Material¶
Defines the unit weight of the wedge and the shear strength criterion on the failure plane.
Common parameters¶
| Parameter | Symbol | Units | Notes |
|---|---|---|---|
| Block unit weight | γ | kN/m³ | rock density × g; typical 22–28 kN/m³ |
Strength criteria¶
Two criteria are available, selected via radio button.
Mohr-Coulomb (linear, default)¶
\[\tau = c + \sigma_n \cdot \tan\varphi\]
| Parameter | Symbol | Units | Typical |
|---|---|---|---|
| Cohesion on plane | c | kPa | 0–50 (often 0 for clean joints) |
| Peak friction angle | φ | ° | 25–45 |
This is the classical Hoek-Bray model. Use it for clean joints where dilation is negligible or for first-pass design.
Barton-Bandis (non-linear)¶
\[\tau = \sigma_n \cdot \tan\left(\varphi_b + JRC \cdot \log_{10}\!\frac{JCS}{\sigma_n}\right)\]
| Parameter | Symbol | Units | Notes |
|---|---|---|---|
| Basic friction angle | φb | ° | smooth rock: 25–35° (siliceous), 30–38° (carbonate). From tilt-test. |
| Joint Roughness Coefficient | JRC | – | 0–4 smooth, 4–8 wavy, 8–12 wavy-rough, 12–16 rough, 16–20 very rough. From Barton-Choubey 1977 charts. |
| Joint Compressive Strength | JCS | MPa | 50–200 MPa intact rock, 5–30 MPa weathered. From Schmidt hammer or ≈ UCS. |
The roughness dilation term ieff = JRC · log₁₀(JCS/σn) is added to φb to obtain the mobilised friction angle. At low normal stress (near the crest) ieff can be very large; it is clamped to 70° to avoid logarithm divergence.
The diagnostics panel under the section view shows σn, ieff, φb+ieff and the resulting τ on the plane.
When to use which¶
- Mohr-Coulomb: cohesion known from test, or rough preliminary analysis.
- Barton-Bandis: rough rock joints at low confining stress (typical in rock slopes), where dilation makes the joint stronger than its base friction suggests. More representative for real rock.