"Shear strength is a term used in soil mechanics to describe the magnitude of the shear stress that a soil can sustain. The shear resistance of soil is a result of friction and interlocking of particles, and possibly cementation or bonding at particle contacts. Due to interlocking, particulate material may expand or contract in volume as it is subject to shear strains. If soil expands its volume, the density of particles will decrease and the strength will decrease; in this case, the peak strength would be followed by a reduction of shear stress. The stress-strain relationship levels off when the material stops expanding or contracting, and when interparticle bonds are broken. The theoretical state at which the shear stress and density remain constant while the shear strain increases may be called the critical state, steady state, or residual strength."
http://en.wikipedia.org/wiki/Shear_strength_(soil)

shear strength -
http://hsc.csu.edu.au/agriculture/production/phys_chem_soil/...

"Unsaturated Soil Shear Strength Parameters Measurements (...) Use shear strength testing to determine parameters for other
shear strength theories. (...) Direct Shear Testing - Usually conducted as drained tests..."
http://www.eng.utoledo.edu/civil/heydinger/unsaturated soils...