Unconfined Compression Test – its 8 [Procedure Steps & Equipment]

Unconfined Compression Test

The unconfined compression test is used to determine the unconfined compressive strength of a rock specimen and an unconfined compression test is a laboratory test.

The value from which the unconsolidated shear strength of the clay under unconfined conditions is calculated is determined by the unconfined compressive strength and the unconfined compression test is used for the determination of this unconfined compressive strength.


Under zero confining stress, unconfirmed compressive strength stands for the maximum axial compressive stress specimen can bear and due to this stress is applied along the longitudinal axis.

In geotechnical design, unconfirmed compressive strength (UCS) is a parameter which is widely used but the strength in-situ may not be represented.

Equipment for the Unconfined Compression Test:

  1. Unconfined compression apparatus.
  2. 1 KN of capacity with 1N accuracy proving ring.
  3. Accuracy of 0.01 mm dial gauge.
  4. Weighing balance and oven.
  5. Stopwatch and sampling tube.
  6. 38mm in diameter and 76mm long spilt mould.
  7. Sample extractor and knife.
  8. Vernier Calipers and large mould.

equipment of unsonfined compression test

The Procedure of Unconfined Compression Test:

There are following steps in the procedure of unconfined compression test as given below:

  1. At the desired water content and density in the large mould place the sampling soil specimen and into the large mould push the sampling tube and remove the sampling tube filled with the soil and push the sampling tube into the clay sample for undisturbed samples.
  2. By a suitable method saturate the soil sample in the sampling tube and with a thin layer of grease coat the split mould lightly and weigh it.
  3. By using the sample extractor and the knife, extrude the sample out of the sampling tube into the split mould and in the split mould trim the two ends of the specimen. With the specimen weigh the mould.
  4. By splitting the mould into two parts remove the specimen from the split mould and with vernier callipers measure the diameter and the length of the specimen.
  5. On the bottom plate of the compression machine, place the specimen and to make contact with the specimen adjust the upper plate. Adjust the proving ring gauge to zero and also adjust the dial gauge.
  6. The to cause an axial strain at the rate of ½ to 2% per minute apply the compression load and to every thirty seconds up to a strain of 6 per cent, record the dial gauge reading, and the proving ring reading. For a strain between 6 per cent, and 12 per cent the reading may be taken every 60 seconds.
  7. Until an axial strain of 20% is reached or failure surfaces have clearly developed, continue the test and if possible measure the angle between the failure surface and the horizontal.
  8. For the water content determination, take the sample from the failure zone of the specimen.

procedure of soil strength

Data Sheet for Unconfined Compression Test:

L is the initial strength of the specimen.

D is the initial diameter of the specimen.

A is the initial area of the specimen.

V is the initial volume of the specimen.

M is the mass of the specimen.

Bulk density =

bulk density in unconfined compression test

G is the specific gravity of solids.

Pd is the dry density

e is the Void ration = Gpw/Pd – 1.

S is the degree of saturation = w G/e x 100.




Elapsed time

Dial gaugeProving ringStrainConcrete

Compressive strength





Test Results:

Qu is the unconfined compressive strength.

s is the shear strength = Qu/2

Advantages of Unconfined Compression Test:

There are the following advantages of the unconfined compression test such as;

  1. This test is very rapid.
  2. Inexpensive.
  3. In this test thin sample allows for rapid drainage of fine-grained soils.


  1. Failure plane forced to occur at a joint in the box.
  2. Only for drained conditions.
  3. Non-uniform distribution of strain and stress.

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