A Simple Equation That Unlocks Your Helical Pile Foundation

November 26, 2021

One of the myths that swirls around helical piles is that they haven't been studied or researched well enough to use in critical infrastructure.

Of course, if you're familiar with helical piles you know this myth has no legs to stand on.

Helical piles have been intensely engineered and improved over the almost two-centuries since their invention. Of all the advancements and discoveries, the most intriguing has been our increased understanding of installation torque.

This humble, often-misunderstood aspect of helical piles has enormous impact on how helical pile foundations are designed and built.

What is Installation Torque?

Installation torque measures the rotational force needed to install a helical pile. The more torque it takes to install, the more resistance that pile is receiving from the soil.

The capacity-to-torque relationship can be expressed in this formula:

Q_t=K_tT

Where:

Q_t is the ultimate capacity of the helical pile

K_t is the capacity-to-torque correlation factor (more on this in a moment)

T is the average installation torque

Since the 1960's, engineers have understood there was a relationship between installation torque and ultimate pile capacity. But, it wasn't until 1989 that an empirical relationship between torque and capacity was established. As a result capacity-to-torque correlation factors were first defined, represented by Kt in the formula.

This empirical relationship would continue to be studied and improved by academics and companies. Capacity-to-torque correlation factors were researched and adopted into publications like the Canadian Foundation Engineering Manual (CFEM) and ICC-ES AC358. Today, the relationship between helical pile torque and ultimate capacity is well-understood.

There are certain instances where capacity-to-torque correlations aren't as accurate. But these cases are due to specific site and soil conditions. Any honest helical contractor will be transparent about the soil and how it will affect the foundation design. If capacity-to-torque correlations don't apply to the site, a helical contractor has several options to overcome the limitation. I don't have space to dive into those options here, but I may write a future article covering them. Let me know if you'd be interested in that article!

For now, back to the main point.

By using capacity-to-torque correlations, designers can calculate in advance how much torque the helical pile needs to reach desired capacity. That alone is useful for the design and planning of a foundation.

But installation torque can do a whole lot more for your project.

What Installation Torque Can't Do

Before we get to all the good stuff, I want to be clear about something:

Some companies will say installation torque confirms capacity.

This is patently untrue.

Installation torque only indicates when desired capacity has likely been reached. The only thing that can confirm helical pile capacity is engineer-monitored load testing.

(Image: A hydraulic jack provides compressive force to test some of VersaPile's high-capacity helical piles)

It might seem like an odd thing to get picky about. Unfortunately I have to be clear about this because some helical pile companies continue to play fast-and-loose with torque. They use it as a shortcut to confirm capacity and get lazy with geotechnical reports.

This is not how installation torque should be used.

Installation torque isn't a shortcut or an excuse to be complacent with getting good geotechnical reports. In fact, without a strong geotech, you won't be able to fully unlock the power of commercial helical piles.

What Installation Torque Can Do

When you combine installation torque with a great geotechnical report, you unlock a powerful team.

Geotechnical engineers are smart, but they can't see underground. So, they have to make assumptions about the soil. These assumptions are based on experience, study, and good science. Despite their best efforts, even the best geotech engineer will make a wrong assumption occasionally.

This is where installation torque is a massive help.

Installation torque can either confirm or refute the assumptions in the geotech report. In a way, it gives us x-ray vision to "see underground" while we install piles. Let me illustrate with a quick example from a recent VersaPile project.

Our geotech report indicated a certain mix of soils on the project site. We designed the foundation according to the reported soil conditions and included extra pile extensions for safety. During install, our crew found torque readings on several piles were lower than expected. The torque readings indicated the soil wasn't exerting enough resistance on the helical pile.

(Image: While they can be expensive, digital torque monitors offer immediate feedback on installation which increases efficiency and safety.)

After a brief engineer consult, they approved adding extensions to the affected piles until the specified torque was reached. After a brief delay our crew was able to install every pile to the required specifications.

If we didn't have a strong geotech report and installation torque to monitor, we wouldn't have known those piles would fail the load-test. Because we did have those two things, we could resolve the issue same-day with zero delays.

Conclusion

Helical pile foundations can be monitored every step of the way - it's one of the reasons they've become so popular in commercial projects. But faster and more accurate installs are only one benefit to monitoring installation torque.

Some of the other benefits of understanding installation torque are:

• The ability to value-engineer a cost-efficient foundation
• Can obtain parameters for ethical design
• Design factors could increase pile capacity (on approval from geotechnical engineer)

Installation torque isn't a magic number that guarantees performance. It's simply one of the (many) tools we can use to build smarter foundations.

And like any tool it's only useful in the right context.

Beware of people who tell you installation torque is just as good as load-testing a helical pile. Toruqe can tell you a lot of valuable things, but guaranteeing safety is not one of them.

We use installation torque, and a whole lot more, to design, plan, and install helical pile foundations. If you'd like to know more about how data can improve your foundation, get in touch with us by clicking here. We'd love to offer any help or advice we can!

 

---

View More