Shear strength
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Shear strength in geology is a term used to describe the strength of materials, especially soil, to resist deformation due to shear stress. Under a load, soil resists shear force by two internal mechanisms.
In fine grained soil, there is a cohesive force (c) caused by diagenetic forces between the soil particles. Apparent cohesion may occur as well, caused by negative pore pressure, which means that it dissipates over time as the negative pore pressure is satisfied and the soil shifts to the drained condition.
The second internal force friction is caused by contact between particles. The size of the resistance from friction depends on the internal friction angle, φ, of the material and the size of the overburden pressure, N. The resistive force from the friction angle is calculated as N*tan(φ), which means that the resistance is proportional to the overburden pressure.
Clays and sands have different shear strengths, since the shear strengths of clays and silt are determined primarily by cohesive forces and the shear strengths of sands are determined by friction. An example of low shear strength soils are the unconsolidated bay muds deposited in estuarine settings.
Shear strength in structural engineering is a term used to describe the strength against the type of structural failure where a component fails by shearing when it splits into two parts that slide past each other. The shear strength of a component is most important for beams but also relevant for e. g. plates. In a reinforced concrete beam, the main purpose of stirrups is to increase the shear strength.
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