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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.
Symbol used in drawings Diagram showing the rod and vane inserted into the soil US Army Corps of Engineers personnel carrying out a shear vane test. 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 ...
Triaxial shear tests This is a type of test that is used to determine the shear strength properties of a soil. It can simulate the confining pressure a soil would see deep into the ground. It can also simulate drained and undrained conditions. Unconfined compression test ASTM D2166. This test compresses a soil sample to measure its strength.
The output of the analysis is a factor of safety, defined as the ratio of the shear strength (or, alternatively, an equivalent measure of shear resistance or capacity) to the shear stress (or other equivalent measure) required for equilibrium. If the value of factor of safety is less than 1.0, the slope is unstable.
A typical reference strain for the approximate occurrence of zero effective stress is 5% double amplitude shear strain. This is a soil test-based definition, usually performed via cyclic triaxial, cyclic direct simple shear, or cyclic torsional shear type apparatus. These tests are performed to determine a soil's resistance to liquefaction by ...