Datasheet No.10
Revised 2009, WKS
MOHAWK COLLEGE OF APPLIED ARTS AND TECHNOLOGY
BUILDING AND CONSTRUCTION SCIENCES DEPARTMENT
Unconfined Compression Test
REFERENCES:
Bowles, J. E. Engineering Properties of Soils and Their Measurement
INTRODUCTION
The unconfined compression test is still used as a means of rapidly evaluating the
shear strength of cohesive soils although it is gradually losing favour. The unconfined
compressive strength is the maximum load attained per unit area or the load at 15%
axial strain, whichever occurs first on a cylindrical sample tested in compression. The
shear strength is taken as equal to 1/2 the unconfined compressive strength. In
practice the test specimens are usually obtained by extruding thin-wall tube samples.
The length to diameter ratio of the specimen should be between 2 and 3.
Two types of loading devices are permitted, either strain controlled or stress controlled. In practice strain controlled loading is now almost universally used for this
test. In the strain controlled test the rate of loading is controlled so as to produce a
uniform axial strain. In the stress controlled test the rate of loading is controlled so
as to produce a uniform increase in axial load. In this laboratory, strain control will be
used and the compression machines will be adjusted to a rate of loading which will
provide 15% axial strain on the sample in approximately 10 minutes.
Sensitivity of a cohesive soil is defined as the undisturbed strength of the soil
divided by the remolded strength. The sensitivity cannot be less than 1. For most
local clays, the sensitivity is between 1 and 3. For some highly sensitive clays it can
exceed 10 and, in some cases, be so high that it cannot even be accurately measured.
The most accurate method of determining sensitivity involves use of the laboratory
vane shear apparatus.
Two devices used to check the unconfined strength or shear strength in the field are
the Torvane and the Pocket Penetrometrer. You will use both and compare the results
with that obtained from the unconfined test. The results of this lab will also be
compared to the results of a triaxial compression test performed in a later lab. It is
therefore necessary to retain a copy of all the results from this lab.
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APPARATUS:
Compression testing machine with proving ring and dial gauge to measure deformation
(0.01 m x 25 mm).
Vernier caliper for measuring sample.
Miscellaneous apparatus: Spatulas, wire saws, balances, moisture tins, etc.
PROCEDURE:
1. The loading device should have been adjusted so that the loading rate will be
approximately that which will produce 15% strain in 10 minutes. (Check with the
instructor that this has been done).
2. Readings of the load and deformation should be taken at increments of 0.5 mm
strain.
3. If a wax coating is on the specimens, carefully remove it. Using a mitre box trim
the ends square and to a length of between 100 and 104 mm; determine and record on
DS 10 the exact length and the average diameter (to 0.1 mm) of the specimen and its
mass to 0.1 g.
Davg = (Dt + 2Dm + Db)/4
where:
Davg = Average diameter of the specimen
Dt = Average diameter at the top of the specimen
Dm = Average diameter at the middle of the specimen
Db = Average diameter at the bottom of the specimen
4. Based on the specimen length, determine the strain dial reading that would
correspond to 15% strain and highlight on DS 10.
5. Insert the specimen in the machine and bring the top platen in contact with the top
of the specimen using the manual feed. Zero the deformation and load dials and
commence the test. Using motor operation, take readings of the load and deformation
dial readings at the specified intervals (0.5 mm).
6. Continue to take dial readings until the loads fall off in two successive readings or
two readings beyond the 15% strain value.
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7. Upon completion cut the specimen in two and carry out:
a) a Torvane test in each new face of the specimen and record the average shear
strength results in kg/cm2.
b) several Pocket Penetrometer tests in the faces and record the average unconfined
compressive strength equivalents in kg/cm2. (Note: the unconfined equivalents will be
approximately double the shear readings).
8. Weigh the specimen again for a percent moisture determination, recording masses
to 0.1 g on DS 10.
CALCULATIONS AND REPORT:
1. Calculate the axial strain, εa for each reading as follows:
εa 
∆L
L0
where:
∆L = change in length of the specimen read from the dial gauge
L0 = initial length of the test specimen
2. Calculate the average cross-sectional area, A', for each reading as follows:
A'
A0
1 - εa 
where:
A0 = the original average area of the specimen in mm2
3. Calculate the load per unit area (unconfined compressive stress), qu for each
reading as follows:
qu 
P
 1000 in kPa
A'
where:
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P = applied axial load in N
4. Prepare a graph showing stress (y-axis) vs. percent strain (x-axis) for the test. The
unconfined compressive strength, quf is determined as the maximum value of load per
unit area in kPa if before 15% strain or otherwise that at 15% strain.
5. The shear strength is equal to half the unconfined compressive strength:
quf
2
where:
τf 
f = shear strength
quf = unconfined compressive strength
6. Calculate the results of the Torvane rests and the Pocket Penetrometer readings in
kPa (1 kg/cm2 = 98.1 kPa). Compare the shear strength determined from the
"Torvane" test with that obtained by halving both the "Pocket Penetrometer"
readings and the maximum unconfined compression test result. NOTE: The maximum
possible Torvane reading is 1 kg/cm2!
7. Using the moisture content found, the pre-test mass of the specimen and assuming
a specific gravity of 2.750, estimate the degree of saturation of the original
specimen. If this assumption appears to produce a negative volume of air in the
specimen then report the specific gravity value obtained by assuming saturation.
Report this together with the moisture content and density in kg/m3.
8. Include labelled sketches of the "Pocket Penetrometer" and the "Torvane" showing
the scales and units of measurement as well as any pertinent dimensions.
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