Unconfined Compression Test

Summary:
The primary purpose of this test is to determine the unconfined

compressive strength, which is then used to calculate the
unconsolidated undrained shear strength of the clay under unconfined
conditions. According to the ASTM standard, the unconfined
compressive strength (Cu) is defined as the compressive stress at which
an unconfined cylindrical specimen of soil will fail in a simple
compression test. In addition, in this test method, the unconfined
compressive strength is taken as the maximum load attained per unit
area, The unconfined compression test is to determine the unconfined
shear strength, qu. Undrained shear strength is interpreted as the
maximum internal resistance of a soil to the applied shear force when it
is sheared at constant volume. The test is done by subjecting rapid
compressive loading on a cylindrical soil specimen so that no drainage
takes place during the shear. Take note there is only vertical load
applied on the soil sample which is the major principal stress (1) and
no radial stress, (3= zero). Precaution, the unconfined compression
test is inappropriate for dry sands or crumbly clays as the materials
would fall apart without some land of lateral confinement. From the
Mohr circle, shear strength
Cu=qu/2
Where: C = Cohesion. The results obtained used for various purposes
especially for foundation design and earth retaining wall design.
Undrained shear strength is used to estimate short-term bearing
capacity of fine-grained soils for foundations and estimate the shortterm stability of slopes.
Objective:
To determine the undrained shear strength (Cu) of find-grained soil.
Test apparatus:
Unconfined compression testing device.
Specimen trimmer and accessories (if undisturbed field specimen
is used).
Harvard miniature compaction device and accessories (if a
specimen is to be molded for classroom work).
Balance.
Oven.
Porcelain evaporating dish.
Data and Results (Calculations):

L =7.6 cm =76mm
D =3.7 cm =37mm
A (mm2)
1075.2
1075.2
1075.2
1075.2
1075.2
1075.2
1075.2
1075.2
1075.2
1075.2
1075.2
1075.2
1075.2
L (mm)
76
76
76
76
76
76
76
76
76
76
76
76
76
L
10
20
30
40
50
60
70
80
90
100
110
120
130
load
1.6
3.8
6.4
9
11.8
14.2
16.6
L(0.01)mm
0.1
0.2
0.3
0.4
0.5
0.6
0.7
18.4
20
21
21.2
20.8
20
0.8
0.9
1
1.1
1.2
1.3
load*(10.2) KN
16.32
38.76
65.28
91.8
120.36
144.84
169.32
187.68
204
214.2
216.24
212.16
204
0.0013
0.0026
0.0039
0.0053
0.0066
0.0079
0.0092
0.0105
0.0118
0.0132
0.0145
0.0158
0.0171
stress and strain

0.2500
0.2000
0.1500
stress and strain
0.1000
0.0500
0.0000
0.0000
0.0050
0.0100
0.0150
0.0200
0.0152
0.0360
0.0607
0.0854
0.1119
0.1347
0.1575
Ac=A/1-
1075.20
1075.20
1075.20
1075.19
1075.19
1075.19
1075.19
0.1746
0.1897
0.1992
0.2011
0.1973
0.1897
1075.19
1075.19
1075.19
1075.19
1075.18
1075.18
qu= 0.2 & Cu= qu/2 = 0.2/2= 0.1
Discussion:
From the laboratory testing investigation on unconfined compressive
strength of soil, the following concluding remarks are drawn. The
unconfined compression test is a type of unconsolidated undrained test
that is commonly used for clay specimens. In this test, the confining
pressure is 0. An axial load is rapidly applied to the specimen to cause
failure. At failure, the total load minor principal stress is zero.
Saturated plastic clays have undrained shear strength greater than
zero. Undrained shear strength can be related to the consistency of soil.
The consistency identifies the kind and degree to which the soil
particles unify and bond in relation to rupture or deformation. It falls
under the classification of soft consistency. With soft consistency the
soil can be easily molded with a light pressure exerted on it. The stress
rapidly increases until it reaches the peak point then gradually
decreases down. The strain softens as it reaches critical shear stress.
In the unconfined test, no radial stress is applied to the sample (3=0).
The plunger load, P is increased rapidly until the soil sample fail, that is
cannot support any additional load. The loading is applied quickly so
that the pore water cannot drain from the soil. The effect stress path is
unknown since pore water pressure changes are not normally
measured. This test is considered as undrained shear test assuming that
there is no moisture loss from the specimen during the test. The
specimen must not certain any fissures, silt seams, verves, or other
defects, this mean that the specimen must be intact, homogenous clay.
Rarely are over-consolidated clays intact, and often even normally
consolidated clays have some fissures.
Since the main limitation for this experiment is the equipments used
execute the experiment a bit faster because other groups will be
waiting. One problem encountered during the experiment was that
there was almost not enough soil to fit in the mold. Make sure that
there is enough cohesive soil before pushing the mold in the soil
otherwise the mold will not be filled. The soil is sticky and can easily be
deform so be careful upon removing the mold. It might get deformed
before testing.
Conclusion:
Unlike the soil sample use in this experiments, in the construction
industry soil used for projects are adequately stiff to minimize the
effect of deformation. The soil must have enough shear strength to
resist the stress induced by loadings. Through this experiment, shear
strength of a soil is determined and examined to know whether the soil
available can be used for construction engineering purposes. This is an
important factor to consider since the soils are the base of horizontal
and vertical structures made. Data gathered from this test is used for
designing foundations and embankment fills.
Based on the value of qu the consistency of a cohesive soil is as follows:
consistency
Very soft
soft
medium
stiff
Very stiff
qu (Ib/ft2)
0-500
500-1000
1000-2000
2000-4000
4000-8000
For many naturally deposited clayey soils the unconfined compression

strength is greatly reduced when the soil is tested after remolding
without any change of moisture content. This is referred as sensitivity
and can be defined as:
S= qu(undisturbed)/qu(remolded)
The sensitivity of most clays ranges from 1-8.

Unconfined Compression Test

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What is the unconfined compression test used to determine?

What is the relationship between unconfined compressive strength and undrained shear strength?

Summary:

The primary purpose of this test is to determine the unconfined

Data and Results (Calculations):

stress and strain

qu= 0.2 & Cu= qu/2 = 0.2/2= 0.1

For many naturally deposited clayey soils the unconfined compression

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