Why Helical Piles?

Today Bob Kaleita and Phil Valachovic installed helical piles for a small privacy fence that will conceal the propane tank, generator, mini split compressor, etc. behind Rosslyn’s carriage barn. I’d like to dilate today’s update with a more detailed explanation of the helical piles concept and utility as an alternative to precast or poured concrete footings. So consider this a continuation of my previous post about installing helical piles (aka helical piers) when building the icehouse decks. This installment will showcase photos from the newly installed enclosure fence posts, but I’ll try to augment this benchmark with useful information for anyone else considering using helical piles for their construction project.

First let’s backtrack a tiny bit for a micro-prologue.

I learned about “earth anchoring” with helical piers (aka helical piles) some years ago when looking at real estate in Santa Fe. This technique for securing (and re-securing if settling, erosion, etc. has undermined structural integrity) foundations, footings, etc. with *giant corkscrews* piqued my interest. (Source: Icehouse Rehab 5.5: Helical Piers – Rosslyn Redux)

I’d spoken with Bob Kaleita over the last few years about helical piles, and I’d encouraged him to consider investing in the equipment. As luck would have it his decision to add helical pile installation to his bag of tricks coincided with my need to build the icehouse decks. Perfect pairing!

What Are Helical Piles?

Let’s start with the basics. Helical piles really are like giant corkscrews. That’s the way I usually explain them, at least. Here’s a better explanation.

Helical piles are hollow steel tubes that are used to stabilize structures by anchoring the load to the surrounding ground, much like a driven pile. However, helical piles are screwed deeply into the earth, section by section if necessary. They are screwed into the ground until it’s proven that the area can properly hold the required bearing capacity of the structure. (Source: Helical Piles 101: Strengthening Foundations)

That’s about as succinct as possible. This next explanation offers a more technical overview (but fair warning: skip if you’d rather not get bogged down in details!)

A helical foundation consists of at least one helix-shaped steel bearing plate attached to a central steel shaft. The shaft is typically solid steel bar (12 to 23 inches square) or heavy wall pipe (2f to 8 inches in diameter). The helix plates are high-strength steel (6 to 16 inches in diameter, d or 2 inch thick). Each helix is circular in plan, formed into a screw thread with a defined pitch (typically 3 inches).

Installation is by hydraulic motors mounted to virtually any type of machine. Portable equipment is available for such tight access areas as crawl spaces, basements, and narrow alleyways. Percussion drilling equipment is not used. The five to 25 rpm high-torque motor provides rotational energy and the machine provides crowd (down pressure), necessary for installation. The helical foundation is rotated (screwed) into the ground to advance one pitch distance per revolution. Helical foundations can be fully extendable; so that the helix plates can be installed to any specified bearing depth.

A helical foundation can be used to resist both uplift and compression loads. Installed to proper depth and torque, the helix plates serve as individual bearing elements to support a load. The central shaft, which transmits torque during installation, now transfers axial load to the helix plates. The central steel shaft also provides resistance to axial load via skin friction and to lateral loads via passive earth pressure. (Source: The Basics of a Helical Deep Foundation)

So now that you understand what helical piles are, let’s consider their utility.

Why Use Helical Piles?

There are actually many reasons why to use helical piles in construction, but I’ll save you my laundry risk by deferring to a better explanation (that offers yet another description of what helical piles are.)

Helical piles (helical piers, screw piles, earth screws, etc.) are a versatile, fast, durable and cost-effective deep foundation technology.

Helical pile foundations are segmented deep foundation systems with helical bearing plates welded to a central round or square steel shaft. The load of a structure is transferred from the shaft to the soil through these bearing plates…  Helical piles work for both compression and tension loads.

Helical piles mean:

• structural stability in loose or poor soils,
• no vibrations installation,
• fast installation (no cure times),
• no spoils,
• using smaller equipment to install,
• no disruption to surrounding area,
• safe for environmentally sensitive areas,
• installation in small or enclosed spaces.

(Source: Earth Anchoring Suppliers)

Let’s augment that list with some additional (and some overlapping) benefits of using helical piles.

The helical pile foundation is preferred over driven piles, drilled shafts, grouted tendons, auger-cast piles, belled piers and other deep foundation systems for a number of compelling reasons:

• Quick installation and immediate loading
• Pre-Engineered
• Eliminates high-mobilization costs associated with driven piles, drilled shafts or auger cast piles
• Instant torque-to-capacity feedback for production control
• Easily modified in the field
• No need for concrete to cure
• Uses common construction equipment that is easily fitted with hydraulically-driven torque motors
• Easy to use in limited access sites, high water tables, and weak surface soils

(Source: Helical Pile Association)

The efficiency of installation was a big driver for my decision to use helical piles for the icehouse decks. And the fact that the earth is not disrupted as it would be by excavation for precast or poured-in-place footers means that they ground doesn’t settle slowly after construction. The ground stays compacted and level, minimizing declivities that can collect water. Both of the benefits served us well today. We were operating in tight quarters with underground gas and waterlines and limited access due to carriage barn and stone walls. And yet the entire installation was started and completed within a few hours, and there was nominal disruption to the site. Plus isolating fence posts from the damp soil, snow, and ice will dramatically extend longevity while the steel structural framework for the fence ensures that it is robust and resilient. Win, win, win.

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. (Source: Helical Piles: An Emerging Two Century Old Deep Foundation Solution)

Fascinating, right? So why did they become less common?

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. […]

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. (Source: Helical Piles: An Emerging Two Century Old Deep Foundation Solution)

Are you sold on the use of helical piles? If you’re interested in more thorough history, technical design, installation, benefits, and limitations, check out this helical pile section in Structural Engineering Basics.

And you might also learn something from this video about how to integrate deck footings with helical piles. It’s quite similar to what we did last winter.

I’ll explain further how we’ll be integrating our privacy fence with the helical piles in a separate post once we get started. More soon!