Civil Engineering

1. Experiment No. 10 Date:
DETERMINATION OF UNCONFINED COMPRESSIVE STRENGTH OF SOIL
Object and scope:
To determine unconfined compressive strength of given soil in the laboratory using a cylindrical soil
specimen.
Reference:
IS: 2720 (Part 10) – 1973
Theory:
The maximum load that can be transmitted to the sub-soil by a foundation depends upon the
resistance of the underlying soil or rock shearing deformation or compressibility. Therefore, it is
important to investigate the factors that control the shearing strength of these materials. The shearing
is commonly investigated by means of compression test in which an axial load is applied to specimen
and increased until failure occurs. The use of compression test to investigate the shearing strength of
materials depends upon the fact that failure in such tests takes place by shear on one or more inclined
plains and this is possible to compute the normal pressure and the shearing stress on such a plane at
the instant of failure. The specimen may or may not be subjected to a lateral pressure during the test.
When lateral pressure is not applied the test is known is unconfined compression test.
Unconfined compressive strength ‘qu’ is the load per unit area at which an unconfined cylindrical
specimen of soil will fail in a simple compression test. If the unit axial compression force per unit
area has not reached the maximum value, the load per unit area at 20% axial strain shall be
considered the value of unconfined compressive strength ‘qu’.
The unconfined compression test is a special case of tri-axial compressive test in which the all round
pressure σ3 = 0. The test is carried out satisfactorily on the samples which can stand without any
lateral support. The test is an un-drained test and based on assumptions that there is no moisture loss
during test.
Specimen of height to diameter ratio 2:1 is normally used. The sample fails either by shear on
inclined plane or by bulging. The material stress at any stage is obtained by the vertical load divided
by cross sectional area.
The cross sectional area of the sample increases in compression. The cross sectional area (A) at any
stage of loading of sample may be computed on the basic assumption that the total volume of the
sample remains the same
Aoho = Ah
Where, Aoho = Initial cross sectional area and eight of sample.
Ah = c/s area and height off sample after compression.
The average vertical stress at any stage loading, σ = P/ Ac = P (1 – ε)/ Ao
Where, P = vertical load at the strain
Stress – Strain curve is plotted and peak value is taken as unconfined compressive strength (qu).
qu = P/ Ac
Where P = axial load at failure for ordinary soils.
If ϕ is the angle of shearing resistance and C is cohesion.
Then
Qu = 2 C tan (45 + ϕ/2)
Qu = 2 C
Shear strength, S = c + σ tan ϕ
As ϕ = 0
Then S = c = qu/ 2

2. Equipment:
1) Loading machine with facility to adjust rate of strain to desired value.
2) Sample ejector
3) Deformation dial gauge with 0.01 mm least count.
4) Vernier callipers suitable to measure dimensions of test specimen to the nearest 0.1 mm
5) Oven thermo-statically controlled, maintaining the temp at 110 °C ± 5 °C.
6) Proving ring to measure axial load applied.
Preparation of test specimen
1) Compacted specimen is prepared at optimum water content and maximum dry density. Tube
sampler is pushed into this compacted soil and then removed by removing the surrounding soil.
Then circular sample is ejected out using sample ejector.
2) After the specimen is formed the ends shall be trimmed perpendicular to the long axis.
3) The specimen has minimum diameter of 38 mm and the largest particle contained within the test
specimen shall be smaller than 1/8 of the specimen diameter. The height to diameter ratio shall
be 2.
Procedure:
1) The initial length, diameter and weight of the specimen shall be measure and the specimen place
on the bottom plate of the loading device. The upper plate shall be adjusted to make contact with
the specimen.
2) The deformation dial gauge shall be adjusted to zero. Force shall be applied so as to produce
axial strain at a rate of 0.5 to 2 percent per minute. Force and deformation reading shall be
recorded at a suitable interval.
3) The specimen shall be compressed until failure surface have definitely developed or the stress
stain curve is well past its peak or 20 percent of axial strain is reached.
4) The failure pattern shall be sketched carefully and shown on stress-strain curve. The angle
between failure surface and the horizontal is measured.
Observations:
Length of soil specimen, L =
Diameter of soil specimen, d =
Cross sectional area of soil specimen, Ao =
Volume of Soil Specimen =
L. C. of Dial gauge =
L. C. of Proving Ring =
Constant of Proving Ring =

3. Observation Table:
Sr.
No.
Dial gauge Proving ring Strain
(Ɛ)
Corrected
cross
sectional area
Ac = Ao/(1-Ɛ)
in mm2
Stress
σ = P/ Ac
in N/ mm2
reading
in
division
Deformation
(∆L) in
mm
reading
in
division
Load
in N
(P)
1 50
2 100
3 150
4 200
5 250
6 300
7 350
8 400
9 450
10 500
11 550
12 600
13 650
14 700
15 750
16 800
17 850
Calculations:
Axial strain, Ɛ = ∆L/ L0
Average cross sectional area, Ac = Ao/ (1- Ɛ)
Compressive stress, σ = P/A
In case of soils which behaves as if the angle of shearing the resistance ø = 0 the shear strength or
cohesion of the soil may be taken to be equal to half of unconfined compressive strength.
i.e. Shear Strength (S) = Cohesion (c) = qu/2
Graph Plotting:
A graph is plotted between stress (σ) and strain (Ɛ). The maximum stress from this plot gives the
value of the unconfined compressive strength. In case no maximum occurs within 20 percent axial
strength the unconfined compressive strength shall be taken as the stress at 20 percent axial strain.
Result:
Unconfined Compressive strength of Soil, qu =
Cohesion, C =
Angle of shearing resistance, ø =

4. Fig. Unconfined Compression strength test setup.