There’s an old saying that "history repeats herself." It seems that even in geotechnical engineering and foundation construction the adage is true. In the middle of the 19th century, screw pile or helical pier foundations became one of the most pre-eminent types of foundations used 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.
A screw pile consists of one or more helical plates attached to a central shaft—the helical plate being one revolution of an Archimedean screw. In nearly every corner of the world, including the United States, screw piles were used to support shallow water and coastal lighthouses, ocean front piers, jetties, and bridges, and 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 and already available in the United Kingdom and the United States. It wasn’t until well after the turn of the century that steel became available.
Screw pile foundations reached their maturity around 1900 and then, shortly after the turn of the century, there was a steady decline in the use of screw piles as other foundation methods became popular. These included open-hole caissons, hand-dug drilled piers, various types of driven piles, enlarged base piers and piles, and a variety of proprietary deep foundations, such as the Gow foundation, Raymond Piles, and Franki Piles. The decline in use of screw piles was also probably related to the rise of the mechanized world of construction and the fact that installing screw piles by hand labor was superseded by machines to drive piles. Although there are several examples of using machines to install screw piles in the late 1800s, these did not really catch on. Much of the rise of deep foundations was the result of significant developments in pile driving equipment that was becoming more readily available. However, within the last 30 years, and even more so in the last 20 years, there has been substantial and steady growth in the use of screw piles in civil construction.
|Installation of multi-helix screw piles is easily accomplished using traditional construction equipment.|
Alexander Mitchell and the first screw pile
Most historians agree that screw pile foundations were introduced as a full-scale practical foundation system by Alexander Mitchell (1780-1868), an Irish builder and brick manufacturer. The concept of screw piles may have come to Mitchell as early as 1831, but there is little doubt that the idea was first associated with the problem of safely mooring ships in harbors at the time and that the idea seems to have then been applied by Mitchell to solve the problem of providing good foundations for lighthouses in soft soil. He considered the use of a single helical blade screwed into the ground for both applications.
The April 1848 issue of the Civil Engineer and Architect’s Journal gives a synopsis of a paper read by Mitchell at the Feb. 22, 1848, meeting of the Institution of Civil Engineers. It states that, "The origin of the screw pile was the screw mooring, which was designed for the purpose of obtaining, for an especial [sic] purpose, a greater holding power than was possessed by either the ordinary pile or any of the usual mooring-anchors or blocks, of however large dimensions."
Mitchell also provided some insight into the mechanics of the bearing power of screw moorings and screw piles. "Whether this broad spiral flange, or ’ground screw,’ as it may be termed, be applied to the foot of a pile to support a superincumbent weight, or be employed as a mooring to resist an upward strain, its 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 beneath the surface. The proper area of the screw should, in every case, be determined by the nature of the ground in which it is to be placed, and which must be ascertained by previous experiment."
In modern geotechnical practice, the design of screw pile foundations considers all of the factors mentioned by Mitchell such as the geometry of the screw pile including size and shape of the shaft, diameter of the helix and 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. And 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. In the United States, commonly available screw pile geometries include both single- and multi-helix configurations with either square or round shafts and with helix diameters ranging from 6 to 24 inches. Common square shafts are typically on the order of 1-1/2 to 2-1/4 inches and common round shafts are between 2-7/8 and 8 inches. Some large piles are available with blade diameters as large as 36 inches for specialty work. Larger shafts and thicker walls in the case of round shafts are capable of supporting higher axial loads and larger bending moments in the case of eccentric loads. Round shaft screw piles may be filled with grout or concrete, or a grout column may be constructed along the square shaft, to form a grouted micropile for added bending capacity. These piles are still smaller than those used historically. To support lighthouses, bridges, and piers in the days of Mitchell, screw piles routinely had helical plates on the order of 2-1/2 to 4 feet in diameter.
During the Second World War, a number of important shipping wharfs in Europe were built rapidly using screw piles with blades up to 8 feet in diameter. While most modern screw piles are smaller, there is good reason to believe that 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 some parts of the world the limitation of size may be the availability of high-capacity torque installation equipment that may need to be retrofitted from the drilled shaft industry to install screw piles. Extension shaft lengths are typically 3 to 10 feet and can be added to the lead section so that the total length of a screw pile is really only limited by the ability to install the pile and the structural strength of the member. In Mitchell’s era, the design of screw piles, like most other foundations, was by trial and error, with a certain amount of failure expected. Today, the design of screw piles, like all foundation design, is based on soil mechanics and sound geotechnical practice.
The installation of screw piles is about as versatile as the available geometries. Historically, a work crew turned a capstan that screwed the piles into the ground, but during the late 1800s this was abandoned for crude steam and hydraulic motors. During World War II, large torque heads driven by electric motors were used to rapidly install screw piles for foundations of wharfs in harbors. 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 foot-pounds of torque.
Another 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 in the length or geometry to be made to ensure that the desired pile capacity will be reached. This also means that a torqued screw pile is a tested screw pile. In fact, 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 to dispose of, and minimal site disruption.
Historically, screw piles were used to support tension loads, compression loads, overturning moments, and combined loading—not unlike loads experienced by many modern foundations. 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. However, screw piles 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.
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. Small-diameter soil screws can be installed quickly into almost any type of soil, except gravel, to provide lateral earth support for a variety of construction applications.
The future of screw piles
There are now, more than ever, 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. Since they can be designed to sustain axial compression and tension loads—as well as lateral loads—they may be used in retrofitting existing structures, such as those damaged by natural disasters including earthquakes, floods, and hurricanes. They may be used as instant foundations for pre-engineered metal buildings and manufactured homes, or for disaster relief, including housing involving prefabricated housing units or temporary bridges. 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. They can be installed in low-head-room or limited-access situations. They generate no soil cuttings and are therefore minimally disruptive to a site. They may be removed and reused in the case of 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. Without a doubt, screw piles have come of age—again.
Alan J. Lutenegger is a professor of Civil and Environmental Engineering at the University of Massachusetts in Amherst, Mass. He can be reached at firstname.lastname@example.org. James Kempker is a territory manager with Chance Civil Construction and Hubbell Power Systems in Centralia, Mo. He can be reached at email@example.com.