Critical State Soil Mechanics
is the area of Soil Mechanics
that encompasses the conceptual models that represent the mechanical behavior of saturated remolded soils based on the Critical State
concept. The Critical State concept is an idealization of the observed behavior of saturated remoulded clays in triaxial compression tests
, and it is assumed to apply to undisturbed soils. It states that soils and other granular materials, if continuously distorted (sheared) until they flow as a frictional fluid, will come into a well-defined critical state. At the onset of the critical state, shear distortions
occur without any further changes in mean effective stress
, deviatoric stress
(or yield stress,
, in uniaxial tension according to the von Mises yielding criterion
), or specific volume
However, for triaxial conditions
All critical states, for a given soil, form a unique line called the Critical State Line (CSL) defined by the following equations in the space :
are soil constants. The first equation determines the magnitude of the deviatoric stress
needed to keep the soil flowing continuously as the product of a frictional constant
) and the mean effective stress
. The second equation states that the specific volume
occupied by unit volume of flowing particles will decrease as the logarithm of the mean effective stress increases.
In an attempt to advance soil testing techniques, Kenneth Harry Roscoe of Cambridge University
, in the late forties and early fifties, developed a simple shear apparatus in which his successive student attempted to study the changes in conditions in the shear zone both in sand and in clay soils. In 1958 a study of the yielding of soil based on some Cambridge data of the simple shear apparatus tests, and on much more extensive data of triaxial tests at Imperial College, London, led to the publication of the critical state concept . Roscoe obtained his undergraduate degree in mechanical engineering and his experiences trying to create tunnels to escape when held as a prisoner of war by the Nazis during WWII introduced him to soil mechanics. Subsequent to this 1958 paper, concepts of plasticity were introduced by Schofield and published later in a classic text book . Schofield was taught at Cambridge by a Prof. Baker, a structural engineer who was a strong believer in designing structures that would fail "plastically". Prof. Baker's theories strongly influenced Schofield's thinking on soil shear.
Original Cam-Clay Model
The Original Cam-Clay model is based on the the assumption that the soil is isotropic, elasto-plastic, deforms as a continuum
, and it is not affected by creep.
Modified Cam-Clay Model
For a general overview of "Harvard soil mechanics" and also for a good comparison between the critical state and the steady state see this review of Prof. Andrew Schofield's book "Disturbed Soil Properties and Geotechnical Design"