Unconsolidated Undrained) test, the undrained shear strength (Su) is established as being equal to half of the unconfined compressive strength. This relationship is derived from the mechanics of saturated cohesive soils under undrained conditions.

During a UU test, a cylindrical soil sample is subjected to axial loading until failure without any drainage or stress release. The unconfined compressive strength is defined as the maximum axial stress that the sample can sustain in this undrained condition. The undrained shear strength, which is a critical parameter for assessing the stability of saturated clay soils, is then calculated as half of this unconfined compressive strength.

Understanding this relationship is central to geotechnical engineering, particularly in assessing the behavior of soils under rapid loading conditions where drainage is not allowed, such as during excavation or the construction of foundations. This principle helps engineers predict the soil's performance and stability during construction and ensure safety in design.