Shear Strength Shear Strength of Cohesive of Cohesive (Clay) Soil(Clay) Soil

IntroductionIntroduction

Soil failure usually occurs in the Soil failure usually occurs in the form of “shearing” along internal form of “shearing” along internal surface within the soil.surface within the soil.

Thus, structural strength is primarily Thus, structural strength is primarily a function of shear strength.a function of shear strength.

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

Slope Stability: Failure is Slope Stability: Failure is an Example of Shearing an Example of Shearing

Along Along Internal SurfaceInternal Surface

Mass Wasting: Shear Mass Wasting: Shear FailureFailure

Shear Failure: Earth Shear Failure: Earth DamDam

Shear Failure Under Shear Failure Under Foundation LoadFoundation Load

What is Shear Strength?What is Shear Strength?

Shear strength in soils is the Shear strength in soils is the resistance to movement between resistance to movement between particles due to physical bonds particles due to physical bonds from:from:

a.a. Particle interlockingParticle interlocking

b.b. Atoms sharing electrons at surface Atoms sharing electrons at surface contact pointscontact points

c.c. Chemical bonds (cementation) such as Chemical bonds (cementation) such as crystallized calcium carbonate crystallized calcium carbonate

Influencing Factors Influencing Factors on Shear Strengthon Shear Strength

The shearing strength, is affected by:The shearing strength, is affected by: soil compositionsoil composition: mineralogy, grain size and grain : mineralogy, grain size and grain

size distribution, shape of particles, pore fluid size distribution, shape of particles, pore fluid type and content, ions on grain and in pore fluid. type and content, ions on grain and in pore fluid.

Initial stateInitial state: State can be describe by terms such : State can be describe by terms such as: loose, dense, overconsolidated, normally as: loose, dense, overconsolidated, normally consolidated, stiff, soft, etc. consolidated, stiff, soft, etc.

StructureStructure: Refers to the arrangement of particles : Refers to the arrangement of particles within the soil mass; the manner in which the within the soil mass; the manner in which the particles are packed or distributed. Features such particles are packed or distributed. Features such as layers, voids, pockets, cementation, etc, are as layers, voids, pockets, cementation, etc, are part of the structure. part of the structure.

Shear Strength of SoilShear Strength of Soil

In reality, a complete shear strength In reality, a complete shear strength formulation would account for all formulation would account for all previously stated factors.previously stated factors.

Soil behavior is quite complex due to Soil behavior is quite complex due to the possible variables stated.the possible variables stated.

Laboratory tests commonly used:Laboratory tests commonly used: Direct Shear TestDirect Shear Test Unconfined Compression Testing Unconfined Compression Testing

(TODAY’S LAB)(TODAY’S LAB)

Since so many variable do exist, the Mohr-Coulomb failure criterion is the most common failure Since so many variable do exist, the Mohr-Coulomb failure criterion is the most common failure criterion used in soil mechanics. criterion used in soil mechanics.

The Mohr-Coulomb criterion defines shear strength:The Mohr-Coulomb criterion defines shear strength:S = c + S = c + σμσμ

c is strength due to cohesion (c=qc is strength due to cohesion (c=quu/2)/2) qqu u is unconfined compressive strengthis unconfined compressive strength σμσμ strength due to friction (zero for clays). This term applies to sand/granular materials strength due to friction (zero for clays). This term applies to sand/granular materials

Shear Strength of Shear Strength of Cohesive SoilsCohesive Soils

S = c + S = c + σμσμ

““c=qc=quu/2” and since we have a cohesive /2” and since we have a cohesive soil, “soil, “σμσμ” can be assumed zero.” can be assumed zero.

We can determine S by the unconfined We can determine S by the unconfined compressive test and yield:compressive test and yield:

S = c =qS = c =quu/2/2