Shear Strength of Soil.
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Shear Strength of Soil.

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Shear Strength of Soil.

 Introduction.

The collapse of the soil by shearing is known as the sliding of a limited mass of soil over a fixed part of the land. Soils of any type have a certain resistance to shearing, and this resistance of course has a maximum, and so that no shear collapse occurs, the maximum shear stress on the soil must not exceed the maximum resistance to shearing  of this kind of soil, with providing an adequate safety factor against the occurrence of this type of collapse. 

The shear strength of the soil is attributed to and derived from two properties of the soil:

  • Friction resistance and the interaction of particles with each other.
  •  Resist cohesion between granules.

Each type of soil resists shearing with one or both of these properties combined.

For example, in sandy or slippery soils (inconsistent), the upper part cannot move sliding on the lower part except with a force capable of overcoming the friction at the points of foundation of the grains on each other, as well as the extent to which these grains overlap with each other and the resistance to friction increases in sandy soil With the increase in vertical tension on the surface of the slide.

In clay (cohesive) soils, where tiny, crusty clay grains stick to each other with the help of moisture, the sliding of the upper part over the bottom is only done with a force that can overcome the cohesion property between the clay grains, and that this cohesion force does not depend on the vertical effort On the surface of the slip, as it depends only on the amount of attraction, bonding and cohesion between the grains, which depends on the crystal structure of the granules and the size of their surface area in relation to their thickness and the chemical composition of the water

Calculation shear strength of soil .

The shear strength of the soil is expressed by the following Coulomb equation

τ = C + σ tan φ

where

τ = Shear strength in units of stress kg/cm2

C = Resistance to soil cohesion

σ = The amount of vertical stress on the slide surface kg/cm2

tan φ = Tangent to the angle of internal friction of the soil, which depends on the size, roughness, and surface condition of the particles

Shear coefficients for soil.

Shear coefficients for any type of soil are C and φ (angle of internal friction of the soil and Resistance to soil cohesion) and these are the two factors that determine the shear strength of any type of soil (τ) when subjected to any value of the vertical stress (σ).

It should be noted that the Coulomb equation represents the state of total stresses on the soil, but in fact the shear strength of the soil is related to the effective stress and the action on the soil where the soil coefficients are taken in this case are C’ and φ’ corresponding to this case from the effective stress, which led to the development Coulomb equation to become

τ = C’ + σ’ tan φ’

Where,

C’ = the amount of soil cohesion corresponding to the effective stress.

σ' = The amount of vertical stress on the slide surface. 

tan φ’ = the angle of internal friction of the soil corresponding to the effective stress.


Soil types according to shear strength.

Soil can be divided according to shear strength based on the values of shear coefficients C and φ as follows:

- Soil cohesive (frictionless soil) where φ = 0 As saturated silt and clay in case of nom-drained. 

- Inconsistent soils where its resistance to friction is high and its resistance to cohesion is zero (C = 0), such as sand and gravel.

- Mixed soil and called it (C-φ soil). It is a mixture of the two previous soils as it has resistance to both friction and cohesion, such as silt, sandy clay, clay sand, etc.












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