Understanding how load capacity works in screw piling and why it is verified differently to concrete methods
Load capacity in screw piling is achieved, calculated, and verified in a fundamentally different way to concrete piling methods, and understanding those differences is important context for anyone commissioning or managing a screw piling project. The helical plates bearing against the surrounding soil, the torque to capacity relationship that allows real time verification during installation, and the influence of ground conditions on the reliability of that relationship all need to be understood clearly before pile capacity is relied upon as the basis for a structural foundation design. We work closely with structural engineers to make sure the capacity assessment is right for every project, and for clients without a structural engineer our in house design team can manage the full capacity assessment and pile design from the ground investigation data upwards.
The key principles behind screw pile load capacity
Screw pile capacity comes from the bearing resistance of the helical plates against the soil they are bearing on and the shaft friction developed along the pile length between the plates. The helical plate bearing resistance is the dominant contributor in most ground conditions, and it increases with the plate area, the number of plates, and the shear strength or density of the soil at the plate depth. Shaft friction between the plates contributes a smaller but still meaningful proportion of the total capacity in cohesive soils.
Capacity is verified in real time during installation through the torque to capacity relationship, which correlates the measured installation torque to the static load capacity of the pile in the specific ground conditions and pile geometry. This means every pile on the project has a documented and objective capacity verification record produced during installation, without the need for a separate post installation load testing programme in most cases. This is one of the most practically distinctive aspects of screw piling and one of the reasons it is increasingly specified on projects where foundation performance needs to be demonstrated clearly and efficiently.
How screw pile capacity is calculated and verified during installation
The structural engineer calculates the design capacity of each screw pile using the soil parameters from the ground investigation and the specific pile geometry, including the shaft diameter, the helix diameter, the number of plates, and the design depth. In cohesive soils the calculation uses the undrained shear strength values from the investigation to estimate the plate bearing resistance and shaft friction contribution at each plate depth. In granular soils, SPT N values or CPT cone resistance values are used with appropriate empirical correlations for the specific soil type and pile geometry.
The torque to capacity relationship is then derived from the pile geometry and the ground conditions, giving a minimum installation torque that must be achieved at or beyond the minimum design depth for each pile to be accepted. During installation the torque is monitored continuously and the final torque is checked against the minimum criterion before the pile is accepted and the rig moves on. The installation record for each pile documents the torque profile with depth and the final torque achieved, providing the structural engineer and client with a complete and objective record of capacity verification across the full pile group.
Where a higher level of verification is required, or where the ground conditions introduce uncertainty into the torque to capacity relationship, load testing can be specified on a proportion of piles to provide direct confirmation of capacity. This is more common on larger commercial projects or on sites with variable or uncertain ground conditions where the engineer requires a greater level of confidence before relying on the torque criteria across all piles.
How capacity requirements vary across different project types and structural applications
On a residential project with modest structural loads in cohesive ground, the capacity requirements are relatively straightforward to achieve with screw piles at the shaft diameters and depths we work with. The torque to capacity relationship in London Clay is well established and reliable, and the installation records provide clear and objective evidence of capacity for every pile without the need for supplementary testing in most cases.
On a commercial project with greater structural loads or more complex loading conditions, the capacity assessment is more detailed and the pile specification may involve larger helix diameters, multiple plates, or greater depths to achieve the required capacity in the specific ground conditions. Where lateral loads are significant, the capacity assessment needs to consider the pile's resistance to lateral forces and bending moments as well as its axial capacity, which involves a more detailed structural analysis of the pile behaviour in the ground.
The technical parameters that govern screw pile capacity calculations
Screw pile capacity calculations follow established methods developed specifically for helical piles, with the cylindrical shear method and the individual plate bearing method being the two most commonly used approaches depending on the pile geometry and the spacing between helical plates. Where the plate spacing is less than three times the plate diameter, the cylindrical shear method is typically used, treating the soil enclosed between the plates as part of the pile. Where the spacing is greater, the individual plate bearing method is used, calculating the bearing resistance of each plate independently.
Using shaft diameters of 60mm, 76mm, and 89mm with helix configurations tailored to the ground conditions, the capacity range we can achieve covers the structural requirements of most residential and light to medium commercial applications. The specific capacity for any pile position depends on the soil parameters at that location, the pile geometry, and the design depth, and it is calculated for each pile position individually rather than assumed to be uniform across the site.
What to be aware of when relying on torque based capacity verification for screw piles
The torque to capacity relationship is reliable when it has been correctly derived for the specific pile geometry and ground conditions and when the installation equipment is performing consistently throughout the programme. Where the ground conditions are more variable than the investigation predicted, where obstructions affect the installation at certain pile positions, or where the installation equipment is not well maintained and the torque readings are not accurate, the relationship between measured torque and actual pile capacity becomes less reliable and the installation records need to be reviewed more carefully.
On sites where the ground conditions introduce genuine uncertainty into the torque to capacity relationship, supplementary load testing on a proportion of piles provides the additional confidence needed before the full pile group is accepted. The cost of this testing is modest relative to the cost of foundation remediation if pile performance is subsequently found to be inadequate, and on projects where the structural loads are significant or the ground conditions are variable it is a worthwhile investment. If you are unsure whether supplementary testing is appropriate for your project, it is a question worth raising with your structural engineer at the design stage, and something our in house team can advise on if no structural engineer is yet in place.
