The document provides an overview of geotechnical engineering concepts focused on soil shear strength determination through various laboratory and field tests, including the triaxial shear test and its advantages over traditional methods. It outlines the procedure for conducting these tests, the interpretation of results, and practical applications in construction scenarios like earth dams and embankments. References to foundational texts in geotechnical engineering are included for further study.

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 # 5
20-Sep-2017

2. 2
DETERMINATIONOFSOILSHEARSTRENGTH
(c, φ or c’, φ’)
Laboratory tests
on representative samples
Field tests
Direct shear test
Unconfined compression test
Triaxial shear test
Simple shear test
Torsional ring shear test
Plane strain triaxial test
Laboratory vane shear test
Laboratory fall cone test
Vane shear test
Torvane
Pocket penetrometer
Fall cone
Pressuremeter
Static cone penetrometer
Standard penetration test
Assignment: Brief intro of each

3. 3
z
σv
σv
σhσh
Before construction
Representative soil
sample
z
σv + Δσ
σhσh
During and after
construction
σv + Δσ
FIELD STRESS CONDITIONS

4. 4
Step 1
Application of initial
stress condition
σv
σv
σh
σh
Representative soil
sample from site
0
00
0
Step 2
Application of
disturbing/structural load
σv + Δσ
σhσh
σv + Δσ
t
σv
σv
τ
τ
LAB SIMULATION OF
FIELD CONDITIONS

5. 5
Developed by Casagrande to overcome some serious disadvantages
of direct shear test.
TRIAXIAL SHEAR TEST
• Advantages over DST and UCCT
– More versatile
– Drainage can be well controlled
– No rotation of principal stresses
like DST
– Failure plane is the weakest one
– Field stress conditions can be
replicated through confinement

6. 6
TRIAXIAL COMPRESSION TEST
t tan cf

7. 7
z
σv
σv
σhσh
Before construction
Representative soil
sample
z
σv + Δσ
σhσh
During and after
construction
σv + Δσ
FIELD STRESS CONDITIONS

8. 8
3
3
3
3
All-around cell pressure 3
Step 1
Deviator stress
( = q)
Shearing (loading)
Step 2
1=3+q
TRIAXIAL COMPRESSION TEST
Normal Stress (1)  Confining Pressure (3) + Deviator Stress (q)
3 3
3
3
Major
principal stress
Minor
principal stress

9. 9
Proving ring to
measure the
deviator load
Dial gauge to
measure vertical
displacement
qf
DeviatorStress,q(kPa)
Axial Strain, e (%)
Deviator Stress at failure
TRIAXIAL COMPRESSION TEST
- Analysis of Results -
Shear Failure Bulging Failure

10. 10
Deviatorstress,q
Axial strain
Shearstress,t


Mohr – Coulomb
failure envelope
qfa
Confining stress = 3a
qfb
Confining stress = 3b
qfc
Confining stress = 3c
3c 1c
3a 1a
qfa
3b 1b
1 = 3 + qf
3
TRIAXIAL COMPRESSION TEST
- Analysis of Results -
c
How to determine shear
strength parameters c and  ?

11. 11
TRIAXIAL COMPRESSION TEST
t tan cf

12. 12
Is the drainage valve open?
yes no
Consolidated
sample
Unconsolidated
sample
Is the drainage valve open?
All-around cell pressure c (or c)
c
c
c
cStep 1
Deviatoric stress
( = q)
Shearing (loading)
Step 2
c c
c+ q
Types of Triaxial Tests
yes no
Drained
loading
Uudrained
loading

13. 13
Is the drainage valve open?
yes no
Consolidated
sample
Unconsolidated
sample
Is the drainage valve open?
yes no
Drained
loading
Undrained
loading
Under all-around cell pressure c
Step-1
Shearing (loading)
Step-2
Types of Triaxial Tests
CD test
CU test
UU test

14. 14
t
t = in situ drained
shear strength
Soft clay
Embankment constructed very slowly, in layers over a
soft clay deposit
CD Test
Practical Applications
Use cd
and d.

15. 15
Earth dam with steady state seepage
t = drained shear
strength of clay core
t
Core
CD Test
Practical Applications
Use cd
and d.
• CD test simulates the long term condition in the field
• cd and d used to evaluate long term behavior of soils

16. 16
t t = in situ undrained
shear strength
Soft clay
Embankment constructed rapidly over a soft clay deposit
with significant time gap between placement of layers.
CU Test
Practical Applications

17. 17
Rapid drawdown behind an earth dam
t = Undrained shear strength
of clay core
Core
t
CU Test
Practical Applications

18. 18
t t = in situ undrained
shear strength
SOFT CLAY
Embankment constructed rapidly over a soft clay deposit
UU Test
Practical Applications

19. 19
Large earth dam constructed rapidly with no change in water
content of soft clay
CLAY
CORE
t = Undrained shear strength
of clay core
t
UU Test
Practical Applications
Note:
UU test simulates the short term condition in the field.
UU can be used to analyze the short term behavior of soils

20. 20
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
Essentials of Soil Mechanics and Foundations – Basic Geotechnics – (7th Edition)
David F. McCarthy
Chapter #11