Unconfined Compressive Strength of Cohesive Soil


The unconfined compressive strength ( clip_image001 ) basically refers to the load per unit area at which the cylindrical sample of a cohesive soil falls in compaction.


Where P= axial load at failure,

A= corrected area =  clip_image003 ,

where  clip_image004  is the initial area of the specimen,

clip_image005 = axial strain = change in length/initial length.

The undrained shear strength (s) of the soil is equivalent to the one half of the unconfined compressive strength,


Tools and equipment:

1. Unconfined compression apparatus, proving ring type

2. Proving ring, capacity 1 kN, accuracy 1N.

3. Dial gauge, accuracy 0.01 mm.

4. Weighing balance

5. Knife

6. Large mould

7. Oven

8. Stop watch

9. Sampling tube

10. Split mould, 38mm diameter, 76mm long.

11. Sample extractor


1. Put the sampling soil sample at the coveted water proportion and thickness in the extensive mould.

2. Push the testing tube into the vast mould and evacuate the inspecting sampling loaded with the soil. For undisturbed specimens, push the examining tube into the mud sample first.

3. Soak the soil specimen in the examining tube by a reasonable strategy.

4. Coat the split mould daintily with a thin layer of oil. Measure the mould.

5. Expel the sample out of the examining tube into the split form, utilizing the specimen extractor and the blade.

6. Trim the two closures of the sample in the split shape. Measure the mould with the sample.

7. Expel the sample from the split mould by part the form into two sections.

8. Measure the length and width of the example with vernier calipers.

9. Put the sample on the base plate of the pressure machine. Alter the upper plate to reach the sample.

10. Alter the dial gauge and the proving ring gauge to zero.

11. Apply the pressure load to make a pivotal strain at the rate of ½ 2% every moment.

12. Record the dial gauge reading, and the demonstrating ring reading at regular intervals up-to a strain of 6%. The reading might be taken after like clockwork for a strain between 6%, 12% and each 2minutes or so past 12%.

13. Proceed with the test until failure surfaces have plainly created or until a pivotal strain of 20% is come to.

14. Measure the edge between the failure surface and the even, if conceivable.

15. Take the example from the failure zone of the sample for the water content assurance.


Data Sheet for Unconfined Compression Test

Initial length of the specimen  clip_image001[4] =

Initial diameter of the specimen,  clip_image002[4] =

Initial area of the specimen,  clip_image003[4] =

Initial volume of the specimen,  clip_image004[4] =

Mass of empty split mould =

Mass of split mould + specimen =

Mass of the specimen M =

Bulk density,  clip_image005[4] =

Water content =

Dry density  clip_image006[4] =

Specific gravity of solids, G=

Void ratio,  clip_image007

Degree of saturation,  clip_image008


Plot a curve between the compressive stress as ordinate, and axial strain as abscissa.



From the plot, unconfined compressive strength,  clip_image012 =

Shear strength,  clip_image013  =

Share it on: