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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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.

  • Shear strength is a soils’ ability to resist sliding along internal surfaces within the soil mass.






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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


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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.


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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)


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  • 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


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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


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