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Shear Strength of Cohesive (Clay) Soil

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Shear Strength of Cohesive (Clay) Soil. Introduction. Soil failure usually occurs in the form of “shearing” along internal surface within the soil. Thus, structural strength is primarily a function of shear strength.

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  • Soil failure usually occurs in the form of “shearing” along internal surface within the soil.
  • Thus, structural strength is primarily a function of shear strength.
  • Shear strength is a soils’ ability to resist sliding along internal surfaces within the soil mass.
what is shear strength
What is Shear Strength?
  • Shear strength in soils is the resistance to movement between particles due to physical bonds from:
    • Particle interlocking
    • Atoms sharing electrons at surface contact points
    • Chemical bonds (cementation) such as crystallized calcium carbonate
influencing factors on shear strength
Influencing Factors on Shear Strength
  • The shearing strength, is affected by:
    • soil composition: mineralogy, grain size and grain size distribution, shape of particles, pore fluid type and content, ions on grain and in pore fluid.
    • Initial state: State can be describe by terms such as: loose, dense, overconsolidated, normally consolidated, stiff, soft, etc.
    • Structure: Refers to the arrangement of particles within the soil mass; the manner in which the particles are packed or distributed. Features such as layers, voids, pockets, cementation, etc, are part of the structure.
shear strength of soil
Shear Strength of Soil
  • In reality, a complete shear strength formulation would account for all previously stated factors.
  • Soil behavior is quite complex due to the possible variables stated.
  • Laboratory tests commonly used:
    • Direct Shear Test
    • Unconfined Compression Testing (TODAY’S LAB)
Since so many variable do exist, the Mohr-Coulomb failure criterion is the most common failure criterion used in soil mechanics.
  • The Mohr-Coulomb criterion defines shear strength:

S = c + σμ

    • c is strength due to cohesion (c=qu/2)
    • qu is unconfined compressive strength
    • σμ strength due to friction (zero for clays). This term applies to sand/granular materials
shear strength of cohesive soils
Shear Strength of Cohesive Soils

S = c + σμ

  • “c=qu/2” and since we have a cohesive soil, “σμ” can be assumed zero.
  • We can determine S by the unconfined compressive test and yield:
  • S = c =qu/2