What is unconfined compressive strength test of soil?


The unconfined compression test is the most popular laboratory test used to determine the Unconfined Compressive Strength (UCS) of a rock specimen. The unconfined compression test method is used primarily for saturated, cohesive soils recovered from thin-walled sampling tubes. But the test is inappropriate for dry sands or crumbly clays because these materials would fall apart without some land of lateral confinement.

Unconfined Compressive Strength (UCS) stands for the maximum axial compressive strain that a specimen can bear under zero confining stress.


The unconfined compressive strength (σ) is the maximum load per unit average cross-sectional area at which the cylindrical specimen of soil falls in compression.


What is the typical loading rate for the unconfined compression test?
The typical load rate for the unconfined compression test should be 0.5 MPa/s to 1.0 MPa/s (in case of a stress-controlled load device) and the failure must occurs in approximately 10 minutes.

Unconfined Compressive Strength Test of Soil, UCS test

Unconfined Compressive Strength Test of Soil, UCS test, lS: 2720 (Part 10): 1991


Scope: To determine the Unconfined Compressive Strength (UCS) of Soil.


  • IS:  2720 (Part 10): 1991


Apparatus Required For Unconfined Compressive Strength Test of Soils:-
  • Unconfined compression apparatus, proving ring type
  • Proving Ring of 2 kN capacity and 1N accuracy
  • Deformation Dial Gauge with 0.01mm
  • Loading frame consists of two metal plates
  • Sample Extractor
  • Sampling tube
  • Vernier Callipers
  • Stop watch
  • Oven–thermostatically controlled with interior of non-corroding material to maintain the temperature between 105ºC & 115ºC.
  • Weighing balances sensitive to 0.01g
  • Split mould, 38 mm diameter, 76 mm long
  • Specimen trimming tools
  • Large mould
  • Water content cans

Unconfined Compressive Strength Test of Soil, UCS test

Procedure of Unconfined Compressive Strength Test of Soil:-


Preparation of specimens:-
*The moulded specimen for the test will have a minimum diameter of 38 mm. The height to diameter ratio will be within 2 to 2.5.

*Measurement of height and diameter will be made with Vernier Callipers or any other suitable measuring device to the nearest 0.1mm.

For Undisturbed Specimens: Sample are extracted from the tube with the extruder such that the degree of disturbance is negligible. The specimen will be of uniform circular cross-section with ends perpendicular to the axis of the specimen.

For Remoulded Specimens: The specimen is prepared from a disturbed soil sample. The material will be wrapped in a thin rubber membrane and thoroughly worked with the fingers to assure complete remoulding. Care will be taken to avoid entrapped air, to obtain a uniform density.

For Compacted Specimens: When compacting disturbed material, it will be done using a mould of circular cross-section with diameter of 36mm. After the specimen is formed, the ends will be trimmed perpendicular to the long axis and removed from the mould.

Unconfined Compressive Strength Test of Soil, UCS test

PROCEDURE:-
  1. The soil specimen will be placed at the desired water content and density in the large mould.
  2. The sampling tube will be pushed into the large mould.
  3. The sampling tube will be removed fulfilled with the soil specimen. For undisturbed samples, the sampling tube will be pushed into the clay sample.
  4. The soil sample will be saturated in the sampling tube by a suitable method.
  5. The split mould will be coated lightly with a thin layer of grease. The mould will be weighed.
  6. The sample will be extracted out of the sampling tube into the split mould, using the sample extractor and the knife.
  7. The two ends of the specimen will be trimmed in the split mould. The mould with the specimen will be weighed.
  8. The specimen will be removed from the split mould by splitting the mould into two parts.
  9. The initial length, diameter and weight of the soil specimen will be measured with Vernier Callipers and the soil specimen will be placed on the bottom plate of the loading device.
10. The upper plate is adjusted to make contact with the specimen.
11. The dial gauge is adjusted to zero and the proving ring gauge seated.
12. Compression load is applied so as to produce axial strain at a rate of 0.5 to 2 percent per minute causing failure with 5 to 10.
13. The dial gauge reading, and the proving ring reading will be recorded. The reading may be taken at strains of 0, 0.1, 0.2, 0.5, 1, 2, 3, 4, 5, 6, 8, 10, 12, 14, 16, 18 and 20 percent. The reading may be taken after every thirty seconds up to a strain of 6 percent and after every 60 seconds for a strain between 6% to 12% and after every 2 minutes or so beyond 12%.
14. The compression load (force) reading will be taken at interval of 0.5 mm of the deformation dial reading.
15. The test will be continued until failure surfaces have definitely developed or the stress-strain curve is well past its peak or until an axial strain of 20% is reached. 16. The stress-strain curve is plotted. The compressive stress is taken as ordinate, and axial strain as abscissa.
17. If possible, the angle between the failure surface and the horizontal is measured.
18. The sample from the failure zone of the specimen is taken and the water content of the specimen is determined.


Observation:-

Compressive stress-axial strain curve:-

Unconfined Compressive Strength Test of Soil, UCS test
Fig: Mohr’s Circle for Unconfined Compression Test


Calculation:-

a)
Unconfined Compressive Strength Test of Soil, UCS test

Where,
e = Axial strain,
∆L = the change within the specimen length as read from the strain dial indicator, and
Lo = the initial length of the specimen.


b)
Unconfined Compressive Strength Test of Soil, UCS test

Where,
A = Corrected area of the specimen and
Ao = the initial average cross-sectional area of the soil specimen.


c)
Unconfined Compressive Strength Test of Soil, UCS test

Where,
σ = Unconfined compressive strength,
P = the compressive force (axial load at failure), and
A = average cross-sectional area.


The undrained shear strength (Sᵤ) of the soil is equal to the one half of the unconfined compressive strength of the soil,
Unconfined Compressive Strength Test of Soil, UCS test

Report:-
The observations of the test will be suitably recorded and reported.


The Unconfined Compression Test is an important laboratory test. It is used to determine the unconfined and undrained compressive strength of a cohesive soil in an inexpensive manner. Unconfined Compressive Strength (UCS) stands for the maximum axial compressive stress that a cohesive soil specimen can bear under zero confining stress. Unconfined compression test is one of the fastest and cheapest methods of measuring shear strength of soil.