Calculator Inputs
Formula Used
| Item | Formula |
|---|---|
| Initial area | A0 = πd² / 4 |
| Corrected area | Ac = A0 / (1 - εa) |
| Deviator stress | q = P / Ac - membrane correction |
| Major total stress | σ1 = σ3 + q |
| Effective stresses | σ1′ = σ1 - u, and σ3′ = σ3 - u |
| Mean effective stress | p′ = (σ1′ + 2σ3′) / 3 |
| Maximum shear stress | t = (σ1 - σ3) / 2 = q / 2 |
| Skempton failure parameter | Af = Δu / q |
| Effective failure line | q = M p′ + b |
| Friction angle estimate | sin φ′ = 3M / (6 + M) |
| Cohesion estimate | c′ = b(3 - sin φ′) / (6 cos φ′) |
Example Data Table
| Specimen | σ3 kPa | u kPa | q kPa | p′ kPa | Use |
|---|---|---|---|---|---|
| CU-01 | 150 | 80 | 260 | 156.67 | Low confining pressure |
| CU-02 | 300 | 150 | 430 | 293.33 | Middle confining pressure |
| CU-03 | 450 | 215 | 590 | 431.67 | High confining pressure |
How to Use This Calculator
- Enter the specimen diameter and height before shearing.
- Add wet mass, dry mass, and specific gravity when density checks are required.
- Enter cell pressure, initial pore pressure, and pore pressure at failure.
- Add axial load and axial strain at the selected failure point.
- Use membrane correction if your lab sheet provides one.
- Add companion specimens to estimate effective strength parameters.
- Press Calculate to show results above the form.
- Use CSV or PDF download buttons to save the report.
About the Consolidated Undrained Triaxial Test
A consolidated undrained triaxial test studies soil strength after drainage during consolidation and no drainage during shear. The specimen is first saturated, then consolidated under a selected cell pressure. During shearing, water cannot leave. Pore pressure is measured while axial load rises. This makes the test valuable for clays, silts, and saturated fills.
What This Calculator Measures
This calculator converts laboratory readings into stresses used in design review. It corrects the specimen area for axial strain. It calculates deviator stress, major total stress, effective stresses, mean stress, stress ratio, and Skempton A at failure. It also estimates water content and density indicators when mass data is provided. The output helps compare specimens tested under different confining pressures.
Why Effective Stress Matters
CU testing is useful because pore pressure changes during undrained loading. Total stress alone may hide that response. Effective stress subtracts pore pressure from total stress. That value better represents the contact force carried by soil particles. Engineers use effective paths to judge stability after rapid loading, embankment construction, excavation, and foundation changes.
Statistical Review
The tool can include companion specimens. When at least two points are entered, it fits a simple p prime versus q line. The slope gives an estimated friction angle. The intercept gives an estimated effective cohesion. These values should be treated as screening results. Final design should use verified laboratory procedures, specimen notes, saturation checks, and professional judgment.
Practical Use
Good input data improves every result. Confirm dimensions before consolidation. Use corrected load readings. Record pore pressure at the same failure point used for axial load. Check whether failure was defined by peak deviator stress, strain limit, or critical state behavior. Save the CSV file for spreadsheets. Use the PDF report for lab folders. Recalculate when any reading changes.
Important Limits
The calculator supports review and teaching. It does not replace a certified test report. Real CU interpretation may need membrane correction, area correction method selection, saturation checks, anisotropic consolidation details, filter paper notes, rate review, and equipment calibration. Use consistent units. Compare results with field history. Keep raw readings beside every computed number for traceable decisions later. Document assumptions, because small choices can shift reported strength values.
FAQs
What is a consolidated undrained triaxial test?
It is a soil strength test. The sample drains during consolidation. Then it is sheared without drainage. Pore pressure is measured during loading.
Why is pore pressure important?
Pore pressure changes effective stress. Effective stress controls soil particle contact. That is why CU results often include total and effective stress paths.
Can one specimen give c′ and φ′?
One specimen cannot reliably define a strength envelope. Use two or more valid points. Three or more specimens usually give a better trend.
What does q mean?
q is deviator stress. It equals the major principal stress minus the minor principal stress. In this tool, q is corrected for specimen area.
What is Skempton A at failure?
It is the excess pore pressure divided by deviator stress at failure. It helps describe undrained pore pressure response during shearing.
Should membrane correction always be used?
Use it when your laboratory procedure provides a correction. The effect can matter for softer specimens and small samples.
Why does the corrected area increase?
Undrained shearing is often treated as constant volume. As axial strain shortens the specimen, the cross sectional area increases.
Can this replace a lab report?
No. It supports review and learning. Final results should follow laboratory standards, calibration records, saturation checks, and professional interpretation.