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Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE)

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Chapter 9
Shear Strength of Soil

SHEAR STRENGTH
Shear strength of a soil is the capacity of the soil to resist shearing stress. It can be defined as the maximum value of shear stress that can be defined as the maximum value of shear stress that can be Mobilized within a soil mass.

 Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE)
-Plane a -a is critical plane
c = Angle of critical plane (a – a)
1 and σ3 are stresses on given planes

(i) 
        Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE)

where, βmax = Angle between resultant stress and normal stress on critical plane=Friction angle of soil = ∅ 

Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE)     
 for clay, ∅= 0
      Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE)

(ii)  Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE) , for sands or Φ -soil.


(iii)Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE), for C- Φ soil.


(iv) Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE), for C-soil (clays).


(v) Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE)

for C-∅  soil.


(vi)Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE), for Φ soil.
 (vii) σ1= 2C , for C– soil.

 Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE)

MOHR COULOMBS THEORY
Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE)

where, C’ = Effective Cohesion
Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE) = Effective normal stress and ∅' = Effective friciton angle

 Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE)

DIRECT SHEAR TEST:

Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE)

 Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE)

UNCONFINED COMPRESSION TEST

-qu = (σ1)f
where, qu = unconfined compressive strength.
Here, σ3= 0
Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE)
- (σ1)f =2C , for C-soil
 -Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE)  for clay’s or C-soil.

VANE SHEAR TEST:
-It is suitable for sensitive clays.

 Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE)
Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE)

 Lab sizeField Size
Height of vane (H)20 mm10 to 20 cm
Dia of vane (D)12 mm5 to 10 cm
Thickness of vane (t)0.5 to 0.1 mm2 to 3 cm


Shear Strength:
Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE)

-When top and bottom of vanes both pertain in shearing.
 Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE)

-When only bottom of vanes pertain in shearing.
 Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE)

where Sf = Sensitivity 

PORE PRESSURE PARAMETER:
Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE) 

where,
B = Pore pressure parameter
ΔUc = Change in pore pressure due to in crease in cell pressure
Δσ= Δσ3 = Change in cell pressure.
-0 ≤ B ≤ 1
-B = 0, for dry soil.
-B = 1, for saturated soil.

(ii) 
Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE)
where A = Pore pressure parameter
Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE)

where ,
 ΔUd = Change in pore pressure due to deviator stress.
 Δσd = Change in deviator stress

 (iii) ΔU=ΔUc +ΔUd
ΔU = change in pore pressure
(iv)

 Shear Strength of Soil - Notes | Study Civil Engineering SSC JE (Technical) - Civil Engineering (CE)

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