Shear Strength in Soil Mechanics

Shear Strength

• Shear strength is the soil's resistance to sliding along internal layers.
• Soil fails when applied shear stress exceeds its shear strength, triggering landslides.
• Shear stress acts parallel to a material's cross-section, causing deformation or sliding.
• Compressive loads on soil indirectly induce shear stresses as particles transfer loads at angles.
• Normal stress acts perpendicular to the cross-section, causing stretching or compression.

Sources of Shear Strength

• Friction between particles resists sliding, especially in sandy soils.
• Cohesion and adhesion bind particles together (clay) and to surfaces respectively.
• Interlocking of particles creates a strong structure that resists deformation (dense sandy soils).

Mohr's Circle

• Mohr's Circle graphically represents stress at a point in a material.
• It determines normal and shear stresses on any plane angle.
• Normal stress is plotted on the x-axis, shear stress on the y-axis.
• The circle's center represents the midpoint of normal stresses on principal planes.
• The circle's radius shows the maximum shear stress on any plane.
• Principal stresses ($\sigma_1$, $\sigma_2$, $\sigma_3$) are maximum, intermediate, and minimum normal stresses on shear-free planes.
• Mohr's Circle helps find principal stresses and their orientations.
• The failure plane is where shear stress surpasses shear strength, leading to failure.
• Mohr's failure envelope is a line touching multiple Mohr's circles, representing failure points for soil samples.
• Mohr-Coulomb theory links normal and shear stress at failure with cohesion (C) and internal friction angle ($\phi$).
• Cohesion (C) resists shearing due to particle attraction.
• Internal friction angle ($\phi$) represents particle friction.

Limitations of Mohr-Coulomb Theory

• It assumes a linear relationship between shear strength and normal stress.
• It approximates the curved failure envelope with a straight line.
• It disregards the intermediate principal stress's impact.
• It assumes uniform properties (isotropy) but soil properties are often non-uniform (anisotropy).

Factors Influencing Landslides

• Rainfall increases pore water pressure, reducing shear strength and adding weight to soil, increasing landslide likelihood.
• Increased weight and reduced shear strength increase landslide risk.
• Other factors include geological features (faults, weak layers), human activities (deforestation, construction), earthquakes, and volcanic activity.

This quiz covers the fundamental concepts of shear strength in soil mechanics, including its definition, sources, and the role of shear and normal stress in soil behavior. Test your understanding of how soil resists sliding and the parameters affecting its stability.