The document provides an overview of shear strength lab determination methods in geotechnical engineering, focusing on unconfined compression tests for cohesive soils. It includes explanations of shear strength parameters, sample calculations, and concepts of sensitivity and thixotropy in clay. Reference materials for further reading are also cited.

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 # 4
15-Sep-2017

2. 2
SHEAR STRENGTH
- LAB DETERMINATION -
1. Direct shear test
2. Unconfined compression test
3. Triaxial compression test
Unconfined Compression Test
• “Unconfined” sample
• For cohesive soils only

3. 3
UNCONFINED COMPRESSION TEST
 tan cf
Proving Ring

4. 4
UNCONFINED COMPRESSION TEST
 tan cf
Proving Ring
1 > 0
3 = 0
Confining pressure, 3 = 0

5. 5
UNCONFINED COMPRESSION TEST
 tan cf
Proving Ring
qu = 1
Stress,(kPa) Axial Strain, e (%)
Unconfined Compressive
Strength

6. 6
 tan cf
Stress,(kPa)
Axial Strain, e (%)
Shearstress,
Normal stress, 
Cu
cu = τf = σ1/2 = qu/2
Undrained Cohesion (CU) = Undrained Strength (SU) OR τf
σ1σ3 = 0
How to determine shear
strength parameters c and  ?
UNCONFINED COMPRESSION TEST
- Analysis of Results -
qu = 1
Unconfined Compressive
Strength
 = 0  τf = cu

7. 7
Dia. Of Sample, D = 1.5in
Length of Sample, Lo = 3in
Orignal x-area of Sample, Ao = 1.767in2
Volume of the Smaple, V = 5.30in3
Wt. of the Specimen, M = 0.1lb
Bulk Density, γb = 0.019lb/in3
Moisture Content, w = %
Proving Ring Constant = 0.8lb/div
DDG L.C. = 0.0005in
Sample#
Cell
Pressure
DDG
Reading
Proving Ring
DG Reading
Sample
Deformation
Axial
Strain
Corrected Area Axial Load Axial Stress
ΔL = Col-3 x
LC
ξ =
ΔL/Lo AC = Ao / (1-ξ )
Col-4 x
PRC Col-8 / Col-7
(kPa) (div.) (div.) (in) (in2) (lb) (psi) (kPa)
Col-1 Col-2 Col-3 Col-4 Col-5 Col-6 Col-7 Col-8 Col-9
1 0
0 0 0.000 0.0000 1.767 0.00 0.00 0.00
10 4 0.005 0.0017 1.770 3.20 1.81 12.46
20 6 0.010 0.0033 1.773 4.80 2.71 18.67
30 7 0.015 0.0050 1.776 5.60 3.15 21.74
40 9 0.020 0.0067 1.779 7.20 4.05 27.91
UNCONFINED COMPRESSION TEST
- Calculations -
Sample Calculations

8. 9
SENSITIVITY AND THIXOTROPY OF CLAY
SENSITIVITY
Loss of strength of clay in
the remolded form.
THIXOTROPY
Gain in strength of clay
with time.
• Most (but not all) clays
are partially thixotropic.
Time

9. 10
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

10. 11
SENSITIVITY AND THIXOTROPY OF CLAY