The University of Toledo
Soil Mechanics Laboratory
1
Unconfined Compression Test
Introduction
Soil, like other materials that engineers use, must be relied upon to satisfy particular
engineering purposes. Soil must have sufficient shear strength to resist shear stresses that
develop during loading. Soil also must have adequate stiffness so that deformations that result
from loading do not cause problems. Therefore, geotechnical engineers test soils to determine
such engineering properties as the drained or undrained shear strength and the stress-strain
behavior. Normally, undisturbed soil samples are obtained using subsurface exploration
equipment. Prepared specimens are then tested in the laboratory using unconfined compression,
triaxial compression or direct shear testing equipment. For this laboratory, samples of soil are to
be prepared by hand compaction using the Harvard miniature device and tested in the unconfined
compression test machine. The unconfined compression strength and soil stiffness are used in
design calculations to determine if the soil has adequate strength and stiffness to support
foundations, slopes and embankment fills.
Apparatus
1. Mixing tub
2. Harvard miniature mold and compactor
3. Spatula
4. Sample ejector
5. Calipers
6. Balance
7. Unconfined compression machine
8. Water content tare and oven
Procedure
1.
2.
3.
4.
5.
6.
7.
8.
1
Obtain enough soil passing the #4 sieve to fill the Harvard miniature mold.
Slowly add water to the soil and mix until the soil will not crumble when squeezed in
the hand but not so much water that the soil becomes sticky.
Compact the soil in Harvard miniature mold as uniformly as possible using 3 layers
with 25 blows per layer.
Trim the ends of the specimen perpendicular to the sides using a spatula.
Eject the soil from the mold, weigh the sample and measure the sample height and
diameter.
Place the sample in the unconfined compression machine and apply vertical loads with
the deformation rate setting at 0500 (0.05 in/min or 1.27 mm/min).
Obtain readings from the load ring gage and the deformation gage at every 20 divisions
on the deformation gage until the sample fails.
Sketch the failed specimen. Use the failed test specimen to obtain the water content.
ASTM D 2166 – 98a
Unconfined Compression - 1
Calculations
The calculations for stress and strain are completed using the equations and Table 1. The
applied stress is computed using the load ring calibration shown below and the cross-sectional
area A’, corrected for sample strain. The stress-strain curve is graphed using the figure below.
s =
P
A'
(1)
A' =
A
1- e
(2)
e =
DL
Lo
(3)
DL = (Deform. Gage Reading – Initial Reading) x 0.001 inch/Div
(4)
Load Ring Calibration
Load (lb) = 0.80 x Dial Divisions
Load (lbs.)
500
400
300
200
100
0
0
100
200
300
400
500
600
700
Dial Divisions
Results
Use Table 1 to compute stress and strain and Figure 1 to plot the stress-strain curve.
Conclusions
Determine the unconfined compression strength
Determine the undrained shear strength.
Would you consider the soil specimen tested soft, medium, stiff or hard?
Estimate a value for the modulus of elasticity for the soil.
Unconfined Compression - 2
Table 1 - Unconfined Compression Test
Unconfined Compression Test
Group __________ Date _________
Soil Description:
Sample Mass:
(g)
Sample Length:
(cm)
(in)
Sample Diameter:
(cm)
(in)
Sample Area:
(cm2)
(in2)
Load Ring Calibration:
0.80 (lb./division)
Deformation Gage Calibration:
0.001 (inch/division)
Deform.
Sample
Load
Gage
Length
Sample
Ring
Corrected
Reading
Change
Strain
Reading
Load
Area
Stress
2
(Div.)
(inch)
(%)
(Div.)
(lbf)
(in )
(lbf/ in2)
Mass of Tare + Wet Soil (g)
Mass of Tare + Dry Soil (g)
Mass of Tare
(g)
Mass of Dry Soil
(g)
Mass of Water
(g)
Water Content, w
(%)
Initial Soil Volume
(cm3)
Initial Wet Density,r (g/cm3)
Initial Dry Density,rd (g/cm3)
Sketch of Failed Specimen
Unconfined Compression - 3
Axial Stress (lbf/in2)
Axial Strain (%)
Figure 1 – Stress-strain for Unconfined Compression Test
Unconfined Compression - 4
Collar
Apparatus
Compactor
Sample
Ejector
Mold
Picture 1 – Harvard Miniature Apparatus
Load
Ring
and
Load
Gage
Deformation
Gage
Load Machine
Picture 2 – Unconfined Compression Machine
Unconfined Compression - 5