Generated by calculatgeek.com Pile Bearing Capacity Estimator
A pile carries load two ways at once: friction along its sides as soil grips the shaft, and bearing at its tip as the pile pushes against the soil below. This calculator estimates both components for a single uniform pile using the alpha method for shaft friction in clay and a choice of clay or sand bearing at the tip, then combines them into an ultimate and allowable capacity.
For piles in clay, shaft resistance is commonly estimated with the alpha method, where an adhesion factor alpha scales the average undrained shear strength along the shaft:
Qs = alpha x cu x perimeter x L
Alpha typically ranges from about 1.0 for very soft clay down to 0.3 or lower for stiff, heavily overconsolidated clay, since stiffer clays tend to develop less shaft adhesion relative to their strength. Published alpha versus cu charts, such as those from API RP 2A or local geotechnical guidance, should be used to select an appropriate value for a specific site.
For a pile tip in clay, Meyerhof’s bearing capacity factor for deep foundations is commonly taken as Nc = 9:
Qp = 9 x cu(tip) x Ap
For a pile tip in sand, end bearing depends on the friction angle and the effective overburden stress at the tip through Meyerhof’s bearing capacity factor Nq:
Qp = Nq x sigma-v’ x Ap, where Nq = e^(pi tan(phi)) x tan^2(45 + phi/2)
Ultimate capacity is simply the sum, Qult = Qs + Qp. Dividing by a factor of safety, commonly 2.0 to 3.0 for static formula estimates without load testing, gives the allowable working load the pile can be designed to carry.
For a 1.5 ft diameter, 40 ft long pile in clay with an average shaft cu of 800 psf, alpha of 0.6, and a tip cu of 1,200 psf, skin friction works out to about 90,500 lb and end bearing to about 19,100 lb, for an ultimate capacity near 109,600 lb. With a factor of safety of 2.5, the allowable capacity is about 43,800 lb, or roughly 22 tons.
This calculator uses a single average soil strength for the shaft and one value at the tip, a simplification of what is usually a layered soil profile requiring a separate calculation for each layer. It also does not account for group effects when piles are installed close together, negative skin friction from settling soil, or driving stresses during installation. Use it for preliminary sizing and to sanity check more detailed calculations, and confirm final capacity with load testing or a full geotechnical report for anything beyond a preliminary estimate.
See also the Bearing Pressure Converter for converting pile capacities between metric and imperial pressure units.
For shallow foundation bearing pressure on the same site, see the Slope Stability Factor of Safety Calculator. Formula reference: Meyerhof pile bearing capacity, deep foundation design.