Failure Range and Overlays
How fit ranges, computed stress maxima, sample counts, and Mohr-Coulomb overlays work.
| Input | Description |
|---|---|
| Application | Chooses General, Tunnels, Slopes, or Custom for Generalized Hoek-Brown. |
| Stress Type | For Mohr-Coulomb, Power Curve, and Barton-Bandis, selects whether the entered range maximum is Sigma3 or SigmaN. |
| Unit Weight, gamma | Unit weight in kN/m^3 for tunnel and slope empirical range estimates. |
| Tunnel Depth / Slope Height | Excavation size H in metres for tunnel and slope empirical range estimates. |
| σ₃ₘₐₓ / SigmaN max | Selected stress maximum for Mohr-Coulomb, Power Curve, and Barton-Bandis. The other chart range is derived from the failure envelope. |
| Custom fit σ₃ₘₐₓ | Direct maximum for Generalized Hoek-Brown when Custom is selected. |
| Samples | Always beside Application (or Stress Type). Number of sampled points used to generate plotted curves; integer 20 to 600. |
| Computed fit σ₃ₘₐₓ | Read-only maximum after application logic is applied; shown for General, Tunnels and Slopes. Custom shows only the editable maximum. |
Range Logic
- General is available for Generalized Hoek-Brown and uses sigmaCi / 4.
- Tunnels and Slopes estimate σ₃ₘₐₓ from rock-mass compressive strength and gamma H.
- Custom uses the user-entered maximum directly.
- Mohr-Coulomb, Power Curve, and Barton-Bandis accept either a Sigma3 or SigmaN maximum and derive the corresponding range for the other chart.
Equivalent Mohr-Coulomb Overlay
The overlay is available for Generalized Hoek-Brown, Power Curve, and Barton-Bandis and adds a separate linear series to both charts. Hoek-Brown retains its closed-form range fit. Power Curve and Barton-Bandis use a bounded least-squares fit that minimizes vertical shear-strength error at uniformly spaced midpoint samples between zero normal stress and the computed fit maximum. Cohesion is constrained to be nonnegative and friction angle is constrained to 0-89 degrees.
For Power Curve and Barton-Bandis, custom cohesion and friction angle can be enabled independently. A custom parameter is fixed while every unchecked parameter remains optimized. Enabling an override captures its current fitted value; disabling it returns to automatic fitting. The two criteria retain separate overrides. Summary output includes the resulting shear RMSE. Overlay settings belong to the selected material, and every enabled visible-material overlay remains on the charts with a material-qualified legend label. Overlay Color affects only that material's extra series.
Instantaneous Mohr-Coulomb Overlay
The instantaneous overlay is available for Generalized Hoek-Brown, Power Curve, and Barton-Bandis. Enter a tangent normal stress within the displayed envelope. JLab evaluates the local shear strength and slope, computes the tangent cohesion and friction angle, and adds the corresponding linear envelope to both charts. A colored point marks the tangency location on the normal/shear chart. Enablement, tangent stress, and color are retained independently for each material.
φi = tan-1(dτ/dσn), ci = τ - σn tan(φi)