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.

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

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


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


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


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

for C-∅  soil.


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

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

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

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

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

DIRECT SHEAR TEST:

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

 Shear Strength of Soil | 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 | Civil Engineering SSC JE (Technical) - Civil Engineering (CE)
- (σ1)f =2C , for C-soil
 -Shear Strength of Soil | 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 | Civil Engineering SSC JE (Technical) - Civil Engineering (CE)
Shear Strength of Soil | 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 | Civil Engineering SSC JE (Technical) - Civil Engineering (CE)

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

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

where Sf = Sensitivity 

PORE PRESSURE PARAMETER:
Shear Strength of Soil | 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 | Civil Engineering SSC JE (Technical) - Civil Engineering (CE)
where A = Pore pressure parameter
Shear Strength of Soil | 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 | Civil Engineering SSC JE (Technical) - Civil Engineering (CE)

The document Shear Strength of Soil | Civil Engineering SSC JE (Technical) - Civil Engineering (CE) is a part of the Civil Engineering (CE) Course Civil Engineering SSC JE (Technical).
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FAQs on Shear Strength of Soil - Civil Engineering SSC JE (Technical) - Civil Engineering (CE)

1. What is shear strength of soil in civil engineering?
Ans. Shear strength of soil in civil engineering refers to the ability of soil to resist shear forces or stresses. It is a measure of the soil's ability to withstand deformation or failure under applied shear loads.
2. How is shear strength of soil determined in civil engineering?
Ans. The shear strength of soil in civil engineering is determined through laboratory tests such as the direct shear test, triaxial shear test, or field tests like the vane shear test. These tests involve applying controlled shear forces on soil samples and measuring the resulting deformation or failure.
3. What factors affect the shear strength of soil in civil engineering?
Ans. Several factors affect the shear strength of soil in civil engineering. These include the soil type (cohesionless or cohesive), soil composition, soil structure, moisture content, density, stress history, and the presence of any reinforcing materials. These factors influence the soil's internal friction and cohesive forces, which ultimately determine its shear strength.
4. Why is shear strength of soil important in civil engineering?
Ans. Shear strength of soil is crucial in civil engineering as it helps determine the stability and safety of various structures such as foundations, retaining walls, slopes, and embankments. Understanding the shear strength of soil allows engineers to design structures that can withstand anticipated shear forces and prevent failure or collapse.
5. How can the shear strength of soil be improved in civil engineering?
Ans. In civil engineering, there are several methods to improve the shear strength of soil. These include adding stabilizing agents like cement or lime, compacting the soil to increase its density, reinforcing the soil with geosynthetics or soil nails, and implementing proper drainage systems to control the moisture content. These techniques enhance the soil's shear strength and overall stability.
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