Pile Capacity

Pile capacity depends on how load is transferred through the pile shaft and base into the surrounding ground. The design must also consider settlement, installation effects, structural capacity, group interaction, drag load and verification testing.

The shaft-plus-base framework and arithmetic example are on the separate Axial Pile Capacity Calculation page.

Pile type and installation matter

Driven, bored, continuous-flight-auger, screw, micropile and other systems disturb and load the ground differently. Installation can change stress, density, pore pressure and interface condition. A resistance method calibrated for one pile type should not be transferred to another without justification.

Shaft resistance

Shaft resistance is commonly calculated by dividing the profile into layers and assigning a unit resistance or method-specific relationship to each layer. The calculation should record layer thickness, pile perimeter, material description, parameter source and any limits applied.

Base resistance

Base resistance depends on the founding material, pile-base area, construction quality, displacement effects and the movement required for mobilisation. Cleanliness and disturbance at the base are particularly important for bored piles.

Settlement and load mobilisation

Ultimate geotechnical resistance and serviceability performance are separate considerations. Shaft and base components may mobilise at different movements. Load-transfer analysis or load testing may be required where settlement controls.

Drag load and pile groups

Settling soil can impose negative skin friction and drag load on a pile. Groups may act differently from isolated piles because of overlapping stress zones, block behaviour, cap interaction and installation sequence. These effects should be checked separately from a single-pile arithmetic sum.

Verification

Verification may include construction records, integrity testing, dynamic testing, static load testing and comparison with ground conditions encountered during installation. The testing strategy and interpretation should be established in the project design basis.

Australian design context

Pile design in Australia commonly requires project-specific application of the current edition of AS 2159 together with structural standards, investigation data and design documentation. Licensed standards should be checked directly before making a compliance claim.

Calculation resources

• Axial Pile Capacity Calculation
• Effective Stress in Soil
• Soil Compressibility and Foundation Settlement
• Pile Assessment: Capacity, Demand and Verification
• Pile Capacity from DCP: Correlation Assessment

Engineering note

Unit shaft and base resistances are method inputs, not universal values. Their selection must be traceable to ground data, pile type, construction method, testing and the approved design procedure.