| Scenario | Method | Bays | Bay length | Width | Depth | Soil | Access | Groundwater |
|---|---|---|---|---|---|---|---|---|
| Small extension support | Mass concrete | 6 | 1.2 m | 0.6 m | 1.5 m | Medium | Moderate | None |
| Urban terrace retrofit | Beam and base | 10 | 1.0 m | 0.7 m | 2.0 m | Hard | Tight | Damp |
| Low headroom basement | Mini-piled | 14 | 0.9 m | 0.6 m | 2.4 m | Rock | Confined | Flowing |
- Total length = Bay length × Number of bays
- Concrete volume (m³) = Total length × Width × Depth
- Rebar (kg) = Concrete volume × Rebar intensity
- Complexity factor = Method × Soil × Access × Groundwater × Excavation multipliers
- Labor hours = (Excavation m³ × hours/m³ + Concrete m³ × hours/m³ + Bays × setup hours) × Complexity
- Equipment days = ceil(Labor hours ÷ (Crew size × 8 hours/day))
- Direct subtotal = Materials + Labor + Equipment + Disposal + Shoring + Fixed allowances
- Grand total = Direct + Contingency + Overheads + Profit
- Select currency and unit system, then choose the underpinning method.
- Enter bays and bay length to set the overall underpinning length.
- Provide width and depth based on design and site constraints.
- Fill in rates for concrete, steel, labor, disposal, and equipment.
- Set soil, access, groundwater, and excavation approach to reflect risk.
- Adjust contingency, overheads, and profit to match your bid strategy.
- Press Estimate cost; download CSV or PDF to share.
1) Why bay quantity drives cost
Most mass-concrete underpinning is built in bays (pins) to control stability and sequencing. Residential projects commonly range from 4 to 20 bays, while small commercial work may exceed 40. Because setup time repeats per bay, doubling bays typically increases labor more than materials in constrained sites.
2) Dimensions and typical volume ranges
Concrete volume is calculated as total length × width × depth. Typical bay lengths are 0.9–1.2 m. Common widths are 0.5–0.75 m, and depths often fall between 1.2–2.5 m depending on bearing strata and adjacent foundations. A 10-bay job at 1.0 m length, 0.6 m width, and 1.8 m depth is about 10.8 m³ of concrete.
3) Rates that influence the estimate
Concrete is usually the largest direct material line, but disposal and temporary works can rival it in urban areas. Disposal is commonly priced per m³ of excavation, and shoring is often estimated per meter of supported trench. Reinforcement may range from 0–120 kg/m³, depending on engineered details and load paths.
4) Productivity and risk multipliers
This calculator applies a complexity factor combining method, soil, access, groundwater, and excavation approach. Hard soils, tight access, and water control can increase labor hours by 15–60% or more. Hand excavation often slows production and increases equipment needs for pumping, breaking, and spoil handling.
5) Contingency, overheads, and profit benchmarks
For early-stage budgeting, contingency commonly sits between 5–20% depending on investigation quality. Overheads may be 10–15% for supervision, insurance, and preliminaries. Profit is frequently modeled at 8–15% to reflect risk and commercial strategy. Use the per-meter and per-bay outputs to compare quotations consistently.
1) What is an underpinning “bay”?
A bay is a short segment of excavation and concrete placed in sequence beneath an existing foundation. Bays reduce instability by limiting open excavation length and allow staged transfer of loads to the new bearing level.
2) Why do soil and groundwater settings change costs?
Hard soils require more effort to excavate, while groundwater can demand pumping, slower sequencing, and added temporary works. Both conditions increase labor hours and equipment usage, raising the direct subtotal and the risk allowance.
3) Should I include reinforcement steel for mass concrete underpinning?
Only include steel if your design details specify it. Some underpinning is unreinforced, while beam-and-base or engineered pins often require reinforcement. If you are unsure, use conservative kg/m³ values until drawings confirm.
4) How accurate is the equipment “days” estimate?
It is a planning proxy based on total labor hours and crew size, assuming 8-hour shifts. Real durations vary with inspections, curing, restricted access, and sequencing constraints. Use it to size rentals, not to finalize schedules.
5) What should I put in shoring allowance per meter?
Enter a rate that reflects trench support, walers, struts, trench sheets, and installation labor. Confined excavations typically need higher temporary works allowances. If temporary works are engineered separately, reduce this line and add a fixed amount.
6) What contingency percentage is reasonable?
For preliminary estimates, 10% is a common starting point. Increase it when investigation data is limited, nearby utilities are uncertain, or water control is likely. Reduce it after confirmed boreholes, method statements, and supplier quotes.
7) Can I use the results for contract pricing?
This tool supports budgeting and comparison, not a substitute for engineered design and contractor takeoff. Validate dimensions, sequencing, temporary works, access plans, and local rates before signing a contract or issuing a final tender.