Foundations · Pile settlement
Axial pile load–settlement by t-z / q-z curves (API RP 2GEO) calculator
Unit shaft friction and end bearing by the API method (α in clay, β in sand), then the pile is marched up from the toe for a range of toe movements using the API t-z and Q-z curves with elastic shortening. Gives the head load–settlement curve, the settlement at the design load and the axial load profile.
Standard: API RP 2GEO / RP 2A-WSD t-z and Q-z curves · AS 2159 · Coyle & Reese (1966) load transfer
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 outside diameter | 0.61 | m |
| Wall thickness (0 = solid) | 0.016 | m |
| Embedded length | 22 | m |
| Pile modulus | 200 | GPa |
| Layers (bottom depth m, 1 clay / 2 sand, γ′ kN/m³, su kPa or φ′ °) | 8, 1, 8, 35 14, 1, 9, 70 30, 2, 10, 33 | |
| Design (service) load | 2500 | kN |
| Allowable head settlement | 20 | mm |
| Geotechnical reduction φg (AS 2159) | 0.55 | |
| Design action N* (strength) | 2000 | kN |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Ultimate shaft resistance | 1,610 | kN |
| Ultimate base resistance | 2,315 | kN |
| Design capacity φg(Qs + Qb) | 2,158 | kN |
| Head settlement at the design load | 12.37 | mm |
| Toe movement at the design load | 5.21 | mm |
| Base load at the design load | 923 | kN |
| Max load on the curve | 3,855 | kN |
- ✓ N* ≤ φg·Rug — 2,000 vs 2,158 kN
- ✓ Settlement at design load ≤ allowable — 12.4 mm
Method, formulas and assumptions
- Clay: f = α su, α = 0.5ψ^−0.5 (ψ ≤ 1) or 0.5ψ^−0.25, ψ = su/σ′v, α ≤ 1; q = 9su. Sand: f = βσ′v ≤ flim, q = Nqσ′v ≤ qlim (API table by φ′).
- t-z: clay curve t/tmax at z/D = 0.0016, 0.0031, 0.0057, 0.008, 0.01 → 0.3 … 1.0, residual 0.9; sand: linear to 2.54 mm. Q-z at z/D = 0.002 … 0.10 → 0.25 … 1.0.
- Marching from the toe: segment mid displacement w + P·h/2AE gives t; P(top) = P(bottom) + t·πD·h; w(top) = w(bottom) + mean P·h/AE.
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.