Foundations · Pad, strip & combined footings
Spread footing from CPT: bearing and settlement calculator
Pad or strip footing designed directly from a CPT, as in GEO5 Spread Footing CPT. The equivalent cone resistance qce is the mean qc from the founding level to 1.5B below, clipped at 1.3 times that mean; the bearing factor kc grows with the equivalent embedment De/B and the footing shape. Design bearing uses φg. Settlement follows Schmertmann's strain-influence method with E = 2.5qc (square) to 3.5qc (strip), depth and creep corrections.
Standard: Direct CPT method (Frank 1999 / EN 1997-2 Annex D): q = kc·qce + q0 · Schmertmann (1978) strain-influence settlement · AS 2159-style φg
Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| CPT: depth (m), qc (MPa) | 0.25, 2.89 0.50, 3.68 0.75, 4.26 1.00, 4.59 1.25, 4.65 1.50, 4.49 1.75, 4.17 2.00, 3.82 2.25, 3.53 2.50, 3.41 2.75, 3.53 3.00, 3.91 3.25, 4.54 3.50, 5.36 3.75, 6.26 4.00, 7.14 4.25, 7.89 4.50, 8.42 4.75, 8.69 5.00, 8.70 5.25, 8.49 5.50, 8.16 5.75, 7.81 6.00, 7.55 6.25, 7.48 6.50, 7.65 6.75, 8.09 7.00, 8.77 7.25, 9.62 7.50, 10.52 7.75, 11.38 8.00, 12.09 8.25, 12.57 8.50, 12.78 8.75, 12.73 9.00, 12.50 9.25, 12.15 9.50, 11.81 9.75, 11.58 10.00, 11.56 | |
| Soil | Sand / gravel (kc0 = 0.14) | |
| Footing width B | 2.4 | m |
| Footing length L (= B for square) | 2.4 | m |
| Founding depth D | 1 | m |
| Soil unit weight | 18 | kN/m³ |
| Geotechnical reduction φg | 0.5 | |
| Design vertical load N* | 1300 | kN |
| Service load (net of soil removed) | 700 | kN |
| Time for creep factor | 25 | yr |
| Allowable settlement | 25 | mm |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Mean qc, D to D + 1.5B | 5.05 | MPa |
| Equivalent qce (clipped at 1.3 × mean) | 4.80 | MPa |
| Equivalent embedment De | 0.80 | m |
| Bearing factor kc | 0.156 | |
| Ultimate bearing kc·qce + q0 | 768 | kPa |
| Design resistance φg·qult·A | 2,212 | kN |
| Net service pressure Δp | 122 | kPa |
| Peak strain influence Izp at 1.20 m | 0.675 | |
| C1 (embedment) / C2 (creep) | 0.926 / 1.480 | |
| Schmertmann settlement | 24.4 | mm |
- ✓ N* ≤ φg·R — 59 %
- ✓ Settlement ≤ allowable — 24.4 mm
- ✓ CPT reaches the bottom of the influence zone (D + 2B)
Method, formulas and assumptions
- qce = mean of min(qc, 1.3·mean) over D to D + 1.5B; De = (1/qce)∫₀ᴰ qc dz.
- kc = kc0[1 + 0.35(0.6 + 0.4B/L)De/B], kc0 = 0.32 clay/silt, 0.14 sand/gravel (simplified from EN 1997-2 Annex D.3 / Frank 1999); qult = kc·qce + γD.
- Schmertmann: s = C1·C2·Δp·Σ(Iz/E)Δz, Izp = 0.5 + 0.1√(Δp/σ′vp), E = 2.5qc square / 3.5qc strip, C1 = 1 − 0.5σ′0/Δp, C2 = 1 + 0.2log(t/0.1).
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.