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The internal angle between the surface of the pile and the horizontal surface is known as the angle of repose and is related to the density, surface area and shapes of the particles, and the coefficient of friction of the material. Material with a low angle of repose forms flatter piles than material with a high angle of repose.
Angle of internal friction for some materials Material Friction angle in degrees Rock: 30 ° Sand: 30 ° to 45 ° Gravel: 35 ° Silt: 26 ° to 35 ° Clay: 20 ° Loose sand 30 ° to 35 ° Medium sand 40 ° Dense sand 35 ° to 45 ° Sandy gravel > 34 ° to 48 °
The relationship between dilation and internal friction is typically illustrated by the sawtooth model of dilatancy where the angle of dilation is analogous to the angle made by the teeth to the horizontal. Such a model can be used to infer that the observed friction angle is equal to the dilation angle plus the friction angle for zero dilation.
The angle of internal friction is thus closely related to the maximum stable slope angle, often called the angle of repose. But in addition to friction, soil derives significant shear resistance from interlocking of grains. If the grains are densely packed, the grains tend to spread apart from each other as they are subject to shear strain.
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:
The gravel is self-interlocking with a high friction angle and is used in road construction, particularly useful in areas of marshy ground or permafrost marginal zones where it can reduce the degradation of the road base.
The results of the tests on each specimen are plotted on a graph with the peak (or residual) stress on the y-axis and the confining stress on the x-axis. The y-intercept of the curve which fits the test results is the cohesion, and the slope of the line or curve is the friction angle. Direct shear tests can be performed under several conditions.
As the roundness of materials increases, the angle of repose decreases since there is less friction between the soil grains. [4] When the angle of repose is exceeded, mass wasting and rockfall can occur. It is important for many civil and geotechnical engineers to know the angle of repose to avoid structural and natural disasters.