Foundations · Pile capacity
Negative skin friction (downdrag) calculator
Drag load on a pile from a settling layer (fill over soft clay) by the β method, and the maximum axial force at the neutral plane.
Standard: Effective-stress β method · drag load in the settling layer
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 |
|---|---|---|
| Pile diameter | 0.6 | m |
| Depth of settling soil | 8 | m |
| Effective unit weight γ′ | 8 | kN/m³ |
| Fill surcharge | 30 | kPa |
| β (clay 0.2–0.3) | 0.25 | |
| Dead load on pile | 900 | kN |
| Structural capacity φNs | 2600 | kN |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Perimeter πD | 1.885 | m |
| Drag load Qn | 234 | kN |
| σ′v at base of settling layer | 94 | kPa |
| Max axial force QD + Qn | 1,134 | kN |
- ✓ QD + Qn ≤ structural capacity — 1,134 kN
Method, formulas and assumptions
- Qn = πDβ∫σ′v dz = πDβ(qHs + γ′Hs²/2)
- Drag load is a structural check at the neutral plane. Check downdrag settlement separately.
- Transient live load and drag load are normally not combined
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.