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The shear strength of soil depends on the effective stress, the drainage conditions, the density of the particles, the rate of strain, and the direction of the strain. For undrained, constant volume shearing, the Tresca theory may be used to predict the shear strength, but for drained conditions, the Mohr–Coulomb theory may be used.
Cohesion is the component of shear strength of a rock or soil that is independent of interparticle friction. In soils, true cohesion is caused by following: Electrostatic forces in stiff overconsolidated clays (which may be lost through weathering) Cementing by Fe 2 O 3, Ca CO 3, Na Cl, etc. There can also be apparent cohesion. This is caused by:
where is the shear strength, is the normal stress, is the intercept of the failure envelope with the axis, and is the slope of the failure envelope. The quantity c {\displaystyle c} is often called the cohesion and the angle ϕ {\displaystyle \phi } is called the angle of internal friction .
The undrained shear strength of remolded soil at the liquid limit is approximately 2 kPa. [ 4 ] [ 10 ] The Plastic Limit is the water content below which it is not possible to roll by hand the soil into 3 mm diameter cylinders.
The shear vane test is a method of measuring the undrained shear strength of a cohesive soil. The test is carried out with equipment consisting of a rod with vanes mounted to it that is inserted into the ground and rotated.
In a 'consolidated undrained' test, the sample is not allowed to drain. The shear characteristics are measured under undrained conditions, and the sample is assumed to be fully saturated. Measuring the pore pressures in the sample (sometimes called CUpp) allows for approximating the consolidated-drained strength.
According to the Mohr-Coulomb equation, the cohesion of a soil is defined as the shear strength at zero normal pressure on the surface of failure. [4] The shear force is a function of cohesion, normal stress on rupture surface, and angle of internal friction. Shear force is significantly impacted by drainage conditions. [5]
Calculation of the capacity of the footing in general bearing is based on the size of the footing and the soil properties. The basic method was developed by Terzaghi, with modifications and additional factors by Meyerhof and Vesić. . The general shear failure case is the one normally analyzed.