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HISTORY

What is a Screw Pile?

A Screw Pile is a foundation pile comprised of high-grade steel pipe segments with one or more helices structurally welded to the pipe shaft, screwed into the ground until a pre-defined bearing capacity is achieved. A screw pile can be used anywhere soil conditions suggest a conventional drive, bored or micro pile could be used, or even in place of conventional concrete footings.

Main advantages of a screw pile over driven, bored or micro piles:

quieter to install
cause negligible subterranean disturbance
can be installed even in the worst soil conditions
quicker to install
can be installed in places where others cannot
can be removed from the ground when no longer needed

Why have I not heard Screw Piles before?

Screw Piles have been used in the construction industry for more than one hundred years but because of high-grade steel was relatively expensive, they were generally only used for foundations where it was impossible to use traditional piles. Three things have happened to change all of that:

The relative price of high-grade steel has dropped,
The relative costs of manpower and concrete have risen, and
The complex computer modeling and engineering, required to optimize design and installation has become fast and affordable.

The History

In 1883, a blind engineer from Ireland called Alexander Mitchell invented the Helical Pile. Helical Piles were first used in 1836 for river moorings for ships.

Helical Piles:

The shape of Helical Piles resemble a steel shaft with a one or more helices welded to that shaft, and are equally suitable for supporting structures in both tension and compression. The technology has been shown to be capable of supporting structures such as bridges, marine piers, single and multi-storey buildings, sign pylons, street light poles, suspended slabs and generally replacing the concrete foundations previously used to anchor these structures. Helical Piles are installed with powerful drilling equipment that is either diesel/hydraulic or electric/hydraulic powered.

The new technology was used to create:

The foundations for the Maplin Sands Lighthouse in the Thames estuary in 1838.
The Brighton Pier in 1863.
Many large scale projects in the UK, USA and Australia in the early 20th century (the majority being marine-based).

Things you didn’t know about screw pilling:

Screw Piles are probably the most underrated foundation engineering feat of the 19th century.

Screw Piles were used for temporary foundations for the 2000 Olympics in Sydney and 2012 Olympics in London (the screw piles could be easily removed after the games).

Helical Pier Foundation (or Screw Piles), became one of the most widely used foundations throughout the world. While the availability of labor popularized other foundation types for a while, the simplicity and effectiveness of screw pile techniques are once again creating demand.

In nearly every corner of the world, screw piles were used to support shallow water and coastal lighthouses, ocean front piers, jetties, and bridges. They were also used to stabilize river banks and underpin buildings. During this period, screw piles were fabricated from cast iron and wrought iron, both of which were in abundant supply at the time. It wasn’t until well after the turn of the century that steel became available.

Although there were several examples of using machines to install screw piles in the late 1800’s, these did not really catch on. The rise of deep foundations was mainly due to developments in pile driving equipment that was becoming more readily available. Within the last 30 years, and more so in the last 20 years, there has been substantial and steady growth in the use of screw piles in civil construction, mainly due to the introduction of torque measuring equipment an tailor-made planetary drives units of varying capacities.

The concept of screw piles came to Mitchell in 1831, and there is little doubt that the idea was first associated with problem of safety mooring ships in harbors at the time. The idea seems to have then been applied by Mitchell to solve the problem of providing good foundations for lighthouses in soft soil. A Screw Pile’s holding power entirely depends upon the area of its disc, the nature of the ground into which it is inserted, and the depth to which it is forced. The proper area of the screw should always be determined by the nature of the ground in which it is to be placed.

In modern Geo-technical practice, the design of screw pile foundations considers all of the factors mentioned by Mitchell. This includes geometry of the screw pile; size and shape of the shaft, diameter of the helix, number of helices, the depth of the installation, and the soil characteristics at the site.

Mitchell’s first full-scale application of screw piles was to support The Trinity House Lighthouse

at Maplin Sands near the mouth of the Thames River. It wasn’t long before screw piles were

being used for a number of lighthouse projects around the coast of England and Ireland.

Soon the method was exported for use in the United States. The first screw pile lighthouse built

in the United States was at Brandywine Shoal, Delaware Bay in 1842 by the U.S. Corps of

Topographic Engineers. Nearly 60 similar structures were built in U.S. coastal waters between

1850 and 1900, along the Eastern seaboard, Florida coast, and the Gulf of Mexico.

Flexibility is the key:

One of the main advantages of modern screw piles for use in civil construction is the wide range of foundation elements and geometries available. During the Second World War, a number of important shipping wharves in Europe were built rapidly using screw piles with blades up to 8 feet in diameter.

While most modern screw piles are smaller, screw piles with helix diameters up to 4 feet may be in common use in the near future to support bridges and other heavy structures. In Mitchell’s era, the design of screw piles was by trial and error. Today, the design of screw piles are based on soil mechanics and sound Geo-technical practice combined with modern torque measuring technology and installation equipment.

Installation monitoring:

The ability to monitor the installation of every pile through checking the installation torque and rotation as the pile advances is a key feature of Screw Piling.

The required installation torque relates to the specific geometry of the pile, including the helical sections and the central shaft. The torque also relates to the interaction between the screw pile and the soil. This is important because it’s not usually possible to have a soil test at each pile location. The installation torque provides a specific log of the conditions at each location and allows for adjustments in the length or geometry to be made. This ensure that the desired pile capacity will be reached. The pile capacity is essentially instantaneous in most soils, and the pile can be used immediately to support design loads. This means that almost instant capacity comes with rapid installation, virtually no soil cuttings, and minimal site disruption.

Applications:

Historically, screw piles were used to support tension loads, compression loads, over-tuning moments and combined loading. In today’s world, the application for screw piles seems limitless and is even broader than in Mitchell’s time. In the last 60 years, screw piles have often been associated with the electric utility industry as helical anchors for guy wires for poles and towers. Screw piles have now found their way into nearly every aspect of civil construction: building foundation support for new construction, pedestrian bridges and walkways in environmentally sensitive wetlands and other areas, slope stability repair, tiebacks in temporary earth support, underpinning foundations for temporary structures, foundations for light and signage structures and wind generators, tension anchors for transmission towers and cell towers, underpinning of existing structures, foundations of bridge foundations, and a variety of other Geo-technical applications.

Since the installation of screw piles is not limited to vertical applications such as a foundation under a building, screw piles are increasingly used as tiebacks to support excavation walls and other retaining structures.

The future of screw piles:

There are now an increasing number of applications for screw pile foundations as we continue to build and develop marginal sites. Screw pile foundations are an attractive alternative to traditional cast-in-place concrete foundations. They can be designed to sustain axial compression and tension loads, lateral loads, and may be used in retrofitting existing structures, such as those damages by natural disasters. They may also be used as instant foundations for pre-engineered metal buildings and manufactured homes, or for disaster relief. Large-diameter screw piles are likely to become more common for large load capacities.

More applications are being found for the use of screw piles as tiebacks for temporary and permanent retaining structures. They may be installed quickly and easily using conventional construction equipment. In most soils they have immediate capacity, and can be installed in low-head-room or limited access situations. No soil cuttings are generated, meaning they are minimally disruptive to a site. They may be removed and reused in temporary construction, providing a sustainable technology. They are readily available in a wide range of sizes for the engineer to choose from.

It all just goes to show that a really good idea never goes out of style, and without a doubt, screw piles have come of age again.