Screw piles are helical columns twisted into the ground to support a load, with their strength depending on the number and diameter of plates, and the characteristics of the ground. Originally used to support lighthouses, they are now used in various applications and offer benefits such as higher load capacity, smaller equipment requirements, and lower environmental impact.
A long strong column driven into the ground to support a load is known in construction as a pile. When at least one plate is attached to the column in a helical fashion and the pile is twisted into the ground rather than hammered, the pile is specifically referred to as a screw pile. While conventional posts are driven into the ground just like nails are driven into a block of wood, screw piles are turned into the ground just like vines are turned into a block of wood.
Originally made from cast or wrought iron, screw posts are now more commonly made from steel. The strength of a screw pile depends in part on the number of plates attached to the column, as well as the diameters of the plates and the shape and length of the column itself. A screw pile helps to transfer the downward force it is subjected to into the surrounding ground, so its bearing capacity also depends on the physical characteristics of the ground and the depth at which it is anchored. Given the large variability in soil characteristics from one location to another, screw piles are designed with the specific local soil characteristics in mind.
Built in the mid-1800s, the first screw piles primarily supported lighthouses in muddy or sandy ground on the coasts of England and Ireland. Lighthouses supported in this way are also sometimes referred to as screw mast lighthouses. The first of these lighthouses was Maplin Sands Lighthouse in the Thames Estuary on the southeast coast of England.
Screw piles have since been used in a number of other applications. They can be used as anchors for cell phone towers and to stabilize sloping embankments and can support bridges, fences and walkways. Screw piles are also used as the foundation for residential and commercial structures, in dock and wharf designs, in flood control systems, to support utility poles, and even as the foundation for roller coasters.
While it may cost more initially, there are a number of benefits to using a screw-in pile instead of the conventional pile. Screw piles typically hold more load than conventional piles of a comparable size. They can be installed in much smaller and more confined spaces because the equipment required to do the job is much more compact than that used for conventional pile driving. Installing screw piles has a much lower environmental impact than installing conventional piles, because no excavation is required and no tailings are produced during the installation process; the installation process also does not involve the noise and vibration associated with conventional pile driving. This can be an especially important consideration where local regulations limit noise nuisances or where nearby structures could be damaged by vibrations caused by driving conventional piles.
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