Example data table
| Scenario | Shape | Key dimensions | Length | Overbreak | Swell | Excavation (m³) | Loose spoil (m³) |
|---|---|---|---|---|---|---|---|
| Metro drive | Circular | D=6.0 m, allowance 0.7 m | 1,200 m | 3% | 15% | 43,011 | 49,463 |
| Utility tunnel | Rectangular | W=4.0 m, H=3.5 m, allowance 0.6 m | 300 m | 8% | 25% | 5,832 | 7,290 |
| Road cross-passage | Horseshoe | W=5.0 m, H=5.5 m, allowance 0.6 m | 120 m | 12% | 30% | 3,115 | 4,050 |
Formula used
- Area depends on the selected shape and any allowances.
- Base volume = Area × Length.
- Excavation volume = Base volume × (1 + Overbreak/100).
- Loose spoil = Excavation volume × (1 + Swell/100).
- Total = per-bore volume × Number of bores.
How to use this calculator
- Select your units, shape, and length input mode.
- Enter tunnel dimensions from drawings or survey sections.
- Enable allowance if lining and fit-up space is required.
- Set overbreak and swell based on method and ground behavior.
- Press Calculate, then export CSV or PDF for reporting.
Key geometric shapes used in tunnel takeoff
The calculator supports circular, rectangular, trapezoidal, and horseshoe sections to match common tunnel drawings. Circular bores are frequent for TBM drives and utility crossings, while horseshoe profiles suit road and rail clearances. Use “Custom area” when your cross‑section comes from survey or CAD takeoff. Check dimensions against excavation class and support system requirements.
Allowance, overbreak, and swell factors
Allowance increases effective excavation dimensions for lining build‑up, waterproofing, and tolerances. Overbreak is added to in‑situ volume; planning ranges often run 3–8% for controlled drill‑and‑blast and 5–12% for mechanical excavation. Swell converts to loose spoil volume; 10–25% is common in competent rock and 20–35% in mixed ground.
Length methods and chainage control
Use “Single length” for a continuous drive, and “Segments” when geometry, ground, or method changes by station. Segmenting captures portal transitions, enlargements, cross‑passages, and different overbreak assumptions. Enter segment lengths as shown on drawings to keep exports aligned with pay items and daily logs. For curved alignments, use centerline length from the latest survey control.
Mass and disposal planning from computed volumes
Converting volume to mass improves haulage planning. Intact rock density is often about 2.4–2.8 t/m³. A 20 m³ tipper loaded at 1.8 t/m³ carries roughly 36 t, so loose spoil volume divided by truck capacity gives first‑pass trip counts. Document density and swell assumptions in the PDF export for review.
Example data snapshot for reporting
Example horseshoe estimate showing how overbreak and swell change outputs.
| Shape | Width | Height | Allowance | Length | Overbreak | Swell | Excavation | Loose spoil |
|---|---|---|---|---|---|---|---|---|
| Horseshoe | 6.0 m | 6.5 m | 0.2 m | 250 m | 8% | 25% | 10,102 m³ | 12,628 m³ |
FAQs
1) What volume does “Excavation” represent?
It is the in‑situ excavated volume: cross‑section area multiplied by length, then increased by the overbreak percentage. It excludes swell, so it suits pay quantities and in‑ground comparisons.
2) What is “Loose spoil” used for?
Loose spoil includes swell, which is useful for transport, stockpile space, and disposal planning. Use it to estimate truck trips, dump fees, and temporary storage requirements.
3) How should I choose an overbreak percentage?
Start with method and ground behavior. Controlled blasting may be 3–8%, mechanical excavation 5–12%, and weak or fractured ground higher. Calibrate using as‑built surveys from similar projects.
4) Does allowance replace lining thickness?
No. Allowance reflects clearance and tolerance space. Measure lining thickness separately for concrete, shotcrete, or segment quantities, and use project specifications for minimum cover.
5) When should I use “Custom area”?
Use it when the section is irregular or already measured in CAD or survey software. Enter the measured area and keep any tolerances inside that value for consistent reporting.
6) How do multiple bores affect totals?
Totals are multiplied by the number of bores. This fits twin‑tube tunnels, parallel headings, or repeated utility drives using the same cross‑section and assumptions.
7) Can I export my scenario for review?
Yes. Export CSV for spreadsheets and PDF for submittals. Both exports include inputs and outputs so reviewers can reproduce and audit the estimate.