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The undrained shear strength of the soft, settling soil layer, in the unit shown beside the field.
Downdrag typically comes from soft compressible soils (like recently placed fill or soft clay) consolidating around the pile.
The empirical adhesion factor for the settling soil.
Careful with the word conservative here, because this page computes a LOAD rather than a resistance and the safe direction is the opposite of the familiar one. In a shaft-capacity calculation a lower α is conservative because it reduces the capacity you are counting on. In a downdrag calculation a lower α reduces the DEMAND on the pile, which is the unsafe direction — so the conservative assumption for negative skin friction is a HIGHER α. Negative skin friction also mobilises fully at very small relative settlement, far less than the movement needed to develop positive shaft friction, so there is little basis for discounting it.
The pile's outer diameter.
Drag load acts on the pile's OUTSIDE surface, so this enters as a perimeter — meaning it scales the downdrag linearly while the end bearing that has to resist it scales with the square. A fatter pile therefore helps twice over. For an H-section the surface soil actually grips may be the block perimeter round the flanges rather than the steel's own developed surface; take whichever the design assumes and keep it consistent.
The length of pile shaft passing through the settling soil layer.
Downdrag develops down to the neutral plane, where relative movement between pile and soil reverses — for a simplified estimate, use the full settling layer thickness.
Downdrag load
20.5 kips
This downdrag load must be ADDED to the pile's structural and geotechnical demand — it's a load, not a resistance, and is easy to overlook in a simple capacity check. A bitumen (asphalt) coating on the shaft through the settling zone can reduce or largely eliminate this effect if it's a design concern.
- Pile perimeter
- 4.19 ft
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Negative Skin Friction (Downdrag) Calculator: 20.47 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)
- Negative skin friction (downdrag) uses the same alpha-method formula as positive shaft friction, but represents an added LOAD on the pile (from settling soil dragging down on the shaft) rather than a resistance: Qnsf = alpha x Su x perimeter x settling-zone length
Inputs used
- Undrained Shear Strength (Su) of Settling Layer
- 626.56 psf
- Adhesion Factor (α)
- 0.3
- Pile Diameter
- 16 in
- Settling Zone Length
- 26 ft
Intermediate steps
- Pile perimeter
- 4.19 ft
Confidence note: This downdrag load must be ADDED to the pile's structural and geotechnical demand — it's a load, not a resistance, and is easy to overlook in a simple capacity check. A bitumen (asphalt) coating on the shaft through the settling zone can reduce or largely eliminate this effect if it's a design concern.
What this calculation does not cover
- The number is an unfactored drag load, not a design action. It applies no load factor, no resistance factor and no combination rule — how drag load sits alongside the live load the same pile carries is treated differently for a structural strength check than for a settlement check, and this figure does not stand in for either.
- It drags the full settling-zone length you type; it does not locate the neutral plane. That plane sits where pile and soil move together, and finding it needs the pile's stiffness, the load it already carries and the settlement profile of the ground, none of which are inputs. On a pile bearing into a stiff stratum the plane can sit at or below the base of the soft layer, so the real drag length is longer than the layer thickness and this reads low.
- One layer, one shear strength, one adhesion factor. A profile of fill over soft clay over a stiffer horizon needs a separate run per layer with the results summed. Nothing here caps a pile group's total drag at the weight of the soil block enclosed by the group, which for closely spaced piles is far less than the sum of the individual shaft drags.
- The perimeter is pi times diameter — a solid round shaft. An H-pile, a square precast pile or a sheet section presents a different drag surface, and for an H-pile it depends on whether soil plugs between the flanges, so entering an equivalent diameter for any of these mis-sizes the shaft area.
- Alpha times Su is one snapshot of a process that runs for months or years. It carries no consolidation time, no dependence on how much further fill is placed or how far the water table drops, and no term for a bitumen slip coat or an isolating sleeve — the usual ways downdrag is actually reduced on site.
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This result is a specification — 20.5 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
- Negative skin friction (downdrag) uses the same alpha-method formula as positive shaft friction, but represents an added LOAD on the pile (from settling soil dragging down on the shaft) rather than a resistance: Qnsf = alpha x Su x perimeter x settling-zone length
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