Why This 180 Year Old Helical Pile Project Still Matters

November 12, 2021

Helical piles have been in recorded use for over 180 years. Like any technology in use for so long, they've been refined and improved. Despite this, helical piles share a surprising amount with their predecessors. Their long heritage highlights the genius of helical piles, and why they've endured for nearly 2 centuries.

Usually, I'm all about looking towards the future. But today, I want to take a step into the past and look at a construction project that would forever change how we build things.

This is the story of the Maplin Sands Lighthouse - the first recorded use of helical piles.

Background

Alexander Mitchell, the inventor of helical piles, is a remarkable character. Born in Ireland in 1780, he would receive little formal education as a child. What education he did receive revealed a keen mathematical mind. Unfortunately, by age 22 he would lose his sight to an illness. Mitchell's lack of education and sight did not dampen his indomitable spirit.

In 1833, Mitchell applied to patent the first-recorded "screw pile". What we now call "helical piles". For simplicities sake I'll refer to them as helical piles from here out.

Mitchell's helical pile design was inspired by the mooring systems designed to secure ships in harbors. "Screw anchors" had been in use for some time as an alternative to weighted anchors, which struggled to hold ships in storms and strong waves. It was discovered that a helical turned into the sand offered exponentially increased resistance to tension forces.

Taking inspiration from these helical anchors, Mitchell applied the idea to a structural foundation. He wisely reasoned the soil conditions on a sandy coast were little different than the sandy floor of a harbor. By adding a helix plate to a cast iron shaft, he judged it would offer firm support to heavy structures in loose and sandy soil.

In 1838, Mitchell would have a chance to prove his new foundation.

Problem

The mighty River Thames flows for 346km through southern England, and even today remains an important passage for goods and people. In the mid-1800's it was determined that a lighthouse was required at the southeastern edge of Maplin Sands, located near Essex, England. The vicious storms and powerful currents made it dangerous for ships and boats to navigate. However, the poor soil had previously prevented the construction of a lighthouse. No foundation technologies of the time were practical for the tough location.

That is, until Alexander Mitchell and his curious invention caught the attention of a leading engineer of the time.

Solution

James Walker was a respected Scottish engineer and a member of Trinity House, the lighthouse authority in England. Walker heard of Alexander Mitchell's new "screw pile" foundation, and was intrigued. He trusted Mitchell's engineering and staked his reputation on the new technology by recommending it to the lighthouse authority.

On Walker's recommendation, Mitchell agreed to install the helical piles at Maplin Sands and test them.

Mitchell and his crew installed nine cast-iron helical piles into the sand at a depth of 25ft. The pile shafts were 5" in diameter with a 3' diameter screw (helical) plate. A heavy raft of wood with a capstan helped turn the piles into the ground.

Including Alexander Mitchell, the crew numbered around 40 workers. Installation began on August 28th, 1838 and finished nine days later.
The total cost of the foundation was £1200 in 1838. If we adjust for inflation and convert to Canadian, it's just over $230,000. That's about seven times what it may cost for a similar foundation today.

To prove the foundation, they loaded 200 tons of stone on the platform. Two years later, the foundation had experienced no creep. Satisfied with the results of the test, they completed the lighthouse by 1840.

Result

The Maplin Sands lighthouse experienced no foundation issues during its life. Its downfall in 1932 would come from the flow of the relentless Thames. Strong tidal streams scoured soil from beneath the lighthouse, undermining the helical foundation. After nearly a century of service, the lighthouse was swept away in a storm.

The legacy of the lighthouse at Maplin Sands can't be understated. What Mitchell invented was more than a foundation for lighthouses and bridges. His "screw pile" invention would fundamentally change how and where we build, and usher in a new age of foundation technology. It jump-started a revolutionary tradition that continues to this day.

And for that, Alexander Mitchell has my thanks.

You might not be building a lighthouse, but you probably have other projects that need support. If you'd like to learn more about helical piles, see if they might fit your project, or just have some questions, contact us! Click here to get in touch with our friendly experts, we'd love to help you find the right answers.

 

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