Shear strength (soil)

Due to interlocking, particulate material may expand or contract in volume as it is subject to shear strains.

The stress-strain relationship levels off when the material stops expanding or contracting, and when interparticle bonds are broken.

The average normal intergranular contact force per unit area is called the effective stress.

As an implication of undrained condition, no elastic volumetric strains occur, and thus Poisson's ratio is assumed to remain 0.5 throughout shearing.

The constant c/p relationship can also be derived from theory for both critical-state [citation needed] and steady-state soil mechanics (Joseph 2012).

c' = is called cohesion, however, it usually arises as a consequence of forcing a straight line to fit through measured values of (τ,σ') even though the data actually falls on a curve.

It is well known that the resulting intercept depends on the range of stresses considered: it is not a fundamental soil property.

The curvature (nonlinearity) of the failure envelope occurs because the dilatancy of closely packed soil particles depends on confining pressure.

In this state the grains being separated are said to be 'tumbling' over one another, with no significant granular interlock or sliding plane development affecting the resistance to shearing.

This is particularly true for most clays that comprise plate-like minerals, but is also observed in some granular soils with more elongate shaped grains.

For large strain deformation, the potential to form a slickensided surface with a φ'r should be considered (such as pile driving).

A major consequence of this is its inability to model strain-softening post peak commonly observed in contractive soils that have anisotropic grain shapes/properties.

Since this is not the case in reality, it is an additional cause of the poor matches to readily available empirical test data.

Additionally, critical state elasto-plastic models assume that elastic strains drives volumetric changes.

Since this too is not the case in real soils, this assumption results in poor fits to volume and pore pressure change data.

Steve J. Poulos Archived 2020-10-17 at the Wayback Machine, then an Associate Professor of the Soil Mechanics Department of Harvard University, built off a hypothesis that Arthur Casagrande was formulating towards the end of his career.

The steady state occurs only after all particle breakage if any is complete and all the particles are oriented in a statistically steady state condition and so that the shear stress needed to continue deformation at a constant velocity of deformation does not change.

In the case where the particles are strongly aligned in the direction of shear, the steady state corresponds to the "residual condition."

This strict definition of the steady state was used to describe soil shear as a dynamical system (Joseph 2012).

The underlying basis of the soil shear dynamical system is simple friction (Joseph 2017).