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Working With Helical Piers

Working With Helical Piers

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    Andy Engel

    The height of the pile above grade is measured.

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    Greg DiBernardo

  • Figure 1. Helical piers have a screw-shaped plate welded to a zinc-coated steel shaft, and are made in different sizes for different soils and applications (A). As the driver turns the pile, it simply screws into the ground until the installer is confident its below the frostline and in soil with sufficient bearing capacity (B). Several types of caps are available to attach piers to framing; some are adjustable in order to fine-tune the elevation (C).

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    Figure 1. Helical piers have a screw-shaped plate welded to a zinc-coated steel shaft, and are made in different sizes for different soils and applications (A). As the driver turns the pile, it simply screws into the ground until the installer is confident its below the frostline and in soil with sufficient bearing capacity (B). Several types of caps are available to attach piers to framing; some are adjustable in order to fine-tune the elevation (C).

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    Andy Engel

    Helical piers have a screw-shaped plate welded to a zinc-coated steel shaft, and are made in different sizes for
    different soils and applications.

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    Andy Engel

    As the driver turns the pile, it simply screws into the ground until the installer is confident it’s below the frostline and in soil with sufficient bearing capacity.

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    Andy Engel

    Several types of caps are available to attach piers to framing; some are adjustable in order to fine-tune the elevation.

  • Figure 2. A piers bearing capacity usually relates to the torque required to drive it. A gauge on the machine measures the hydraulic pressure, which correlates to the torque.

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    Figure 2. A piers bearing capacity usually relates to the torque required to drive it. A gauge on the machine measures the hydraulic pressure, which correlates to the torque.

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    Andy Engel

    A pier’s bearing capacity usually relates to the torque required to drive it. A gauge on the machine measures the hydraulic pressure, which correlates to the torque.

  • Figure 3. Shaft extensions are welded on and the pier driven as deep as needed to reach soil with adequate bearing capacity (A). The pier can be steered around a below-grade rock by moving the drivers boom; once the obstruction is passed, the boom pulls the shaft plumb again (B). Even though this pier penetrates about 13 feet into the ground, theres no pile of excavated soil as there would be with a conventional footing (C).

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    Figure 3. Shaft extensions are welded on and the pier driven as deep as needed to reach soil with adequate bearing capacity (A). The pier can be steered around a below-grade rock by moving the drivers boom; once the obstruction is passed, the boom pulls the shaft plumb again (B). Even though this pier penetrates about 13 feet into the ground, theres no pile of excavated soil as there would be with a conventional footing (C).

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    Andy Engel

    Shaft extensions are welded on and the pier driven as deep as needed to reach soil with adequate bearing capacity.

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    Andy Engel

    The pier can be steered around a below-grade rock by moving the driver’s boom; once the obstruction is passed, the boom pulls the shaft plumb again.

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    Andy Engel

    ven though this pier penetrates about 13 feet into the ground, there’s no pile of excavated soil as there would be with a conventional footing.

  • Figure 4. Above is a typical deck foundation using helical piers, which have greater bearing capacity than concrete piers and install with little disturbance to the landscape. At right, treated posts supporting a porch have been fastened to connector caps on the helical piers.

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    Figure 4. Above is a typical deck foundation using helical piers, which have greater bearing capacity than concrete piers and install with little disturbance to the landscape. At right, treated posts supporting a porch have been fastened to connector caps on the helical piers.

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    Greg DiBernardo

    A typical helical pier foundation has greater bearing capacity than one with concrete piers, and installs with little disturbance to the landscape.

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    Greg DiBernardo

    Treated posts supporting a porch have been fastened to connector caps on the helical piers.

  • Figure 5. Adding a vertical extension to the drilling rig makes it possible to drive piers into steeply sloped sites (A). The machine is also well-suited to foundation stabilization work, as it can fit in tight spaces around existing homes (B). Piers are typically installed every 6 feet along an underpinned foundation (C).

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    Figure 5. Adding a vertical extension to the drilling rig makes it possible to drive piers into steeply sloped sites (A). The machine is also well-suited to foundation stabilization work, as it can fit in tight spaces around existing homes (B). Piers are typically installed every 6 feet along an underpinned foundation (C).

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    Techno Metal Post

    Adding a vertical extension to the drilling rig makes it possible to drive piers into steeply sloped sites.

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    Techno Metal Post

    The machine is also well-suited to foundation stabilization work, as it can fit in tight spaces around existing homes.

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    Techno Metal Post

    Piers are typically installed every 6 feet along an underpinned foundation.

  • Figure 6. The authors small rig excels at going places larger pile-driving equipment cant (A, B, C). When the machine wont fit into a tight area, the drive head can be mounted on a portable bracket that bolts to the structure (D).

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    Figure 6. The authors small rig excels at going places larger pile-driving equipment cant (A, B, C). When the machine wont fit into a tight area, the drive head can be mounted on a portable bracket that bolts to the structure (D).

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    Techno Metal Post

    The author’s small rig excels at going places larger pile-driving equipment can’t.

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    Techno Metal Post

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    Techno Metal Post

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    Techno Metal Post

    When the machine won’t fit into a tight area, the drive head can be mounted on a portable bracket that bolts to the structure.

My deck company builds about 50 projects a year. That’s a lot of footings, especially when you hate digging footing holes and mixing up concrete as much as I do. Since I began building with helical piers, I’ve stopped wearing out shovels and posthole diggers. I no longer worry about the inspector arriving on schedule to look at my footing holes — or about watching them fill up with water if he doesn’t. And once a pier is installed, I know exactly how much weight it can support.

Steel Foundation

A helical pier is a manufactured steel foundation pin that is driven into the soil to a depth below frostline using hydraulic machinery. Helical piers are primarily used in heavy commercial work, but they’re also well-suited for backyard decks, additions, and foundation repairs.

Two years ago, I bought a franchise with Techno Metal Post (see “Dealership” sidebar). Now a big part of my business volume comes from installing piers for other contractors. Most helical piers are driven with a skid-steer or excavator, but Techno Metal Post uses a proprietary machine that’s small enough to fit through a gate and go places larger machines can’t. I can actually drive the rig right onto an existing deck if I need to retrofit additional footings to support a new hot tub.

Typical piers have a 7-foot shaft with a helical bearing plate welded to the end and a cap on top that attaches to the framing. Most piers intended for residential use are hot-dipped galvanized steel. If the soil is particularly corrosive, sacrificial anodes (similar to those used to protect underground LPG tanks) can be added. In most commercial and industrial applications, however, the piers aren’t even galvanized.

The diameter of the helix varies based on soil conditions. Generally, the installer selects a smaller helix for rocky soils and a larger one for marshy and clay soils. Once the pier is set, a variety of caps are available to tie the pier to the framing; some of them have a screw assembly that allows fine-tuning of the elevation.

Bearing Capacity

The load-bearing capacity of a helical pier usually relates to the amount of torque required to install it, a function of both the size of the helix and the soil’s bearing capacity. A pressure gauge on the installation machine reads the torque as the pier is rotated into the ground.

In weaker soil, the pier will be driven deeper to reach stronger soil. (If greater bearing or uplift capacity is required at shallower depths, the project engineer may specify multi-helix piers.) When the helix is below frostline and the pressure gauge hits a high enough number relative to the loading requirements of the structure, the installation is complete. To calculate the actual bearing capacity of the pier, the pressure reading is plugged into a formula called a torque correlation.

When poor soil conditions mandate going deeper than the standard 7-foot-long shaft, we weld on an extension. Sometimes all it takes is a foot more depth to go from terrible soil to firm material. This is particularly relevant if we’re building a freestanding deck where the piers close to the house might start out in backfill. If we were excavating to install a conventional concrete footing, we’d have to dig down to virgin ground at the house foundation level — as much as 7 feet or 8 feet if the house had a basement. It’s far easier to drive a helical pier to this depth.

Also, with a traditional footing, you never really know what lurks an inch below the bottom of your footing excavation. Now that I am in the helical-pier business, I frequently see situations where seemingly good soil turns to mush inches below where I typically would have installed the footing.

Rocks. Normally, we just power through loose rock basketball-size and smaller. The installation machine generates sufficient torque for the helix to push rocks out of the way as it turns. Sometimes, the installer can actually steer the helix around a rock, then use the machine’s boom to pull the pier back into plumb.

If we hit a large rock below frostline, the pile is parked on top of the rock and load-tested (see “Load-Testing” sidebar, right). Assuming it passes the load test — it usually does — we can be confident the pier will never move. If it doesn’t pass the load test, the pier will have to be installed in a different spot. On critical jobs, a soil test has often been done before we get there so we’ll know where there’s ledge or bedrock.

When we encounter a large rock above frostline, it can be drilled and the pier’s shaft pinned to the rock. Occasionally, however, there is so much rock on the job helical piers just won’t work. There are some locations where I don’t even bother trying to install them because every lot on the street was blasted out of bedrock.

In average soil, driving a helical pier takes about 10 minutes, after which it’s ready to build on. A P2 pier with a 2 5/8-inch-diameter shaft — the smallest pier I install — will support a 6,800-pound load. A concrete pier would need to be bigger than 16 inches in diameter in verified 4,000-psi soil to achieve the same capacity. Because of the higher bearing capacity, most projects require fewer piers, although larger beams may be needed for the greater spans.