Enter the effective principal stresses at failure for each test stage (σ3 = confining stress, σ1 = major principal stress), all in the same stress unit. At least two stages with different confining stresses are required.
Demo:
Test Stages
| Stage | σ3 (kPa) | σ1 at failure (kPa) |
|---|
Results
Effective cohesion, c'
-
kPa
Friction angle, φ'
-
degrees
Regression fit, R²
-
Formulation
Failure envelope: \( \tau = c' + \sigma' \tan\phi' \).Regression on the stages: \( \sigma_1 = m\,\sigma_3 + b \), with \( m = \tan^2(45^\circ + \phi'/2) \) and \( b = 2c'\tan(45^\circ + \phi'/2) \).
Hence \( \sin\phi' = \dfrac{m-1}{m+1} \) and \( c' = \dfrac{b}{2\sqrt{m}} \) (negative values are clamped to zero).
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