# Shear Strength of Soil Part 1

By Sachin Singh|Updated : November 29th, 2021

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

= Friction angle of soil = ∅

for clay ∅ = 0

(ii)  (iii) , for C-∅ soil.

(iv)  for C-soil (clays).

(v)  for C-∅ soil.

(vi) , for φ-soil.

(vii)  for C-soil.

Mohr Coulomb's Theory

where, C' = Effective cohesion

= 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)

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

σd = Deviator stress A = Area of failure

where, A0 = Area of beginning

v = Volumetric strain

where, ΔV = Volume of water escaped out

= 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  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
•  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.

When only bottom of vanes take part in shearing.

where sf = Sensitivity

Pore Pressure Parameter

(i)

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)  where A = Pore pressure parameter

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)]

If you are preparing for ESE/ GATE or other PSU Exams (Civil Engineering), then avail Online Classroom Program for ESE and GATE CE:

### Online Classroom Program for ESE/GATE CE

You can avail of BYJU'S Exam Prep Test Series specially designed for all Civil Engineering Exams:

Thanks

All the Best