Shear Strength of Soil

# 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
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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## 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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