How to use the pile bearing capacity calculator
A single pile carries load two ways: end bearing Qp at the toe and shaft friction Qs along the sides. This calculator applies the static formula Qu = Qp + Qs and divides by a factor of safety to give the allowable capacity per pile.
In sand, end bearing uses Qp = Ap·Nq·σ'v with the Prandtl–Reissner bearing factor, and shaft friction uses the beta method with β = (1−sinφ)·tan(2φ/3). In clay the undrained case applies, so Qp = 9·cu·Ap and Qs = α·cu·As, with the adhesion factor near 1.0 for soft clay and dropping toward 0.4 for stiff clay.
Enter the submerged (effective) unit weight for any portion below the water table, otherwise capacity is overstated. Piles in a group also carry less than the simple sum of single-pile capacities, so a group efficiency check is a separate step.
These results assume one uniform soil layer and are a screening estimate only. Boring logs and load test results govern in real projects, and final pile design should be confirmed by a licensed geotechnical engineer.
Frequently asked questions
Static formulas based on soil properties are for design; driving formulas and dynamic monitoring are for verification during installation. A static or dynamic load test is the real confirmation, and test results always govern over calculated capacity.
A factor of safety near 3 is typical when capacity comes from a static formula alone. It can drop toward 2 when a static load test confirms the pile, and should be raised when subsurface conditions vary or data is thin.
Both shaft friction and end bearing are computed from effective stress, so enter the submerged unit weight (roughly total unit weight minus 62.4 pcf) for the saturated portion. Using total unit weight overestimates capacity.