Ground engineering · Tunnels & shafts
Tunnel face stability (Broms & Bennermark) calculator
Stability ratio N = (σv − σT)/su and the support pressure needed to keep N at a target.
Standard: Broms & Bennermark stability ratio
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Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Depth to axis | 15 | m |
| Unit weight | 18 | kN/m³ |
| Surface surcharge | 10 | kPa |
| Undrained strength | 50 | kPa |
| Face support pressure | 100 | kPa |
| Target N | 4 |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| σv at axis | 280.0 | kPa |
| N = (σv − σT)/su | 3.60 | |
| Support for target N | 80.00 | kPa |
- ✓ N ≤ 6 (stable)
- ✓ N ≤ target
Method, formulas and assumptions
- σv at axis = 18*15+10 = 280 kPa
- N = (σv − σT)/su = (280-100)/50 = 3.6
- Support for target N = MAX(0,280-4*50) = 80 kPa
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.