Unconfined compressive strength and cohesion of clay (IS 2720 Part 10).
Water ContentSpecific GravitySieve Analysis (Soil)Free Swell IndexLiquid & Plastic LimitPermeability (Falling Head)Permeability (Constant Head)
Unconfined Compressive Strength (UCS) Calculator: Free Online Tool
The unconfined compressive strength (UCS) test is a quick way to measure the strength of cohesive soil. A cylindrical clay specimen is loaded axially with no confining pressure until it fails, and the peak load is recorded. The unconfined compressive strength, qu, is the peak load divided by the cross-sectional area of the specimen at failure.
Soil laboratory technicians, geotechnical engineers and students use qu to describe the consistency of clay and to estimate its undrained cohesion, which is half of qu. These values feed into short-term bearing capacity checks, slope stability and foundation design on clay. This calculator follows IS 2720 Part 10. It works out the axial strain, corrected area, load, qu in kPa and kgf/cm2, cohesion and the consistency of the clay from your dial reading.
When to use this calculator
Use this calculator after crushing a clay specimen in a compression frame with a proving ring. It suits undisturbed tube samples and remoulded specimens of saturated cohesive soil, but not sand or fissured soil. Enter the specimen diameter and length in mm, the axial deformation at failure, the proving ring constant and the dial reading at peak load. The defaults are a 38 mm by 76 mm specimen, 4.5 mm deformation, a ring constant of 2 N per division and 65 divisions. It is useful for lab reports and quick consistency checks.
How to use the Unconfined Compressive Strength (UCS) Calculator
- Open the calculator above on your phone, tablet or computer. Nothing needs to be installed and no sign-up is required.
- Enter your values in the input boxes: Specimen diameter (mm), Specimen length (mm), Axial deformation at failure (mm), Proving ring constant (N per division), Dial reading at peak load (divisions). The boxes already hold sample values, so overwrite them with your own.
- Press Calculate. The result appears straight away under the button.
- Change any value and press Calculate again to compare options. Press Reset to go back to the sample values.
- Check the result against your drawings, project specification and local code before you order material or approve work.
How the calculation works
The calculator first finds the axial strain as deformation divided by specimen length, and the initial area from the diameter. The corrected area is the initial area divided by (1 - axial strain), which allows for the specimen bulging as it is compressed. The load is the proving ring constant multiplied by the dial reading. If you already know the load in newtons, enter 1 as the ring constant. Then qu = peak load / corrected area, and cu = qu / 2. The result is shown in kPa and kgf/cm2. The consistency uses these limits: very soft below 25 kPa, soft 25-50, medium 50-100, stiff 100-200, very stiff 200-400 and hard above 400 kPa.
Worked example
When you open the page the calculator is filled with sample values. Pressing Calculate with those values gives the result below, which you can use to check that you are reading the output correctly.
| Specimen | |
|---|---|
| Axial strain | 5.92 % |
| Initial area | 11.34 cm² |
| Corrected area | 12.05 cm² |
| Result | |
| Peak load | 130.0 N |
| Unconfined compressive strength, qu (kPa) | 107.8 |
| qu (kgf/cm²) | 1.100 |
| Undrained cohesion, cu = qu / 2 (kPa) | 53.9 |
| Consistency of clay | Stiff |
Tips and common mistakes
- Enter the deformation at failure in mm, not as a dial count. Convert the dial reading using the least count of your deformation gauge first.
- Check the proving ring constant on the calibration certificate. A wrong constant changes the load, and qu, by the same percentage.
- If you already have the load in newtons, enter 1 as the ring constant and the load as the dial reading to avoid a conversion error.
- Use a specimen with a length-to-diameter ratio near 2, as in the default 76 mm by 38 mm, and trim the ends flat and parallel.
- Do not use the test for sands, dry crumbly or fissured clays, where the soil cannot stand unsupported without confinement.
Frequently asked questions
What is the difference between qu and cu?
The unconfined compressive strength qu is the peak axial stress at failure. The undrained cohesion cu is half of qu. In the default example qu is 107.8 kPa and cu is 53.9 kPa.
Why is the area corrected?
As the specimen is compressed it bulges and its area increases. The calculator divides the initial area by (1 - axial strain) so that the stress reflects the area at the time of failure.
How do I use the proving ring constant?
Load equals the ring constant multiplied by the dial reading at peak load. With 2 N per division and 65 divisions the load is 130 N. If the load is already known, enter 1 as the constant.
How is the consistency of the clay decided?
It is read from qu. Below 25 kPa the clay is very soft, 25-50 soft, 50-100 medium, 100-200 stiff, 200-400 very stiff and above 400 kPa hard. The default 107.8 kPa gives stiff clay.
Can the test be used for any soil?
No. It is meant for cohesive soil that can stand without lateral support, such as saturated clay. Granular and fissured soils fail at very low or uneven loads without confinement and give misleading values.
Important note
This tool gives estimates for planning, study and quick checks. It does not replace design by a qualified engineer, site-specific testing, or the requirements of your project specification and local building code. Always confirm final quantities, test results and acceptance limits with the responsible engineer before using them for construction or payment.
