deviatoric stress
Introduction
Critical state soil mechanics is the area of soil mechanics that encompasses conceptual models that represent the mechanical behavior of remoulded saturated soils based on the Critical State concept.
Formulation
The Critical State concept is an idealization of the observed behavior of remoulded saturated clays in a triaxial compression test, and is assumed to apply to unaltered soils. It states that soils and other granular materials, if continually distorted (sheared) until they flow as a frictional fluid, will enter a well-defined critical state. At the beginning of the critical state, shear distortions occur without any other change in the average effective stress, deviatoric stress (or yield stress, in uniaxial tension according to the von Mises yield criterion), or specific volume:.
Where,.
However, for triaxial compression conditions. So,.
All critical states, for a given soil, form a single line called the Critical State Line (CSL) defined by the following equations in (p',q,v) space:
where , , and are ground constants. The first equation determines the magnitude of the deviatoric stress necessary to keep the soil flowing continuously as the product of a frictional constant (uppercase) and the average effective stress. The second equation states that the specific volume occupied per unit volume of the flowing particles will decrease as the logarithm of the average effective stress increases.
History
In an attempt at advanced soil testing techniques, Kenneth Harry Roscoe of the University of Cambridge, in the late 1940s and early 1950s, developed a simple shear apparatus in which his later students attempted to study changes in conditions in the shear zone in both sand and clay soils. In 1958 a study of soil yield based on some data from Cambridge testing in the simple shear apparatus, and much more data from triaxial tests at Imperial College London from research led by Professor Sir Alex Skempton at the Imperial Geotechnical Laboratories, led to the publication of the critical state concept (Roscoe, Schofield & Wroth 1958).
Roscoe earned his bachelor's degree in mechanical engineering[1] and his experiences trying to create escape tunnels when he was a prisoner of war by the Nazis during World War II introduced him to soil mechanics.[1] After his 1958 article, the concepts of plasticity were introduced by Schofield (Schofield & Wroth 1968). Schofield was a student at Cambridge of Professor John Baker, a structural engineer who was a fervent believer in designed structures that would fail "plastically". Professor Baker's theories strongly influenced Schofield's thinking on soil shear. Baker's views were developed from his pre-war work on steel structures and later informed by his wartime experience assessing blast-damaged structures and with the design of the "Morrison Shelter", an air raid shelter which could be placed inside (Schofield 2006).