Pile Capacity Calculator to AS 2159: Shaft and Base Resistance, Alpha and Beta Methods, Design Strength

A first estimate of what a single pile can carry from the soil.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Choose clay or sand and enter the pile size and length, the soil strength parameters and the design load.

For clay the shaft resistance is alpha times the undrained strength over the shaft area, and the base resistance is 9 su times the base area. For sand the shaft is beta times the effective vertical stress, and the base is Nq times the stress at the tip.

The ultimate strength is multiplied by the geotechnical reduction factor phi_g from the AS 2159 risk assessment to give the design strength, which is compared with the design load.

Formulas used

  • R_s = \alpha\,s_u\,p\,L\ \ (\text{clay})
  • R_b = 9\,s_u\,A_b\ \ (\text{clay})
  • R_s = \beta\,\sigma'_{v}\,p\,L\ \ (\text{sand})
  • R_b = N_q\sigma'_{v}A_b\ \ (\text{sand})
  • R_{d,g} = \phi_g R_{d,ug}

Worked example

With the default numbers in the calculator above:

  1. Perimeter and base area of the pile: 1.41 m and 0.16 m2
  2. Unit shaft resistance by the alpha method (AS 2159 Section 4 and Appendix): 25.00 kPa
  3. Ultimate shaft resistance: 424.12 kN
  4. Ultimate base resistance in clay: 71.57 kN (end bearing pressure 450.00 kPa)
  5. Ultimate geotechnical strength: 495.68 kN (shaft 85.56 percent, base 14.44 percent)
  6. Design geotechnical strength (AS 2159 cl 4.3: R*d,g = phi_g R_d,ug): 247.84 kN

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

Which standard is it?

AS 2159:2009 Piling: Design and installation, Section 4, which uses the design geotechnical strength R*d,g = phi_g R_d,ug. The alpha and beta methods are the common textbook methods.

Where does phi_g come from?

From the risk assessment of AS 2159, which depends on the quality of the site investigation, the type of analysis, the pile testing and the installation control. It is between about 0.4 and 0.9.

Does this include the strength of the pile itself?

No. The structural strength of the shaft, settlement, groups, negative skin friction and lateral loads are separate checks.

What soil values do I need?

The undrained shear strength for clay, and the effective unit weight, beta and Nq for sand, all from a geotechnical report.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: AS 2159:2009 Piling: Design and installation; Tomlinson and Woodward, Pile Design and Construction Practice; Fleming et al, Piling Engineering.

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