Shear Strength of Soil

# 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.

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

(i)

where, βmax = 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

-qu = (σ1)f
where, qu = 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.

 Lab size Field Size Height of vane (H) 20 mm 10 to 20 cm Dia of vane (D) 12 mm 5 to 10 cm Thickness of vane (t) 0.5 to 0.1 mm 2 to 3 cm

Shear Strength:

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

-When only bottom of vanes pertain in shearing.

where Sf = Sensitivity

PORE PRESSURE PARAMETER:

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)

where A = Pore pressure parameter

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)

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## Civil Engineering SSC JE (Technical)

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