Bank Volume (BCM) with Geological Overbreak
Excavated Bank Volume (BCM) = Alignment Length (m) × Theoretical Cross-Section Area (m²) × (1 + Overbreak %)A 500 m NATM tunnel with a 60 m² face and 8% rock overbreak yields 500 × 60 × 1.08 = 32,400 BCM.
Tunnel Quantities.
Estimate circular tunnel excavation and lining volumes from geometry, length and an overbreak allowance.
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Heavy Civil & Tunnel Excavation Formulas: BCM to LCM Swell, Overbreak & Shotcrete
In heavy civil earthworks, deep basement excavations, and underground tunneling, drawings specify Bank Cubic Meters (BCM)-the in-situ volume of intact soil or rock. Dump trucks, spoil conveyors, and muck disposal sites must be priced using Loose Cubic Meters (LCM) after excavation expands, plus geological overbreak beyond the theoretical excavation pay line.
Engineering Formulas & Calculation Methodology
Loose Spoil Volume (LCM) & Haulage Truck Trips
Loose Volume (LCM) = BCM × Bulking (Swell) Factor | Truck Loads = ⌈ LCM / Truck Bed Capacity (m³) ⌉Intact rock typically swells by 1.50 to 1.65 when blasted; 32,400 BCM × 1.55 swell = 50,220 LCM.
Primary Shotcrete Support Volume (with Rebound Loss)
Shotcrete Volume (m³) = Tunnel Perimeter Arc (m) × Length (m) × Thickness (m) × (1 + Rebound + Overbreak Fill %)Accounts for 10%–15% wet-mix shotcrete rebound and rock cavity backfill.
Frequently Asked Questions
How do you convert Bank Cubic Meters (BCM) to Loose Cubic Meters (LCM) in excavation?
Multiply the in-situ Bank Cubic Meters (BCM) by the material's swell factor: LCM = BCM × Swell Factor. Common factors are 1.12–1.20 for sand and gravel, 1.25–1.35 for clay and earth, and 1.50–1.65 for blasted hard rock.
How is tunnel overbreak calculated in a civil engineering BOQ?
Tunnel overbreak is the volume excavated outside the theoretical design profile due to jointing, drill-and-blast tolerances, or ravelling. It increases both muck haulage volume and concrete or shotcrete required to backfill the void.
How many dump truck loads are needed for an excavation?
Divide the Loose Cubic Meters (LCM)-never the Bank Cubic Meters (BCM)-by the effective struck capacity of the haul truck and round up to the next whole integer.