Shear Strength of Soil Part 1

By BYJU'S Exam Prep

Updated on: September 25th, 2023

Through Champion Study Plan for GATE Civil Engineering (CE) 2022, we are providing Shear Strength of Soil Part 1 study notes and other important materials on every topic of each subject.

These topic-wise study notes are useful for the preparation of various upcoming exams like GATE CivilIESBARCISROSSC-JEState Engineering Services examinations and other important upcoming competitive exams.

The article contains fundamental notes on the “Shear Strength of Soil”  topic of the “Geotechnical Engineering” subject.

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 mobilized within a soil mass.



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

(i) image003 where, βmax = Angle between resultant stress and normal stress on critical plane.

= Friction angle of soil = ∅


for clay ∅ = 0


(ii) image006 (iii) image007, for C-∅ soil.


(iv) image009 for C-soil (clays).

(v) image010 for C-∅ soil.

(vi) image011, for φ-soil.

(vii) image013 for C-soil.


Mohr Coulomb’s Theory


where, C’ = Effective cohesion

image016 = Effective normal stress

and ∅’ = Effective friction angle


Direct Shear Test




Results of Direct Shear Test



Drained conditions maintained, hence C taken as zero in results (as cohesion doesn’t mobilizes in drained condition)

  • image022
  • image023
  • image024


where, σ3 = Cell pressure or all-round confining pressure

σd = Deviator stress A = Area of failure

image026 where, A0 = Area of beginning

v = Volumetric strain


where, ΔV = Volume of water escaped out


image030 = Initial Volume

∈ = Axial strain

Important Points regarding Triaxial Test

  1. During triaxial, either pore water line is open (to get pore pressure) or Drainage line is open (to get volume change)
  2. UD (not possible in field)
  3. CD – Total = effective stress: To check long term stability of embankment which has been in existence since long ago.


  1. With image032 more σ1 required to canse failure hence mohr circle bigger. for normal consolidated soil, at σ3 = 0, τ = 0
    But for OC soil, at σ3 = 0, τ have some value.
  1. CU – Undrained strength comes higher than in site due to isotropic confining pressure of lab and anisotripic confining in site soil.



(Direct Shear) drained condition → C can’t be modified

(unconfined undrained compression test) → ∅ can’t be modified

  1. UU Test

With ↑ in σ3, effective doesn’t change hence no decrease in void ratio or increase in strength is noted, hence for all σ3, same incremental σ1, will come and only one Mohr’s circle is obtained.

Unconfined Compression Test

  • qu = (σ1)f where, qu = unconfined compressive strength.
    Here, σ3 = 0
  • image037
  • image038
  • image039 for clay’s or c-soil.
  • For clays as sand/coarse grained soil/can’t stand in equipment with no lateral pressure.
  • Used to rapidly assess clay consistency in field.
  • To get sensitivity values of clay.

Vane Shear Test

  • It is suitable for sensitive clays.




Shear Strength


When top and bottom of vanes both take part in shearing.

  • image044

When only bottom of vanes take part in shearing.

  • image045

where sf = Sensitivity

Pore Pressure Parameter

(i) image046

where, B = Pore pressure parameter

ΔUc = Change in pore pressure due to increase in cell pressure

Δσc = Δσ3 = Change in cell pressure.

  • 0 ≤ B ≤ 1
  • B = 0, for dry soil.
  • B = 1, for saturated soil.

(ii) image047 where A = Pore pressure parameter

  • image048

ΔUd = Change in pore pressure due to deviator stress.

Δσd = Change in deviator stress

ΔU = Change in pore pressure

(iii) ΔU = ΔUc + ΔUd

(iv) ΔU = B[Δσ3 + A(Δσ1 – Δσ3)]

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