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The soil's undrained shear strength, from a geotechnical report.
Typically from unconfined compression or vane shear tests on cohesive soil samples.
The empirical adhesion factor relating shaft friction to undrained shear strength.
α decreases as Su increases (stiffer clays develop proportionally less adhesion) — use a site-specific correlation chart (e.g. API RP2A) rather than a single universal value.
The pile's outer diameter. Circular and closed-end pipe piles only — this field is not a square pile's face width.
The outside diameter of the shaft, because skin friction acts on the surface the soil touches. For a closed-end pipe pile that is the pipe's outer diameter; for an H-pile or an open-ended pipe the surface in contact is not a simple circle and this calculation does not describe it. A square pile's perimeter is four times its face width, not π times — entering a face width here understates the shaft area by about 21%.
The pile's length embedded in the soil layer contributing skin friction.
If the pile passes through multiple soil layers with different Su values, run this once per layer and sum the results.
Skin friction capacity
72.2 kips
This is ultimate (unfactored) shaft capacity from one soil layer only — a full pile design also includes end-bearing capacity, applies a safety factor (typically 2-3) or LRFD resistance factors, and sums contributions from every soil layer the pile passes through.
- Pile perimeter
- 4.19 ft
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Driven Pile Skin Friction Capacity Calculator (Alpha Method): 72.17 kips — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Alpha method for pile shaft capacity in cohesive soil: Qs = alpha x Su x pile perimeter x embedded length, where alpha is an empirical adhesion factor (from a site-specific correlation chart, e.g. API RP2A) and Su is the soil's undrained shear strength
Inputs used
- Undrained Shear Strength (Su)
- 1044.27 psf
- Adhesion Factor (α)
- 0.5
- Pile Diameter
- 16 in
- Embedded Length
- 33 ft
Intermediate steps
- Pile perimeter
- 4.19 ft
Confidence note: This is ultimate (unfactored) shaft capacity from one soil layer only — a full pile design also includes end-bearing capacity, applies a safety factor (typically 2-3) or LRFD resistance factors, and sums contributions from every soil layer the pile passes through.
What this calculation does not cover
- Shaft friction only, at ultimate and unfactored. There is no end-bearing resistance at the toe, no factor of safety or LRFD resistance factor, no check that the pile section itself can carry the load, and no check that it can be driven to depth. This is one input to a pile design, not the design.
- The perimeter is pi times the figure you enter, so the pile is treated as a solid circular shaft of constant section. A square precast pile of the same face width has about 27% more shaft area, and H-piles, sheet sections, tapered piles and open-ended tubes all differ from both.
- The adhesion factor is whatever single number you type, held constant over the whole embedded length. It is not derived from Su and effective overburden the way an API-style correlation chart derives it, it does not vary with depth, and it does not know whether the shaft is steel, precast concrete or timber.
- Nothing in this changes with time. Driving remoulds the clay around the shaft and a large part of the capacity returns over days to months as pore pressures dissipate, while cyclic or sustained loading can degrade it. The figure carries no set-up gain and no cyclic degradation.
- It assumes the entire embedded length you enter is in contact with clay of that strength and stays there. Downdrag from a settling layer, scour, softening at the top of the shaft, and any length inside a casing, sleeve or fill that should not be counted are all outside the model, so take those lengths off before entering. Su is read as kPa on both the metric and imperial pages; that field does not convert.
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This result is a specification — 72.2 kips — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Alpha method for pile shaft capacity in cohesive soil: Qs = alpha x Su x pile perimeter x embedded length, where alpha is an empirical adhesion factor (from a site-specific correlation chart, e.g. API RP2A) and Su is the soil's undrained shear strength
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