What is Shear strength

Most problems in geotechnics, such as bearing capacity of shallow and deep foundations, slope stability, retaining wall design, resistance to penetration and soil liquefaction, are affected by soil shear strength. Analytical and numerical analyzes use shear strength values ​​to solve these engineering problems.Shearing strength in soils is the result of resistance to movement at interparticle contacts, due to particle interlocking, physical bonds formed across the contact areas (resulting from surface atoms sharing electrons at interparticle contacts) and chemical bonds (i.e., cementation particles connected through a solid substance such as recrystallized calcium carbonate).

Different criteria can be used to define the point of failure in a stress–strain curve of a particular material. Failure and yield should not be confused. There is no unique way of defining failure. For some materials, failure can be assumed to be the yield point. For soils, ‘failure’ is usually considered to be occurring at 15–20 per cent strain. This deformation usually implies that the function of a particular structure, e.g., a building foundation, might be impaired but not have failed. Failure of the soil does not imply failure of the system. In this sense, the shear strength of soils can be defined as the maximum stress applied on any plane in a soil mass at some strain considered as failure.

Different failure criteria are applied to define failure. The Mohr–Coulomb failure criterion is the most common empirical failure criterion used in soil mechanics.

The stress–strain relationship of soils, and therefore the shearing strength, is affected by the following:

1. Soil composition (basic soil material): Mineralogy, grain size and grain size distribution, shape of particles, pore fluid type and content, ions on grain and in pore fluid.
2. State (initial): Defined by the initial void ratio, effective normal stress and shear stress (stress history). State can be described by terms such as loose, dense, over-consolidated, normally consolidated, stiff, soft, contractive and dilative.
3. Structure: Refers to the arrangement of particles within the soil mass, the manner in which the particles are packed or distributed. Features such as layers, joints, fissures, slickensides, voids, pockets and cementation are part of the structure. The structure of soils is described by terms such as undisturbed, disturbed, remoulded, compacted, cemented; flocculent, honey-combed, single-grained; flocculated, deflocculated; stratified, layered, laminated; isotropic and anisotropic.

1. Loading conditions: Effective stress path – drained or undrained, type of loading, magnitude, rate (static and dynamic) and time history (monotonic and cyclic).

In reality, a complete shear strength formulation would account for all these factors. Laboratory tests, e.g., direct shear test, triaxial shear test, simple shear test, using different drainage conditions (drained or undrained), rate of loading, range of confining pressures and stress history, are used for determining values of shear strength, unconfined compressive strength, drained shear strength, undrained shear strength, peak strength, critical state shear strength and residual strength.