Unconfined Compression Test【IS 2720(Part 10):1991 PDF】

The maximal axial compressive stress that a specimen can withstand under no confining stress is known as unconfined compressive strength. The Unconfined Compression Test is a laboratory procedure for determining the Unconfined Compressive Strength of a rock specimen, clayey soil, and other materials.

Laboratory Procedure/ Method

1. Unconfined Compression Test is a variant of an undrained unconsolidated triaxial compression test with a confining cell pressure equal to atmospheric pressure.
2. It’s useful for assessing the shear strength (S) of saturated cohesive soils (Φ = 0) that hasn’t been drained. Since σ3 = 0 (atmospheric pressure), the mohr circle passes through the origin, and cohesion (C) equals one half of the axial stress (σ1) at failure, the failure envelope remains horizontal in this case.
3. The research is carried out on a cylindrical sample with a 2: 1 ratio in height and diameter.
4. Without any lateral assistance, the sample is mounted between the plates of mechanical load frames.
5. Axial load is used to produce a strain rate of about 2% of sample height per minute.
6. Readings of load and deformation are taken.
7. Loading is continued until the load dial gauge shows a declining or constant load for three or more consecutive readings.
8. The average time to failure is between 10 and 15 minutes.
9. On the basis of the deformed field, the axial stress is determined.
10. Unconfined compressive strength refers to the peak axial stress (qu).

Unconfined Compression Test Apparatus

Any of the following compression devices can be used:
a) A Platfom weighing scale with a yoke that is activated by a screw-jack.
b) Loading unit with hydraulics
c) A proving ring on a screw jack
d) Some other means of loading

All of these loading devices must have enough capacity and strain control to be effective.

 Also Read : Triaxial Shear Test for Shear Strength of Soil
Also Read : Direct Shear Test for Shear Strength of Soil
Also Read : Vane Shear Test [ IS 2720 (Part XXX) – 1980 ]

Observation Table :

The unconfined compressive strength is determined by the following data and observation table :

• Initial Length (Lo) =
• Initial Area (Ao) =
• Initial Density =
• Initial Diameter of Specimen (Do) =
• Initial Water Content =

Sr. No	Elapsed Time (min)	Load (P)	Deforma-tion (Min)	Strain ε (%)	Corrected Area ‘A’ (mm)	Stress

 

Calculations

Compressive Strength from the corrected area ‘A’ is determined using the formula,

Where,
P = compressive force

Between σ and ε, a plot is developed. The values of the unconfined compressive strength ‘qu’ are determined by the maximum stress from the curve. When there is no maximum stress, the ‘qu’ is calculated as the stress at 20% axial pressure.

For the Φ=0 state, the soil’s shear strength or cohesion can be assumed to be half of its unconfined compressive strength.

Undrained Shear Strength of Clays for Various Consistencies :

The table below shows the undrained shear power of clays in different consistencies :

Consist-enct	Very Stiff	Stiff	Firm to Stiff	Firm	Soft to Firm	Soft	Very Soft
Undrained Strength kN/m²	≥ 150	100 – 150	75 – 100	50 – 75	40 – 50	20 – 40	≤ 20

 

To determine the remoulded strength, the failed sample is placed in a small polythene bag with a small amount of soil at the same water content and squeezed and knealed thoroughly. The remoulded soil is placed in a 38 mm diameter tube and then extracted with sample extraction to create a test specimen. The remoulded strength is then finally determined.

Characteristics

• The test is simple and quick to complete.
• Ideally unconsolidated undrained shear strength of saturated clays is determined by this method
• Conducting the test on an undisturbed sample and then on the remoulded sample is a simple way to assess the soil’s sensitivity.
• Fissured clays are ineligible for the test.
• For soils with a non-zero angle of shearing resistance, the test may be misleading.
• The shear strength of such soils is not equal to half of the compressive strength.

IS 2720 ( Part 10 ) : 1991 – Determination of Unconfined Compressive Strength

In this IS code detailed procedure of the calculations along with the apparatus used for performing laboratory unconfined compression test on clayey soil, undisturbed, remoulded or compacted, using controlled rate of strain is given.

You could refer the below IS code for better understanding of the practical procedure.

IS 2720 ( Part 10 ) : 1991 PDF download

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