The City College of New York
Department of Civil Engineering
CE 345: Soil Mechanics
Instructor: Dr. George Mylonakis
LAB EXPERIMENT #6:
UNCONFINED COMPRESSION TEST
Introduction
The unconfined compression test is used to measure the unconfined shear strength of fine-grained soils,
which is an approximate value of their undrained shear strength. This test is applicable only to cohesive
soils such as saturated clays or cemented soils that retain an intrinsic strength without a confining pressure.
It is not applicable to dry or cohesionless soils, such as gravels and sands. This test can be done for both
undisturbed and remolded specimens. Here you will perform the test on a remolded specimen.
Equipment
1.
Loading device
2.
Loading ring
3.
Dial indicator
4.
Scale
5.
Trimmer
6.
Watch
7.
Sampling tube with extractor
8.
Harvard miniature compaction device
9.
Balance
10. Moisture cans
11. Oven
Preparation of specimen
Weigh 115g of dry disturbed soil. Add water to reach water content of 28%. Mix thoroughly to get
uniform water content. Compact the sample in Harvard miniature compaction mold. Carefully extrude the
specimen.
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Procedure
1.
Determine the initial length and diameter of the specimen.
2.
Immediately place the sample on the loading device. Lower the bottom platen or raise the bottom
platen so that the upper platen barely touches the specimen and triggers a slight response of the load
sensor. Attach the dial indicator to the loading device to measure the axial deformation of the
specimen.
3.
Adjust the rate of axial displacement to obtain a strain rate of 1% per minute. The complete loading
lasts about 20 min. to apply 20% of axial strain.
4.
Simultaneously record the axial displacement and load. Stop the test when load remains constant or
after 20% axial strain.
5.
Record the time to peak strength and failure pattern (shear or bulge failure) and sketch the failed
specimen.
6.
After the test, determine the water content, w, of the specimen.
Calculation
1.
Water content w is given by
w = (Wi / Wd - 1) x 100 (%)
Wi = initial weight
Wd = dry weight
2.
The axial strain  is
 = H / Ho
H = change of specimen height
Ho = initial specimen height
3.
The stress  is given by
 = F / Ac
Ac = Ai / (1 - )
where,
F = applied load
Ac = corrected area
Ai = initial area
General Comments
Plot a stress-strain diagram using same scale. Determine shear strength of soil. Discuss the soil tested
based on results.
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DATA SHEET: Unconfined Compression Test
Description of soil:
Group No.:
Date:
_______________________________
______________________________________
_____________________________________________
Moisture content, w:
_______________________________ %
Initial length of specimen, Ho:
________________________ in.
Initial diameter of specimen, Do:
________________________ in.
Initial area of specimen, Ai:
________________________ in.2
Corrected area of specimen, Ac:
________________________ in.2
Force, F
(lb)
Displacement, H
(in.)
Strain, 
Stress, 
(psi)
Shear Strength,  ________________ psi
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