Thursday, 7 September 2017

The evolution of screw piling.

It is said that the first recorded use of a screw pile was by Alexander Mitchell, a brick maker and engineer from Ireland who coined the term ‘screw-pile’ in 1833.

Initially the concept of screw piling was used to provide solid foundations for lighthouses in soft soil and provided enough holding power to safely moor ships in the harbour.

The original screw pile concept cannot be officially confirmed but in the April 1848 issue of the Civil Engineer and Architects Journal the article highlights the mechanics of the bearing power of screw moorings and screw piles which Alexander Mitchell is argued to have introduced: 
"The origin of the screw-pile was the screw mooring, which was designed for the purpose of obtaining, for an especial purpose, a greater holding power than was possessed by either the ordinary pile or any of the usual mooring -anchor blocks, of however large dimensions.... whether this broad spiral flange, [or ’ground screw,’ as it may be termed] were fixed upon a spindle, and forcibly propelled by rotary motion to a certain depth into the ground, an enormous force would be required to extract it by direct tension"
It wasn’t long after this that screw piles were being installed all around the coast of England and Ireland and soon after that the method was being exported to the United States of America.

During Mitchell’s era, the design and application of screw piles was by trial and error and it wasn’t until later down the line that the strategic use of screw piles begins to take place.

Screw piles were used to support tension loads, compression loads, overturning moments and combined loading – as we see in many modern foundations today.
Over the last 60 years while screw piles have often been associated with the electric utility industry as helical anchors for guy wires for poles and towers, they have 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 geotechnical applications.

A key feature of screw piles is the ability to monitor the installation of every pile by careful monitoring of the installation torque and rotation as the pile advances. Even though 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. Therefore, the resulting installation torque record provides a means of direct quality control and assurance that can be used to verify soil conditions at each pile location, as well as to provide an estimate of pile capacity through correlations between torque and capacity. This is particularly important since it is usually not possible to have a soil boring at each pile location. The installation torque provides a specific log of the conditions at each location and allows for adjustments to be made to ensure that the desired pile capacity will be reached. This also means that each pile is tested and in most cases can be used to immediately support design loads.
Australian screw piling expert and business owner Brodie Houghton of Solidity from Victoria, told Digga that they see helical piling as the next big shift in building technology, with a much needed flow on effect to both profitability of the building industry and housing affordability in general. Houghton explained: 
"Builders who are conscientious of project economics are already making the shift to helical piling in hordes. Screw piling [with Solidity*] not only eliminates costly delays to the building program by offering all weather installation ability, but further simplifies the building process by eradicating the need for additional base stage inspections and doing away with dewatering as a concern. Adding to this the reduced material cost of helical pile supply over concrete, helical piling can produce savings of up to 20%* over an equivalent bored pier foundation system."
There are a number of advantages to screw/ helical piling which we discuss in more detail here, (Advantages of screw piling) but Houghton goes on to tell us:
"When you consider the environmental impact of a project credence is often not given to a well designed and executed foundation system which significantly reduces the wider environmental impact of a project, along with the immediate effects on local ecology. The simple disturbances to the environment brought about by the excavation of 0.5-1CUM of spoil, which is then relocated to either a tipping site, or another area on the subject building site can have massive effects on local flora and fauna. Being that helical piles are a type of displacement pile, there is little to no spoil disturbed during installation." 
Today, almost any conventional piece of standard excavation equipment, such as a track or wheeled excavator, a mini excavator, a backhoe or a skid steer may be easily fitted with a low-speed, high-torque hydraulic head to provide the required torque for installation. Even in areas of limited access or low head room such as inside the basement of a structure, a small hand-held portable hydraulic torque head and a torque reaction bar can be used to install screw piles inside buildings, providing up to 5,000 nM of torque.

Digga see the installation of screw piles as a versatile alternative to traditional methods of foundations and as more applications are being found for the use of screw piles they are fast becoming an attractive alternative to traditional cast in place concrete foundations. 

You can find more advantages of the installation of screw piles here

For further reading on the evolution of screw piling check out this article which we referenced when putting this post together for you: Civil and structural engineer news article.