Shear strength tests

The determination of the shearing properties of soil is critical for any engineering application. Not only to determine the shearing stress needed to shear the material on purpose, e.g. for ploughing and/or soil removal. Yet also to understand the soil behaviour and limitations under load. As soil is composed of individual particles, movement of these particles among each other (i.e. shearing past each other) can occur when the soil is put under stress. A large quantity of geotechnical failures involve a shear-type failure of the soil.

Shear tests cause distortion of the soil while measuring the resulting force and deformation to determine the material’s response. The latter depends on several parameters, such as rate of distortion, initial stress conditions and the direction of loading.
 Put very simply, the shear strength of soils can be divided between cohesionless (coarse-grained) soils and cohesive (fine-grained) soils. Compared to the determination of shear strength of cohesionless soils, the shear strength of cohesive soil is much more complex: Besides the clear distortion of the material, cohesive soil has (in general) the tendency to change in volume during shearing. Their shear strength is therefore divided into drained and undrained. In addition the shear strength of cohesive soils can be determined as peak value on undisturbed samples or as residual value on remoulded samples. Cohesionless soil samples are generally re-constituted.

Depending on the type of soil, the requested soil properties and the planned direction of loading, a number of methods can be used to determine the shear strength of soil. For horizontal loading either the Direct Shear (DS) or the Direct Simple Shear (DSS) test can be used; by which the DSS is mainly for peat-soils and the DS for coarse-grained soils. It is possible to use the DS for cohesive soils, yet due to the low permeability of the material it is more convenient to use a triaxial test instead. The triaxial tests are used to obtain the stress-strain behaviour and shear strength under triaxial compression. Within the triaxial tests a wide range of setups can be chosen.