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Pile Foundation Calculator

Calculate pile load capacity using static formulas. Supports driven piles (CPT, SPT) and bored piles. End bearing, skin friction, and total capacity.

Foundation Engineering Geotechnical engineers, structural engineers Commercial Intent: VERY HIGH

Engineering Formulas

End Bearing Capacity

Qp = Ap × qp
qp = 40N (driven) or 20N (bored) in kPa

Q_p: End bearing capacity (kN)

A_p: Pile toe area (m²)

N: SPT N-value at toe

Skin Friction Capacity

Qs = Σ(α × su × π × D × ΔL)
Qs = Σ(fs × π × D × ΔL) for granular

Q_s: Skin friction capacity (kN)

α: Adhesion factor

s_u: Undrained shear strength (kPa)

D: Pile diameter (m)

ΔL: Layer thickness (m)

Total & Allowable Capacity

Qult = Qp + Qs
Qallow = Qult / FS
FS = 2.5 typical

Q_ult: Ultimate capacity (kN)

FS: Factor of safety

Worked Example

Driven Pile in Dense Sand

pileType: drivendiameter: 0.5length: 15sptN: 30avgSu: 50alpha: 0.5

Base Area

Ap = π × 0.5² / 4 = 0.196 m²

End Bearing

Qp = 0.196 × (40 × 30) = 0.196 × 1200 = 236 kN

Perimeter

Perimeter = π × 0.5 = 1.571 m

Skin Friction

Qs = 0.5 × 50 × 1.571 × 15 = 589 kN

Total

Qult = 236 + 589 = 825 kN

Allowable

Qall = 825 / 2.5 = 330 kN

Result: Qult = 825 kN, Qall = 330 kN for a 0.5m × 15m driven pile

Engineering Notes

Always verify calculated capacity with a static load test on at least 1% of production piles.

For piles in rock, end bearing governs — set to at least 1m socket into sound rock.

Group efficiency for piles at 3D spacing ≈ 0.7 for friction piles in clay.

Pile driving formulas (Hiley, Danish) can verify capacity during installation but are less reliable.

For seismic areas, check pile ductility and consider liquefaction effects on skin friction.

Assumptions

• Soil is homogeneous (simplified single-layer analysis)

• Static formula method (not dynamic)

• Factor of safety: 2.5

• No group effects

• Pile is vertical (no batter)

• No negative skin friction

Common Mistakes

✕ Using unreduced SPT N-values for bored piles (apply 0.5 factor)

✕ Forgetting to include pile self-weight in structural design

✕ Using total stress (undrained) parameters for long-term loading in clay

✕ Not applying appropriate factor of safety for the design method

✕ Ignoring effects of pile installation on adjacent structures

Frequently Asked Questions

What is the difference between driven and bored piles?

Driven piles are pre-formed and hammered into the ground, displacing soil. Bored piles are drilled and cast-in-place, removing soil. Driven piles generally have higher skin friction.

What is a typical factor of safety for piles?

Factor of safety of 2.5 is typical for static formula methods. For load tests, FS = 2.0 is often used. For dynamic formulas, FS = 3.0–4.0.

How does SPT N-value relate to pile capacity?

For driven piles, end bearing qp ≈ 40N kPa. For bored piles, qp ≈ 20N kPa. Skin friction fs ≈ 2N kPa (driven) or 1N kPa (bored). These are empirical relationships.

What is the adhesion factor alpha?

Alpha (α) relates undrained shear strength to skin friction. For soft clay: α ≈ 1.0. For stiff clay: α ≈ 0.3–0.5. It accounts for soil disturbance during pile installation.

When should I use a pile foundation?

Piles are used when: shallow soils are weak, water table is high, heavy column loads require deep support, or settlement control is critical.

What is negative skin friction?

Negative skin friction (downdrag) occurs when surrounding soil settles more than the pile, dragging downward. It adds to the load and reduces effective capacity.

How are piles tested?

Piles are tested by: static load test (most reliable), PDA/CAPWAP (dynamic), or PIT (integrity). Static load test is the definitive method but expensive.

What is a pile group efficiency?

Pile group efficiency accounts for interaction between closely spaced piles. For spacing < 3D, efficiency < 1.0. For spacing > 3D, efficiency ≈ 1.0 for most soils.