What this calculator does
A pile carries most of its load along its shaft and the rest at the toe. This calculator splits those two contributions and divides the total by your safety factor, using the standard α-method for clay and an effective-stress approach for sand.
How to use it
- Select the soil type: cohesive clay uses the α-method with the undrained tip factor, sand uses effective stress.
- Enter the pile diameter and length.
- For clay, enter the undrained shear strength and the adhesion factor α, typically 0.5–0.8 depending on the installation method.
- For sand, enter the effective unit weight, the friction angle and the cone factor Nq.
Governing formula
Qs = α·c·π·D·L ; Qp = Nc·c·(πD²/4) (clay) Where:
-
α= Adhesion factor [–] -
c= Undrained shear strength [kPa] -
D= Pile diameter [m] -
L= Pile length [m]
Derived Equations:
Nc = 9 Q_ult = Qs + Qp Q_all = Q_ult / FS Worked example — 600 mm pile, 15 m deep in 50 kPa clay
The figures below were produced by running this page's own engine with the example inputs (Pile diameter (m), Pile length (m), Soil type, Undrained shear strength (clay) (kPa), Adhesion factor α (–), Effective unit weight (sand) (kN/m³), Friction angle (sand) (°), Cone factor Nq (sand) (–), Factor of safety (–)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Ultimate capacity | 1117 kN |
|---|---|
| Allowable capacity | 446.73 kN |
| Skin friction component | 989.6 kN |
| End bearing component | 127.23 kN |
| Method | Cohesive clay (α-method) |
| Assessment | Cohesive clay (α-method): skin 990 kN + tip 127 kN → Qu 1117 kN, Qall 447 kN at FS 2.5. Field load testing is recommended. |
- Shaft friction: 0.7 × 50 × π × 0.6 × 15 ≈ 990 kN.
- End bearing: 9 × 50 × 0.283 ≈ 127 kN — the shaft does most of the work in clay.
- At a factor of safety of 2.5 the allowable load is under 450 kN, so confirm by load test.
Assumptions & limitations
Engineering calculation. Single-pile capacity in homogeneous soil using the α-method for clay and a simplified β-method for sand. No layered profile, negative skin friction or group effects.
- Single pile in homogeneous soil — layered profiles, very soft underlying layers or negative skin friction change the answer substantially.
- No settlement of the pile group or of the surrounding soil.
- Both methods need field confirmation; a static load test is the only reliable verification for production piles.
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 — pile foundation design.
- Tomlinson, Pile Design and Construction Practice.
- API RP 2SK — offshore pile design (α and β methods).