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Unconfined Compressive Strength Test

To determine the unconfined compressive strength of cohesive soil in the laboratory.

• IS 2720 (Part-10)-1973 Methods of test for soils: Part 10 – Determination of unconfined compressive strength.
• IS 2720 (Part-2)-1973 Methods of test for soils: (Part 1) – Determination of water content.


Unconfined Compressive Strength Test Apparatus
Fig 1: Unconfined Compressive Strength Test Apparatus

The soil specimen to be used for test shall be depend on the purpose for which it is tested and may be compacted, moulded or undisturbed.

The specimen for test shall have a minimum diameter of 38 mm and the largest particle contained within. The test specimen shall be smaller than 1/8 th of the specimen diameter. If after completion of test on undisturbed sample, it is found that larger particles than permitted for the particular specimen size tested are present, it shall be noted in the report of test data under remarks. The height to diameter ratio shall be within 2 to 2.5. Measurements of height and diameter shall be made with Vernier callipers or any other suitable measuring device to the nearest 0.1 mm.

Undisturbed specimens shall be prepared from large undisturbed samples or samples secured in accordance with IS 2132: 1986.

Undisturbed sample shall be prepared from the drive sampling tube. The ejecting device shall be capable of ejecting the soil core from the sampling tube within the same direction of travel during which the sample entered the tube and with negligible disturbance of the sample. Conditions at the time of removal of the sample may dictate the direction of removal but the principle concern should be to stay the degree disturbance negligible.

The specimen shall be handled carefully to stop disturbance, change in cross section or loss of water.

The specimen shall be of uniform circular cross section with ends perpendicular to the axis of the specimen.

Specimen of required size could also be carved from large undisturbed specimens.

Where the prevention of the possible development of applicable capillary forces is required the specimens shall be sealed with rubber membranes, thin plastic coatings or with coating of grease or sprayed plastic immediately after plastic immediately after preparation and through the whole testing cycle.

Representative sample cuttings taken from the tested specimen shall be used for determination of water content.

The specimen could also be prepared either from a failed undisturbed specimen or from a disturbed soil sample. just in case of failed undisturbed specimen, the fabric shall be wrapped during a thin rubber membrane and thoroughly worked with the fingers to assume complete remoulding. Care shall be taken to avoid entrapped air, to get uniform density, to remould to an equivalent void ratio as that of undisturbed specimen and to preserve the natural water content of the soil.

When compacting disturbed material, it shall be done employing a mould of circular cross section. Compacted specimen could also be prepared at any predetermined water content and density.

After the specimen is made, the ends shall be trimmed perpendicular to the long axis and faraway from the mould. Representative sample cuttings shall be obtained or the whole specimen shall be used for the determination of water content after the test.

The load per unit area at which a cylindrical specimen of a cohesive soil fails in compression is called UCS (qu).

qu = P/A
where P = axial load at failure

A = Corrected area = A0/ (1-Ε),
where A0 is the initial cross-sectional area of the specimen,
Ε = axial strain = (Change in length)/ (Original Length)

The undrained shear strength (Su) of the soil is equal to one half of the UCS i.e. Su = qu / 2.


Sl No. Consistency of Clay Unconfined Compressive Strength (KN/m2)
1 Very Soft < = 25
2 Soft 25 – 50
3 Medium 50 – 100
4 Stiff 100 – 200
5 Very Stiff 200 – 400
6 Hard > = 400


Details of the specimen:
Details of the specimen:
I. Initial D0 (mm) =
II. Initial length, L0 (mm) =
III. Initial area, A0 (cm2) =
IV. Initial volume, V0 (cm3) =
V. Initial mass of the specimen (g) =
VI. Initial density of the specimen (g/cm3) =
VII. Proving ring (stress gauge) constant =
VIII. Dial Gauge (strain gauge) constant =

Deformation Dial Reading Axial deformation (mm) Axial Strain, Ε Area (cm2), A= A0/ (1-Ε) Proving ring dial reading Axial force (N) Compressive Stress (KPa) Remarks

The maximum unconfined compressive strength of soil sample is _____________.

A graph is drawn between compressive stress as ordinate and strain as abscissa. The maximum stress from the curve gives the worth of unconfined compressive strength, qu. If no maximum value of stress is out there, the strain at 20% strain is taken as unconfined compressive strength. This test provides an immediate value of the compressive strength of soil in the remoulded condition, it is carried out within a short time to ensure that no drainage of water is permitted into or out of the specimen. In very plastic soils the axial stress does not readily reach a maximum value. The Mohr circle can be drawn from stress conditions at failure. As the minor principle stress is zero, the Mohr circle passes through origin. The failure envelop is horizontal. The cohesion intercept is equal to the radius of circle.


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Kanwarjot Singh

Kanwarjot Singh is the founder of Civil Engineering Portal, a leading civil engineering website which has been awarded as the best online publication by CIDC. He did his BE civil from Thapar University, Patiala and has been working on this website with his team of Civil Engineers.

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