Free Web Tool

Pile Capacity (Axial)

α-method for cohesive layers, β-method for cohesionless layers, plus end bearing. Layered soil profile, capacity vs depth chart, and allowable load with FS.

Inputs

m
m
2.0–3.0 typical
m below ground
kN/m³
For clay: c = undrained shear strength su, last = α (or 0 to auto). For sand: φ in degrees, last = β (skin) override or 0 for K·tanδ default.
Methods
  • α-method (clay, undrained): fs = α·su; α from API: 1.0 if su≤25 kPa, 0.5 if ≥75 kPa, linear between.
  • β-method (sand, drained): fs = β·σ'v; β = K·tan(δ), default β = 0.27–0.50 by φ.
  • End bearing (clay): qp = 9·su,tip (Skempton).
  • End bearing (sand): qp = σ'v,tip·Nq with Nq from φ (Vesic).
  • Effective stress σ'v uses buoyant γ below water table.
  • Qult = ΣQs + Qp; Qallow = Qult/FS.

Results

Pile Perimeter—
Pile Tip Area Ap—
σ'v at tip—
Skin Friction Qs—
End Bearing Qp—
Ultimate Capacity Qult—
Allowable Qallow—
LayerTypeDepthfs (kPa)Qs,layer (kN)

About this calculator

The RHCES Pile Capacity Calculator estimates the ultimate and allowable axial capacity of a single circular or square pile through a layered soil profile. It combines shaft friction — the α-method in clay, the β-method in sand — with end bearing at the tip, then divides by a factor of safety for an allowable working load. It suits preliminary sizing by foundation/structural engineers and students, not final design.

Method & Formulas

Skin friction is integrated layer by layer along the pile shaft and multiplied by the pile perimeter; end bearing is computed once at the pile tip:

Worked Example

Circular pile, D = 0.40 m, L = 10 m, homogeneous clay with su = 50 kPa (α interpolates to 0.75):

  1. Perimeter = πD = 1.257 m; tip area Ap = (π/4)D² = 0.1257 m².
  2. fs = α·su = 0.75 × 50 = 37.5 kPa → Qs = fs×perimeter×L = 37.5 × 1.257 × 10 ≈ 471 kN.
  3. qp = 9×50 = 450 kPa → Qp = qp×Ap = 450 × 0.1257 ≈ 56.5 kN.
  4. Qult = 471 + 56.5 ≈ 528 kN. With FS = 2.5, Qallow = 528 / 2.5 ≈ 211 kN.

Assumptions & Limitations

FAQ

Does the calculator include group efficiency for pile clusters?
No — it computes the capacity of a single, isolated pile. Group capacity needs a separate group-efficiency check (e.g., Converse–Labarre) and a block-failure check for closely spaced piles.
How are clay and sand layers handled together in one profile?
The tool reads each layer's type from your input and automatically switches between the α-method (clay) and β-method (sand) at every depth increment, summing every layer's contribution along the shaft.
What factor of safety should I use?
2.0–3.0 is typical for static axial capacity, depending on the level of site investigation and load-test verification — check your governing geotechnical code or project specification for the required minimum.