Triaxial Test Calculator

Calculate principal stresses, deviator stress, and corrected area. Estimate cohesion and friction angle from tests. Review results with plots, exports, explanations, and practical guidance.

Enter Triaxial Test Data

Use three tests for stronger cohesion and friction angle estimation. Required units are kPa, kN, and mm.

Corrected area is common in compression analysis. Initial area is useful for comparison or simplified review.

Test 1

Test 2

Test 3

Clear Form

Example Data Table

Use this sample set to test the calculator and visualize how the failure envelope changes with confining pressure.

Test Cell Pressure σ3 (kPa) Axial Load (kN) Diameter (mm) Height (mm) Displacement (mm) Pore Pressure u (kPa)
Test 1 100 2.70 38 76 8.0 18
Test 2 200 4.20 38 76 8.8 34
Test 3 300 5.90 38 76 9.3 49

Formula Used

1) Initial specimen area

A₀ = πd² / 4

2) Axial strain

εa = Δh / h₀

3) Corrected area

Ac = A₀ / (1 − εa)

4) Deviator stress

q = P / Ac

5) Major principal stress

σ1 = σ3 + q

6) Effective stresses

σ3′ = σ3 − u and σ1′ = σ1 − u

7) Mean effective stress

p′ = (σ1′ + 2σ3′) / 3

8) Mohr-Coulomb triaxial compression relationship

σ1′ = σ3′[(1 + sinφ) / (1 − sinφ)] + [2c cosφ / (1 − sinφ)]

The calculator fits a straight line to effective principal stresses from at least two valid tests, then derives cohesion c and friction angle φ.

How to Use This Calculator

  1. Enter up to three triaxial test rows using consistent units.
  2. Choose the area method. Corrected area is usually preferred for compression interpretation.
  3. Provide cell pressure, failure load, specimen size, displacement, and pore pressure.
  4. Click the calculate button to show the results above the form.
  5. Review q, σ1, effective stresses, p′, and undrained shear strength values.
  6. Use two or more valid rows to estimate cohesion, friction angle, and failure plane angle.
  7. Inspect the Plotly chart to visualize effective-stress Mohr circles and the fitted envelope.
  8. Export the current analysis as CSV or PDF for reports and documentation.

Frequently Asked Questions

1) What does this triaxial test calculator estimate?

It calculates axial strain, corrected area, deviator stress, total stresses, effective stresses, mean effective stress, undrained shear strength, and estimated Mohr-Coulomb strength parameters.

2) Why is pore pressure included?

Pore pressure allows conversion from total stress to effective stress. Effective stress is essential when estimating friction angle and cohesion for saturated soil behavior.

3) When should I use corrected area?

Use corrected area when axial deformation materially changes specimen geometry. It usually gives a better estimate of stress during compression than the initial area alone.

4) How many tests should I enter?

One complete row gives detailed stress results. Two or three valid rows are better when you want the calculator to estimate friction angle and cohesion from a fitted envelope.

5) What units does the calculator expect?

Enter cell pressure and pore pressure in kPa, axial load in kN, and specimen dimensions and displacement in mm. Keep all rows in the same unit system.

6) What does the Mohr circle graph show?

The graph plots effective-stress Mohr circles for each test. When enough rows are entered, it also draws the fitted failure envelope for quick interpretation.

7) Is undrained shear strength always equal to q / 2?

This calculator reports su as q / 2 for quick interpretation. That shortcut is commonly used for undrained compression, but lab method and specimen condition still matter.

8) Can I use this for final design decisions?

Use it for rapid checks, teaching, and report preparation. Final design should still rely on validated laboratory procedures, engineering judgment, and project-specific standards.

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Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.