Formula used
General: Ultimate axial capacity is the sum of shaft resistance and base resistance.
Qu = Qs + QbQa = Qu / FS
As = P · L,P = πD,Ab = πD²/4Qs = f_skin · α · su_avg · AsQb = f_tip · Nc · su_tip · Ab
σ'v(z)from unit weight and water table depthQs = f_skin · β · P · ∫₀ᴸ σ'v(z) dzQb = f_tip · Nq · σ'v(L) · Ab
How to use this calculator
- Select Clay or Sand based on governing soil behavior.
- Enter pile geometry: diameter D and embedded length L.
- Set a safety factor FS to obtain allowable capacity.
- For clay, provide su,avg, su,tip, α, and Nc.
- For sand, provide β, Nq, unit weights, and water table depth.
- Adjust fskin and ftip for installation effects.
- Press calculate to see results above the form, then export if needed.
Example data table
| Case | Soil model | D (m) | L (m) | Key inputs | FS | Qu (kN) | Qa (kN) |
|---|---|---|---|---|---|---|---|
| A | Clay | 0.60 | 18 | su,avg=50 kPa, su,tip=70 kPa, α=0.6, Nc=9 | 2.5 | ≈ 3,240 | ≈ 1,296 |
| B | Sand | 0.50 | 20 | β=0.25, Nq=25, γ=18, γsat=20, WT=2 m | 2.5 | ≈ 3,000 | ≈ 1,200 |
Examples are illustrative; site-specific parameters can change capacity significantly.
Practical notes
- Use appropriate resistance factors and design codes for your region.
- Consider negative skin friction, setup, and downdrag where relevant.
- For layered soils, use segmented analysis and verified correlations.
Professional guide to axial pile capacity estimates
1) What this calculator solves
This tool estimates axial pile capacity by combining shaft resistance and base resistance, then applies a safety factor to report allowable load. It supports clay behavior using undrained shear strength and sand behavior using effective stress, helping you compare scenarios quickly during early sizing.
2) Capacity components and reporting
Ultimate capacity Qu equals Qs + Qb. The calculator reports both parts in kN and also converts to metric ton-force for quick field communication. Tracking Qs and Qb separately is useful because construction changes often affect them differently.
3) Clay behavior: alpha method inputs
For clays, shaft resistance uses α·su,avg over the shaft area, while base resistance uses Nc·su,tip over the base area. Typical α values range from about 0.3 to 1.0, while Nc near 9 is commonly adopted for deep foundations.
4) Sand behavior: beta method and stresses
For sands, the calculator integrates effective vertical stress with depth and multiplies by β and pile perimeter to estimate Qs. Base resistance uses Nq times the tip effective stress. Many preliminary designs use β near 0.15–0.35 and Nq in the tens, depending on density and interface behavior.
5) Water table and effective unit weight
Because effective stress controls sand capacity, groundwater position can materially change results. Above the water table, stress increases with the moist unit weight. Below it, the calculator uses submerged unit weight (γsat − 9.81 kN/m³). A deeper water table generally increases σ′ at the toe and raises Qb.
6) Installation effects and reduction factors
Driven piles often develop higher interface stresses and improved shaft performance, while bored piles can suffer from disturbance, smear, or imperfect base cleaning. The inputs fskin and ftip let you apply practical reductions, such as 0.7–0.9 for shaft and 0.6–0.8 for base when conditions are uncertain.
7) Choosing safety factors for allowable load
Allowable capacity is Qa = Qu / FS. A higher FS reduces permissible load but increases reliability when soil variability, construction tolerance, or limited testing exists. Early-stage sizing often uses FS around 2 to 3, then refines it with better subsurface data and load testing plans.
8) Interpreting results and next steps
Use the output to compare pile diameter, length, and soil parameter sensitivity. If Qb dominates, toe condition and bearing factors deserve attention; if Qs dominates, interface behavior and installation matter most. For final design, check settlement, group efficiency, negative skin friction, and applicable local standards.
FAQs
1) What units should I use?
Enter geometry in meters, strengths in kPa, and unit weights in kN/m³. The calculator returns capacity in kN and also shows metric ton-force for convenience.
2) Should I choose clay or sand model for layered soils?
Select the model that best represents the governing resistance. For layered profiles, run multiple scenarios or segment the pile by layers using averaged parameters, then validate with site-specific methods.
3) What do fskin and ftip represent?
They are multipliers applied to shaft and base resistance to reflect installation and construction quality. Use lower values when disturbance, poor base cleaning, or limited verification is expected.
4) How does groundwater affect the sand method?
Groundwater reduces effective stress below the water table by using submerged unit weight. Lower effective stress reduces both Qs and Qb, especially for deeper piles with significant below-water embedment.
5) Are α, β, Nc, and Nq fixed constants?
No. They depend on soil type, density/strength, interface roughness, and design approach. Use correlations from your geotechnical report or standards, then perform sensitivity checks.
6) Why might calculated capacity differ from load test results?
Soil variability, setup effects, disturbance, installation method, and boundary conditions can change measured resistance. Load tests also reflect displacement behavior and mobilization, not just ultimate theoretical strength.
7) Does this tool check settlement or group effects?
No. It focuses on axial capacity only. For design completion, evaluate settlement, pile group interaction, downdrag, and structural capacity of the pile and connection details.
Verify inputs, then share results with your team confidently.