About Pile Foundation Design Calculator
The pile foundation design calculator estimates the axial capacity of a single driven or bored pile from skin friction and end bearing, using the alpha method in clays and the beta method in sands and gravels. It is used by geotechnical engineers to check pile capacity, settlement, and negative skin friction against Eurocode 7, ASCE/FHWA, and AS 2159.
Define the pile type, diameter, and length, build a layered soil profile with cohesion or friction angle, and enter the applied axial load. The tool returns the shaft and base capacity, the ultimate and allowable capacity, the axial utilisation, an estimated settlement, and a per-layer breakdown of the shaft contribution, all in real time.
How It Works
- For each cohesive layer compute the unit shaft friction qs = alpha*cu, and for each granular layer qs = beta*sigma_v, where beta = K*tan(delta).
- Sum the shaft force Qs = qs * pi*D*deltaL over all layers and add the base capacity Qb (Nc*cu*Ab for clay, Nq*sigma_v*Ab for sand).
- Form the ultimate capacity Qu = Qs + Qb and divide by the safety factor (2.5 for EC7/AS 2159, 2.0 for ASCE/FHWA) to get the allowable capacity.
- Compare the applied load with the allowable capacity, estimate pile-head settlement, and add negative skin friction when a water table lies within the pile length.
Worked Example
A 0.6 m driven concrete pile, 10 m long, in stiff clay with cu = 75 kPa under EC7 (safety factor 2.5). alpha = 0.55 - (75 - 50)*(0.10/50) = 0.50, so Qs = 0.50*75*pi*0.6*10 = 706.9 kN. Base area Ab = pi*0.3^2 = 0.2827 m2, so Qb = Nc*cu*Ab = 9*75*0.2827 = 190.9 kN. Then Qu = 706.9 + 190.9 = 897.7 kN and Qallow = 897.7 / 2.5 = 359.1 kN.
Formulas
- Shaft friction (alpha method, cohesive)
Qs = alpha * cu * pi * D * deltaL- Shaft friction (beta method, granular)
Qs = K * tan(delta) * sigma_v * pi * D * deltaL- Base capacity
Qb = Nc*cu*Ab (clay) ; Qb = Nq*sigma_v*Ab (sand)- Ultimate and allowable capacity
Qu = Qs + Qb ; Qallow = Qu / SF
Standards & References
- Eurocode 7 (EN 1997)
- ASCE/FHWA
- AS 2159
Frequently Asked Questions
What is the difference between the alpha and beta methods?
The alpha (total stress) method applies to cohesive soils and gives the unit shaft friction qs = alpha*cu, where the adhesion factor alpha falls as the undrained strength cu rises. The beta (effective stress) method applies to granular soils with qs = beta*sigma_v and beta = K*tan(delta).
How is end bearing calculated?
For a pile bearing in clay the base capacity is Qb = Nc*cu*Ab with Nc = 9 for deep foundations. For a pile bearing in sand it is Qb = Nq*sigma_v*Ab, where Nq follows the Meyerhof/Berezantsev form and is capped to avoid over-prediction at high friction angles.
How do driven and bored piles differ in the calculation?
Bored piles use a reduced shaft adhesion (about 0.8 of the driven value) and a lower earth pressure coefficient K = 1 - sin(phi) with delta = 0.7*phi, while driven piles use K = 1 + sin(phi) and delta = 0.85*phi, reflecting the looser interface bond of bored construction.
What safety factor should I use?
The tool applies a global safety factor on the ultimate capacity: 2.5 for Eurocode 7 and AS 2159, and 2.0 for the ASCE/FHWA approach. You can override the factor manually to match project-specific or load-test-verified requirements.