Excavation Overrun Calculator

Excavation conditions drive overruns. This calculator sizes the risk.

Excavation budget

Rock risk (CY)

Rock premium $/CY

Dewatering months

Dewatering/month

Total overrun risk

$138,000

Rock risk dollars

$66,000

Dewatering cost

$72,000

How the math works

Rock risk = rock CY × premium. Dewatering = months × rate. Total = both.

Lock the excavation subcontract with a separate rock line item at a fixed $/CY. Changes orders on rock blasting executed mid-excavation routinely cost 2-3x what they would have if negotiated upfront — the contractor has pricing power once the crew is on site and schedule is moving.

How to Use

Enter excavation budget.
Enter rock risk CY and rock premium.
Enter water table risk days and dewatering rate.
Enter shoring contingency.
Read total overrun risk.

Frequently Asked Questions

Rock cost?

Standard dirt excavation: $8-15/CY. Rippable rock: $20-40/CY. Blasted rock: $40-120/CY. Rock risk on unknown site: budget 15-30% of bulk excavation at rock premium. Geotech report is required input.

Dewatering?

Light dewatering (well points): $3-10k/month. Moderate (sump + pump): $10-30k/month. Heavy (dewatering system): $30-100k/month. Duration typically 2-8 months during excavation + foundation. Budget explicitly — it's not in base CY rate.

Shoring?

Open-cut excavation feasible at 1:1 slope typically. Urban sites require shoring (soldier pile + lagging, secant pile, shotcrete). $80-300/SF of wall. 20-50% of basement excavation cost in urban infill.

How often should I rerun this?

Rerun this calculator whenever inputs change materially — new rent roll data, rate moves, loan balance updates, or quarterly operating data. For active deals, monthly refresh is typical. For stabilized assets under monitoring, quarterly is fine. Treat the output as a decision tool, not a one-time answer — market conditions evolve and so should your analysis.