Helical Piles: An Emerging Two Century Old Deep Foundation Solution

A helical pile is a deep foundation solution suitable for supporting light and heavy structures. They are engineered to support compressive loads less than 1kN (225 lb) or in excess of 5,000kN (1.125 million lb) - from a small landing to a 32 story tower, helical piles are an option. Another common name for helical piles are "screw piles".

A helical pile consists of a round steel shaft and one or more helical bearing blades which are welded to the shaft. The length, thickness and diameter of a shaft varies based on several factors as will the number, thickness and diameter of helical piles. Regardless of dimensions, the helical should be formed with a consistent pitch (plane or angle) to ensure reliable design and installation methods which are consistent with generally accepted engineering guidelines.

What is the History of Helical Piles?

Helical piles are often considered the new kid on the block and the local engineering network might not have received much training or education on the design, installation or general engineering principles specific to helical piles. However, helical piles have nearly two centuries of historical use. Invented in the 1830's in the UK by Alexander Mitchell, helical piles were first engineered to support marine structures such as lighthouses and boat moorings in the ocean.

In the late 1800's and early 1900's hundreds of lighthouses were constructed along the eastern seaboard of North America on helical piles - remarkably, some of them still stand today.

There are few projects where the soil conditions would prove more challenging than building in an ocean. Recognizing that helical piles were performing well in such a challenging environment, the engineering community in the late 1800's began to leverage helical piles mainland.

In the early 1900's nearly all deep foundation options required a lot of labour to install and helical piles were no exception. Cap stands were constructed and powered by many men or animals who would turn in circles to advance helical piles. By 1920 a new and slick steam powered pile driver was available and made it very efficient, for the era, to lift and drop heavy weights to pound timber into the ground. The result was that helical piles which were growing in popularity to this point, fell out of favour and were replaced by easier pile driving methods.

A variation of helical piles did emerge and is known today as a ground anchor. The utility sector was expanding rapidly and required lateral support of transmission poles with guywires secured by anchors. Helical piles were well known for their ability to support compressive and tensile loads but, as mentioned above, they were laboursome to install. The utility sector found that if they greatly increased the pitch of the helical, they became much easier to install. The ability of a ground anchor vs a helical pile to support compressive loads was greatly diminished as the engineering community found ground anchors were prone to self-advancement under compression.

After several decades of declining usage, helical piles once again emerged as a viable option as hydraulics and planetary gears and high torque drives came to market. Slowly, helical piles emerged back into the deep foundation market.

In 2011, when I was researching helical piles and considering starting a helical pile contracting business, I learned that the US Army Corps of Engineers included helical piles in their foundation design manual back in the 1980's. Helical piles were widely used in 2011 in the light residential market but also on the opposite end of the scale in the ultra high capacity and high liability energy sector. Two years prior to my entering the helical pile market, Howard A. Perko, PhD, PE published the very comprehensive engineering manual "Helical Piles - A Practical Guide to Design and Installation".

Fast forward to 2021, helical piles have grown exponentially over the past decade across various markets. You will now see helical piles in International Building Code and the authority on piles, the Deep Foundations Institute (DFI), assembled a group of 50 helical pile experts and released the "Helical Pile Foundation Design Guide". Perko's and DFI's principles for designing and installing helical piles compliment each other well and have now been applied and proven millions of times across North America and the World.

How are Helical Piles Designed

In designing any foundation, it is a best practice to engineer to the weakest link which is predominantly the soil, and therefore it is highly recommend to start with a geotechnical investigation and analysis. Armed with soil data, a geotechnical engineer, using empirical methods outlined in the DFI and Perko manuals as well as foundation engineering manuals in various countries, can reliably design helical piles to satisfy design criteria.

A benefit to helical piles is increased predictability of performance due to an empirical relationship between installation torque and soil capacity. The relationship between installation torque and capacity is very reliable but, when liability is high, it is not a replacement to a geotechnical investigation. A good comparison would be a driven pile is installed to resist a certain amount of force which makes them a reliable pile in terms of performance - again, the installation force is not a replacement for geotechnical investigation. When capacities and liabilities are low, such as a residential deck, it would be uncommon to justify the cost of a geotechnical investigation and in this situation an engineer might design helical piles according to installation torque.

Most pile options are simple in design when you compare them to helical piles. Imagine you have a board with dials in front of you which you can turn up or down. Your dials for designing a precast concrete pile are simply pile diameter and length; it's pretty straightforward. For a helical pile the board has several dials, shaft diameter, shaft thickness, shaft length, number of helices, diameter of helices, thickness of helices as well. Therefore, helical piles are quite customizable and the experience of a design engineer with the technology will really play a factor in getting the most efficient pile possible to site that will satisfy the final design and the forces which will be encountered during installation.

How are Helical Piles Installed

Today, helical piles are installed with a wide range of hydraulic equipment. Light capacity helical piles are installed with portable hydraulic torsion bars or towers, mini-skid steers, skid steers, mini excavators, midi excavators, and utility sector style auger trucks (Digger Derricks). High capacity helical piles are installed primarily with 15 tonne or larger excavators but sometimes with modified drill rigs or custom equipment.

The machine chosen should be appropriate for the anchor drive which will turn the pile in the ground. The swing resistance, auxiliary hydraulic flow and pressure, reach, break-out force and downward axial force will all play a role in ensuring a helical pile is properly installed. High capacity helical piles are generally installed to refusal, a machine like a skid steer will not have the downward axial force for the helical pile to bite into hard layers like till - soft till is particularly difficult to penetrate without a proper machine.

A popular image is of a mid capacity helical pile being installed with a machine as small as a skid steer or tracked loader. It is cool that a pile can be installed with such small equipment but it's not necessarily the right equipment. A skid steer, by design is made for lifting, they lack the swing resistance and force required to accurately and correctly install helical piles in many situations and could result in piles that are out of plumb and that auger the soil during install. Ideally a helical pile is within 2 degrees of plumb and installed in a manner that the pile advances in direct proportion to the pitch of the helical blade(s) disturbing the least amount of soil possible. Excavators are made to dig in and are much more capable of applying downward force to achieve a professional installation. Hoyt and Clemence tested helical piles installed in an unprofessional manner to those installed by proper equipment in a professional manner, the same soil and same depth, however there was a 50% difference in performance in tension!

When liabilities are high, the addition of on-site confirmatory load tests will further increase the confidence in the design and installation. A site where piles are being load tested can result in leaner design based on the actual performance of the piles.

An additional step we might also see on higher liability project sites is the presence of a third party engineer to supervise the installation. If you are working with a helical pile contractor who you do not have a well-established relationship, it is a very good idea to supervise the installation and verify that the piles are not auguring the soil and the installation torques required are being achieved.

A piling report with achieved depths, torques and any observations which might relate to performance, should always be completed by a helical pile contractor regardless of the type of project.

Example of installing light capacity helical piles to satisfy 22kN (5000 lb) loads.

Example of installing high capacity helical piles to satisfy 1,000kN (225,000 lb) loads.

To view an example of helical piles being installed to till in Winnipeg, MB for a large custom home, click this link: https://youtu.be/koP0mgNto_U

The Benefits of Helical Piles

Helical piles are growing in popularity in an exponential manner due to the many advantages they offer. In short, helical piles are a very versatile deep foundation and this versatility results in a long list of benefits, including but not limited to:

• Efficient installation of approximately 30 seconds per lineal foot of pile on a typical project site;
• Ability to instantly load the piles - no curing or heating and hoarding in the winter;
• They are clean - there is no predrilling or tailings/spoils, the pile is simply turned into the ground;
• Easily remediate the project site - helical piles can be easily uninstalled, reused or recycled;
• Quiet and low vibration installation will not disrupt neighbours or wildlife;
• Installation with smaller, more nimble equipment than traditional piles means savings on mobilization and easier accessibility of problematic project sites;
• Extending helical piles is fast and cost-efficient - instead of potentially exponentially higher costs to go deeper, helical piles are proportionally more expensive;
• Ability to pass through saturated, sloppy and sloughing soils layers without issue;
• Superior resistance to frost heave;
• Work well in tension and compression;
• May be designed as a floating pile or installed to refusal as an end bearing pile;
• Can be galvanized, made thicker or have anode systems added to increase the life of the piles by centuries;
• Easily modified in the field to suit unforeseen site conditions (example: a helical can be made smaller in diameter if soils are more dense than predicted);
• Various installation methods and equipment make it possible to install helical piles under existing structures or in very limited access locations;
• Light weight and easy to transport;
• A wide range of pile caps and transitions can be engineered for helical piles to support nearly any beam or structural element;
• Ability to install through frost for efficient winter building;
• Empirical relationship between installation torque and capacity mean predictable performance;
• The ability to uninstall a pile means you can relocate a pile which meets an obstruction like a boulder vs abandoning it;
• The ability to change many design aspects often results in a value engineered pile that is effective as well as efficient;
• Installation on a batter to combat high lateral forces or to combine pile groupings in a tight space; and
• Other advantages not listed.

The Disadvantages or Limitations of Helical Piles

There are no silver bullets in the world of deep foundations and helical piles are no exception. The advantages of helical piles are many but some of the limitations or drawbacks, include but are not limited to:

• Helical piles are not often included as regular curriculum for structural or geotechnical engineers and therefore expertise in helical piles is considered niche;
• Universal guidelines are available but not always utilized which results in manufacturers relying on proprietary data which often lacks testing in a wide range of soils conditions (this is avoided when insisting on universally accepted engineering principles for helical piles);
• The existence of other variations of screw piles, such as augered or duckbill piles, ground anchors and ground screws add confusion to the market, these piles lack standard design and universal engineering principles do not exist or apply to these similar but very different types of screw piles;
• Helical piles are not a viable option when there is shallow refusal (impenetrable very dense strata) at or before 1.5 meters from grade;
• The predictability of torque and capacity when installing in boulders is lowered, load testing would be required in this situation;
• Performance may be greatly influenced by the ethics of the helical pile contractor, their experience, technical knowledge/engineering pedigree and equipment utilized;
• Helical piles have smaller shafts compared to other pile options resulting in lower resistance to lateral and/or bending forces (this can be overcome with various strategies including pile caps, larger shafts near surface, grillages or battered resistance piles);
• When pile must be installed through very dense layers to achieve desired install depths, helical piles may need to be constructed more robustly to resist the installation torques or predrilling may be required;
• Helical piles can not penetrate bedrock;
• Require quality mill grade steel and certified welds; and
• Other disadvantages vs alternative pile options depending on a range of factors such as the availability of steel, familiarity of local engineers or building authority with helical piles, etc.

Helical Piles in Summary

The versatility of helical piles combined with sound established engineering principles means that helical piles are an excellent arrow for any design engineer's quiver. Helical piles are growing at an exponential pace as is the confidence in helical piles within the general engineering community. Helical piles have nearly two centuries of engineering history with a great track record for performance, however universal guidelines for helical piles are available but lacking in terms of enforcement. It is important to choose a strong helical pile team (install contractor, expert engineer, and reputable manufacturer) to ensure all the advantages of helical piles will be made available to your project.

To view the article on Structural Engineering Basics website, please visit: https://structuralengineeringbasics.com/helical-piles