SHEAR strength of soil

⇒Shear strength bparameters (C , ϕ ) are not inherent properties ,they are related to type of test & condition under measured

⇒penitration test → field test ( τ ) 

★failure on soil occurs by shearing never by crushing

★τ ∝ eff stress ( not total stress)

⇒Coulomb eqn : {τ = C + σ tanϕ}

⇒ Terzaghi modification :{τ = C' + ⁻σ tanϕ'}

⇒{ ⁻σ = σ - u } , u ← pore water pressure 

  ★cohesionless soil (C=0) → τ ∝ σ ∝ rate of loading

⇒angle of failure {Ø= 45 + ϕ/2} ← with major principal plane.

→Clay ϕ= 5°-20°

A} DIRECT SHEAR TEST :

→ sand & gravel not for clay

→eff σ = total σ ( bcz U = 0)

→{τ = C' + ⁻σ tanϕ'}

→quick, inexpensive & simple

→shear & vertical deflection are measured (dial gauge)

Disadvantages: 

 a) drainage condition uncontrolled & pore water pressure can't measured

 b) failure plane predetermined & always horizontal

 c) non-uniform stress distⁿ

 

B} VANE SHEAR TEST : 

⇒soft clay , sensitive soil , cohesive soil , plastic clay soil

⇒lab test : silty sand shear parameters

⇒used to find sensitivity also {sensitivity = Cu undisturbed / Cu remoulded }

{Cu = qu/2}

{shear strength→ τf = Cu ←undrained cohesion}

⇒τ f = T /πd²(½h +⅙d) ← 2way shear top & bottom & if nothing is give..

 τ f = T /πd²(½h +¹/12d) ←one end shear from bottom

 

C}TRIAXIAL TEST:

→all type of soil 

→ Lenght / dia = 2-2.5

→pore water pressure & vol. Change can be measured

→drainage condtⁿ best controlled

→Axial strain & deviator stress are determined

→{ σ1 = σ3 tan²(45+ϕ/2) + 2C tan(45+ϕ/2) } & ϕ=0←undrain 

Stage 01 : cell pressure or consolidatⁿ test

Stage 02 : shear stage or deviater stage

{(σ 1)f = σ 3 +(σ d)f} & {σ 3= σ c}

{Sin ϕ = (σ 1 - σ 3)/(σ 1 + σ 3)}

⇒confined compressive strength = σ d at failure = P/Af

Type of TT

UU - clay ,quick test(15min)

CD - sand ,slow test 

CU - dam embankment

UD - physically impossible 

Representation of TT

⇒pure clay ϕ = 0 ←undrain test on sat clay

⇒CD test on NC clay then clay behave like sand (C=0)

⇒for C-ϕ soil →CD test on OC clay behave like silt

D} UNCONFINED COMPRESSⁿ TEST: 

⇒a undrained test

⇒cohesive soil ( saturated clay & silt) not for coarse grain soil

⇒special case of TT σ 3 = σ c = 0

⇒no moisture loss

⇒qu = 2Ctan (45 + ϕ/2) for clay ϕ = 0

⇒

PORE PRESSURE coeff : 

⇒B = ∆Uc/∆σc = 0 - 1 dry =0, fully saturated soil = 1

⇒A = -0.5 for OC soil & = 3 for loose soil

Posted by : Jagmohan Singh Panwar Tags : SOIL and FOUNDATION

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