Unconfined Compressive Strength Test

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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 Compression apparatus, (loading frame) proving ring type.
  • Proving capacity 1 KN
  • Dial gauge accuracy 0.01 mm
  • Weighing balance
  • Oven
  • Stopwatch
  • Sampling Tube
  • Split mould (38 mm diameter, 76 mm long)
  • Sample extractor
  • Knife
  • Vernier Calliper
  • Large mould
  • Stopwatch
  • Grease

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.


  • To determine the shear strength of a soil triaxial shear test is conducted. Also, it’s quick and straightforward to perform.
  • The consistency of clay is often determined using the worth of unconfined compressive strength of soil.
  • This should be evaluated as unconfined compression test is inappropriate for dry sands or crumble clays because the materials would disintegrate without some land of lateral confinement.
  • Shear strength of a soil the foremost important engineering property. to settle on the simplest material for the embankment, one has got to conduct strength tests on the samples selected.
  • If it’s not evaluated then it’ll be needed to conduct the bearing capacity test within the field which isn’t always possible.
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


  • The soil specimen is prepared at the desired water content and density in the large mould.
  • The sampling tube into the large mould is pushed and the sampling tube which is filled with the soil is removed.
  • The soil sample in the sampling tube is saturated by a suitable method.
  • The split mould is lightly coated with a thin layer of grease. The mould is weighed.
  • The sample is extracted out of the sampling tube by a suitable method into the split mould, using the sample extractor and the knife.
  • The two ends of the specimen are trimmed off in the split mould.
  • The mould with the specimen is weighed.
  • The specimen is removed from the split mould by splitting the mould into two parts.
  • The length and diameter of the specimen is measured with Vernier callipers.
  • The specimen is placed on the bottom plate of the compression machine.
  • The upper plate is adjusted to make contact with the specimen.
  • The dial gauge and the proving ring gauge is adjusted to zero.
  • The compression load is applied to cause an axial strain at the rate of ½ to 2% per minute.
  • The dial gauge reading is recorded and the proving ring after every 60 seconds for a strain between 6% to 12% after every 2 minutes or so beyond 12%.
  • The test is continued until failure surfaces have clearly developed or until an axial strain of 20% is reached.
  • The angle between the failure surface and the horizontal is measured if possible.
  • The sample from the failure zone of the specimen for the water content determination is taken.

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.


  • The type of soil in this test depends on the purpose for which it is tested and may be compacted, remoulded or undisturbed. The specimen has minimum diameter of 38 mm. Compacted specimen could also be prepared at any predetermined water content and density.
  • Due to lack of stopwatch the readings taken may include some error. Moreover, the specimen was already prepared so nothing can be said much about the type of soil specimen.
  • Both the ends of the sample are shaped so that it should sit properly on the bottom plate of loading frame. Rate of loading of the sample should be constant. Readings should be taken with proper attention so as to have accurate results. Reading should be taken perpendicularly so as to remove parallax.
  • Unconfined Compressive Strength Test is a special type of Unconsolidated Undrained (UU) test that is commonly used for clay specimen. It is special in case of triaxial compression test.
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