Class notes of Geotechnical Engineering course I used to teach at UET Lahore. Feel free to download the slide show. Anyone looking to modify these files and use them for their own teaching purposes can contact me directly to get hold of editable version.

1. 1
Geotechnical Engineering–II [CE-321]
BSc Civil Engineering – 5th Semester
by
Dr. Muhammad Irfan
Assistant Professor
Civil Engg. Dept. – UET Lahore
Email: mirfan1@msn.com
Lecture Handouts: https://groups.google.com/d/forum/geotech-ii_2015session
Lecture # 1
6-Sep-2017

2. 2
SOIL STRENGTH
Construction Materials
Concrete, Steel, Bricks, Wood, Aggregate,
Soil, etc.
Most important property for Civil
Engineers?
• Concrete, Brick, Wood 
• Steel 
• Soil ?
Material Strength
Compressive
strength
Tensile Strength

3. 3
SOIL STRENGTH
SOIL
• Mostly loaded in compression
• But fails mostly in shear
Embankment
Strip footing
Failure surface
Mobilized shear
resistance

4. 4
• Greatest shear stress a material can sustain before failure
• Safety of geotechnical structure dependent on soil shear strength
• Failure of soil → Failure of whole structure
SHEAR STRENGTH
Typhoon triggered landslide
Wakayama, Japan (September 2011)

5. 5
• Resistance to shearing stresses
• Shear failure occurs due to sliding or rolling of particles past each
other.
• Sources of soil shear strength
– Cohesion
• Cementation between sand grains
• Electrostatic attraction between clay particles
– Frictional resistance
• Interlocking between sand grains
SHEAR STRENGTH OF SOILS
Sliding of particlesRolling of particles
(stress independent component)
(stress dependent component)
 Cohesion (c)
 Angle of internal
friction (f)

6. 7
SHEAR FAILURE OF SOIL
Embankment
Strip footing
Soils generally fail in shear
At failure, shear stress/resistance along failure surface (mobilized
shear resistance) reaches the shear strength.
Failure surface
Mobilized shear
resistance

7. 8
SHEAR FAILURE OF SOIL
Retaining wall
Soils generally fail in shear

8. 9
SHEAR FAILURE OF SOIL
Retaining wall
Mobilized shear
resistance
Failure
surface
At failure, shear stress/resistance along failure surface (mobilized
shear resistance) reaches the shear strength.
Soils generally fail in shear

9. 10
SHEAR FAILURE MECHANISM
• Soil grains slide/roll
over each other along
the failure surface.
• No crushing of
individual grains.
Failure Surface
At failure, shear stress/resistance along failure surface ()
reaches shear strength (f).
X
Y
Difference in shear strength of X & Y?

10. 11
The relationship between normal and shear stress on the failure
plane
f tan cf
f = shear strength
c = cohesion
 = normal stress
Φ = angle of internal friction
)( f
σ1
σ3
f
Friction angle
f
 

Graphical
representation
Cohesion
c
MOHR-COULOMB FAILURE CRITERIA

11. 12
MOHR-COULOMB FAILURE CRITERIA


f
Non-cohesive/Granular Soils
(c = 0; f > 0) 

Cohesive Soils
(c > 0; f = 0)
c


c-f Soils
(c > 0; f > 0)
c
f

12. 13
N
T
Area: A
For a continuous material
Normal stress: σ = N / A
(compression: +ve)
Shear stress: τ = T / A
(counter-clock-wise: +ve)
Basic Concepts
Principle Stress: Max. and min. value of normal stresses
Principle Plane: Plane on which principle stresses act
• Normal stresses are either max. or min. on principle planes
• Shear stresses are zero on principle planes
MOHR-COULOMB FAILURE CRITERIA

13. 14
MOHR-COULOMB FAILURE CRITERIA

f is the maximum shear stress the soil can take without failure,
under any particular normal stress of .

f tan cf
c
f
Cohesion
Friction angle
f

In terms of Total Stress

14. 15
MOHR-COULOMB FAILURE CRITERIA
u 

’
f  tancf
f’
Effective
friction angle
c’
Effective
cohesion
f
’
 = Total stress
u = Pore water
pressure
In terms of Effective Stress
f is the maximum shear stress the soil can take without failure,
under any particular normal effective stress of ’.

15. 16
CONCLUDED
REFERENCE MATERIAL
Principles of Geotechnical Engineering – (7th Edition)
Braja M. Das
Chapter #12
Geotechnical Engineering – Principles and Practices – (2nd Edition)
Coduto, Yueng, and Kitch
Chapter #12