How to Design Screw Piles According to Building Codes

Displacement piles are characterized by being installed without excavation or soil removal—except for minor ground heaving or vibrations during obstruction removal or to assist with installation. The soil that accumulates on the surface during the installation of displacement piles is a result of the displacement effect and the lack of ground loading near the surface. It does not constitute a significant amount of removed soil.

Classification of screw pile foundations - photo caption.

Screw pile (en: screw pile, de: Schraubpfahl) - a pile in which the pile or the pile casing has several auger flights at the bottom end and is embedded by the simultaneous action of torque and pushing force.
Screw piles are made of closed circular steel profiles with multiple threads that provide a self-tapping mechanism during installation.

Screw piles should be designed taking into account:

geotechnical capacity (external),
element capacity (internal), and
durability (corrosion resistance during use).

Design of screw pile foundations - photo caption

Geotechnical design is usually based on one of two popular methods - test loads or analytical calculations. Geotechnical surveys of appropriate quality are essential for the proper design of foundations using either method.

Methods for verifying external capacity - photo caption

Static pile load tests, during which the pile is subjected to a static compressive or tensile (pullout) load acting along its axis to determine the load-displacement relationship. The testing procedure according to EN ISO 22477-1 can be a single-cycle or multi-cycle procedure.

Single-cycle procedure - The load should be applied incrementally, with the same duration for each load step. The test load should be applied in at least eight load steps. When approaching the ultimate capacity, the size of the load step can be reduced to more accurately determine the pile settlement under the load corresponding to the ultimate capacity.

Multi-cycle procedure - The load should be applied in at least two cycles. The maximum load in the first cycle should correspond to the characteristic value of the pile reaction (Fc,k). The maximum load in the last cycle corresponds to the test load (Fp). The test load should be applied in at least eight load steps.

Analytical method, is the verification of the axial capacity of the pile based on skin friction and toe resistance. The determination of axial pile capacity based on empirical data must be performed in such a way that it is possible to reliably assign characteristic empirical data obtained from previous load tests to the results of the pile toe capacity qb,k and pile surface friction qs,k (empirical data). For this purpose, EC 7-1 provides the following basic equations:

The suitability of the given empirical data, as well as other general data from experience, must be confirmed by a geotechnical expert or geotechnical designer with respect to the proposed application.

The calculations should also take into account the so-called model factors or model coefficients $\eta$ which depend on the type of pile toe and shaft. They result from the pile installation technology.

If an additional reinforcing module in the form of a plate with an external helix (the so-called vertical booster) is used, its capacity should also be additionally taken into account in the calculations.

There are many factors influencing the final capacity of screw piles, most of which require detailed consideration during the design phase. The best results can be achieved by using high-quality Winkelmann Foundation Screw piles.