Shear Strength

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Braja M Das, Principal of GeotechnicalEngineering , fourth edition, PWS PublishingCompany, 1998, Boston

Muni Budu, Soil Mechanics and Foundations , 2nd

edition, John Wiley & Sons, 2007, USA R.F. Craig, Soil Mechanics , (English & terjemahan

Prof.Dr.Ir Budi S Supandji)

Cernica, J.N., Soil Mechanics , John Wiley & Sons,1995

Holtz, R.D., Kovacs, W.D., An introduction toGeotechnical Engineering , Prentice Hall, N.J.,1981

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Definitions Mohr’s Diagram

Failure Envelope

Stress Path Shear Strength Tests

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Pn = W

Pr

φ

No Friction

α = 0 < φ

No Slip

Pr

φ

Ps

P’f

α

α

Partial Friction

α < φ

No Slip

Pn = W

Pr

φ

Ps

Pf

α

α

Full Friction

α = φ

Slip to Right

Pn = W

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Amount of Available Friction Depends Directlyon Stress & Friction Angle

If Obliquity of Applied Stress < Friction Angle,Only Part of Friction Acting

(No Danger of Slip) If Obliquity = Friction Angle, All Friction

Acting(Slip Imminent)

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Soils generally fails in shear

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At failure, shear stress along the failure surfacereaches the shear strength



The maximum internal resistance to appliedshearing forces (Budu 2007)

From Das (1988) Mohr presented a theory ofrupture in materials that a material failsbecause a critical combination of normalstress and shearing stress. The functionalrelationship between normal stress and shearstress on failure plane can be expressed as:

'tan'' φ σ τ += c f

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Mohr-Coulomb failure criterion

τf : shear strength

c : cohesion

σ f : normal stress

φ : angle of shearing resistance

' ' tan ' f f cτ σ ϕ = +

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( )

( ) ( ) θ σ σ σ σ σ

θ σ σ τ

2cos''2

1''

2

1'

2sin''21

3131

31

−++=

−=

f

f

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'vσ 'hσ vσ hσ

u

Effective stresses

Total stresses

, 'σ σ

τ



'1

σ '3

σ

σ σ σ ∆+= '' 31

'3

σ

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p-q Diagram( )

( )''2

1

''2

1'

31

31

σ σ

σ σ

−=

+=

q

p

p'

q

'1σ '3σ

(p’,q)

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p-q Diagram

p'

q

'1σ '3σ

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Direct Shear Test Triaxial Test

Unconfined Compression Test

Vane Shear Test

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The direct shear test is the oldest and simplestform of shear test arrangement

The disadvantages of the test : the drainage conditions cannot be controlled

only the total stress can be determined shear stress distribution on the failure plane is not

uniform the area under the shear and vertical loads does

not remain constant

The advantage of the test : the test is simple to perform

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Unconsolidated Undrained (UU)Consolidated Undrained (CU)Consolidated Drained (CD)

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onsolidated Drained. Drainage of the specimen is permitted under

a specified all round pressure until consolidation is complete; withdrainage still being permitted, the principal stress difference is thenapplied at a rate slow enough to ensure that the excess pore waterpressure is maintained at zero.

onsolidated–Undrained

Drainage of the specimen is permittedunder a specified all-round pressure until consolidation is complete;the principal stress difference is then applied with no drainage beingpermitted. Pore water pressure measurements may be made duringthe undrained part of the test.

Unconsolidated–Undrained. The specimen is subjected to a specifiedall-round pressure and then the principal stress difference is appliedimmediately, with no drainage being permitted at any stage of thetest.

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Shear Strength

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