What is the neutral plane?

The depth at which the pile and the settling soil move together — above it soil settles more than the pile and drags it down (negative skin friction); below it the pile settles more than the soil and is supported. The maximum axial force in the pile occurs there, and pile settlement is governed by the soil settlement at that depth.

Should dragload be subtracted from pile capacity?

No — that common shortcut is what the unified method corrects. Dragload is a structural load to check the pile section against at the neutral plane; geotechnical capacity below the neutral plane is unaffected by it. FHWA GEC-12 and Fellenius are explicit on this point, and PileCalc reports the quantities accordingly.

Where does the ground-settlement profile come from?

From your consolidation or fill-settlement analysis — the tool takes it as input (uniform, linear or point-by-point with depth) and solves the pile's response against it, so any settlement source can drive the analysis.

Fellenius unified method · FHWA GEC-12

Downdrag by the neutral plane method, done properly.

Impose the ground-settlement profile, and the analysis finds the neutral plane where pile and soil settle together — dragload, maximum axial force, and the full force and settlement profiles, per Fellenius and FHWA GEC-12.

Try it free

When the ground settles around a pile — new fill, soft-clay consolidation, dewatering — the upper soil drags the pile down instead of holding it up. The correct framework is the neutral plane: above it, negative skin friction adds dragload; below it, positive shaft resistance and toe bearing carry the pile. Treating dragload as a simple capacity deduction gets both the force and the settlement wrong.

PileCalc implements the Fellenius unified method as adopted by FHWA GEC-12: you impose a ground-settlement profile, and the analysis solves for the depth where pile settlement equals soil settlement. The output is the full axial-force profile (with the maximum at the neutral plane), the dragload, and the pile settlement — the quantities the geotechnical report actually needs to state.

Neutral plane located, not assumed

The equilibrium depth where pile and soil settlement match is solved from the imposed settlement profile — not picked by hand.

Force & settlement profiles

The complete axial-force distribution, peaking at the neutral plane, alongside the pile and soil settlement profiles.

Imposed ground settlement

Enter the settlement profile from your consolidation analysis — uniform, linear or point-by-point with depth.

Dragload reported separately

Dragload is a structural demand at the neutral plane, not a geotechnical capacity deduction — and it is reported that way.

Validated against published benchmarks

Every figure below is produced by the engine on the cited published problem — closed-form solutions, design-manual tables, or independent codes — and reproduced by the test suite on every release.

FHWA GEC 12 neutral-plane method with layer-specific t-z/q-z transfer

Quantity	PileCalc	Reference	Agreement
Neutral-plane depth — rigid pile, uniform skin, no toe, no head loadFellenius, Basics of Foundation Design — force-equilibrium closed form	≈ 0.5·L	0.5·L (20 ft)	within 10% (t-z mobilization)
Dragload Qn = f·π·D·(L/2)Fellenius closed form (f = 1000 psf, D = 1 ft, L = 40 ft)	≈ 62.8 kip	62.8 kip	within 10% (t-z mobilization)
Maximum axial force = permanent head load + dragload + effective pile self-weightFHWA GEC-12 §7.3 equilibrium identity	P + Qn + Wp	P + Qn + Wp	exact

See the full per-tool validation report

The numbers, published

PileCalc's engine is checked term-by-term against the reference codes. A representative sample of the benchmarks — every intermediate value is visible in the app so you can reproduce them yourself.

Benchmark

Quantity

PileCalc

Reference

Agreement

Free-head pile (EI 1.43×10⁶, D 1 m, L 24.4 m)Liang et al. (2014) exact solution / RSPile

Max moment

792.2 kN·m

792.1 kN·m (exact)

within 0.1%

Fixed-head pile (EI 1.43×10⁶, D 1 m, L 24.4 m)Liang et al. (2014) exact solution / RSPile

Head moment

−581.1 kN·m

−581.0 kN·m (exact)

within 0.1%

Cantilever pile (EI 320, D 0.1 m, L 5.25 m)Liang et al. (2014) exact solution / RSPile

Head deflection

8.75 mm

8.750 mm (exact)

within 0.1%

Cantilever pile (EI 320, D 0.1 m, L 5.25 m)Liang et al. (2014) exact solution / RSPile

Max moment

2.335 kN·m

2.334 kN·m (exact)

within 0.1%

Circular pile, API sand (D 0.5 m, L 10 m, H 100 kN)RSPile verification manual

Deflection & moment profile

RSPile 2018 problems #1/#2/#8/#15

chart-reading tolerance

within 1–3%

Steel pile, multi-layer slope stabilization (17 m)RSPile / TZPile / LPile

Lateral resistance

607 kN

582 kN

within ~4%

Read how we validate — and why against two independent codes

Downdrag / negative skin friction: common questions

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