What this calculator does
Foundations rarely fail by bearing capacity; they fail by settling. This calculator estimates the immediate settlement from elasticity and the long-term consolidation from the compressibility of the soil beneath the footing, then adds them so you know whether the total is within what the structure will tolerate.
How to use it
- Enter the net bearing pressure and the footing dimensions.
- Enter the soil parameters: Poisson ratio, elastic modulus, compression index, void ratio and the initial vertical stress.
- Set the thickness of the compressible layer beneath the footing.
- Compare the total against the differential settlement the structure can accept — usually 25 mm total for reinforced concrete frames.
Governing formula
δe = q·B(1−ν²)·If / E ; sc = Cc·H/(1+e₀) · log₁₀((σ₀′+Δσ)/σ₀′) Where:
-
q= Net bearing pressure [kPa] -
If= Influence factor (0.88 for a square footing at centre) [–] -
Cc= Compression index [–] -
H= Compressible layer thickness [m]
Worked example — 2 m square footing at 150 kPa over 6 m of soft clay
The figures below were produced by running this page's own engine with the example inputs (Net bearing pressure (kPa), Footing width (m), Footing length (m), Poisson ratio (ν) (–), Elastic modulus (E) (MPa), Compression index (Cc) (–), Initial void ratio (e₀) (–), Thickness of compressible layer (m), Initial vertical stress at layer mid-depth (kPa)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Total settlement | 409.95 mm |
|---|---|
| Immediate (elastic) settlement | 12.01 mm |
| Consolidation settlement | 397.94 mm |
| Stress increase at layer mid-depth | 90 kPa |
| Assessment | Immediate 12 mm + consolidation 398 mm ≈ 410 mm for a 6 m compressible layer. Bearing-pressure stress at layer mid-depth 90 kPa. |
- Immediate settlement: 150 × 2 × (1 − 0.3²) × 0.88 / 20,000 ≈ 12 mm.
- The consolidation term uses the stress increase at mid-layer depth with the compression index.
- Add the two and compare with the tolerance of the structure — if the total exceeds about 25 mm, a raft or ground improvement is worth considering.
Assumptions & limitations
Engineering calculation. Splits settlement into an immediate elastic component (Janbu influence factor) and a one-dimensional consolidation component. No secondary consolidation or creep.
- The consolidation term is one-dimensional only — sand that consolidates almost immediately is over-predicted and clay that is stiff at depth may be under-predicted.
- No secondary (creep) settlement, which matters for very soft clays and organic soils.
- Differential settlement between footings is not calculated; only the average under one footing.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- BS EN 1997-1 — serviceability limit states and settlement.
- Terzaghi, Peck & Mesri, Soil Mechanics in Engineering Practice.
- Das, Principles of Geotechnical Engineering, chapters on settlement.