Shear Strength of Soil | Civil Engineering SSC JE (Technical) - Civil Engineering (CE) PDF Download

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.

 
-Plane a -a is critical plane
-θ_c = Angle of critical plane (a – a)
-σ₁ and σ₃ are stresses on given planes

(i) 
       

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

     
 for clay, ∅= 0
     

(ii)   , for sands or Φ -soil.

(iii), for C- Φ soil.

(iv) , for C-soil (clays).

(v)

for C-∅  soil.

(vi), for Φ soil.
 (vii) σ₁= 2C , for C– soil.

MOHR COULOMBS THEORY

where, C’ = Effective Cohesion
 = Effective normal stress and ∅' = Effective friciton angle

DIRECT SHEAR TEST:

UNCONFINED COMPRESSION TEST

-q_u = (σ₁)_f
where, q_u = unconfined compressive strength.
Here, σ₃= 0

- (σ₁)f =2C , for C-soil
 -  for clay’s or C-soil.

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

Shear Strength:

-When top and bottom of vanes both pertain in shearing.
 

-When only bottom of vanes pertain in shearing.
 

where S_f = Sensitivity 

PORE PRESSURE PARAMETER:
 

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

(ii) 

where A = Pore pressure parameter

where ,
 ΔU_d = Change in pore pressure due to deviator stress.
 Δσ_d = Change in deviator stress

 (iii) ΔU=ΔU_c +ΔU_d
ΔU = change in pore pressure
(iv)