Helical Piers: How They Work And Where To Use Them

Helical piers present contractor and engineers with a valuable tool for new construction in bad soils or remediation of structures that have sustained settlement damage from loss of support by the underlying soils.

What they are:

Basically, a helical pier is a giant screw that you drive into the earth to resist compressive or uplift loads, or both.  It consists of a helix, a shaft, and a bracket of some sort at the top.  The pier can have more than one helix, and the shaft can be solid square steel or a pipe section.

Depending on the size, helical piers are driven with portable or with equipment-mounted rotary drive heads.  The drive head screws the pier into the ground while monitoring how much torque it takes to drive the pier.

How they work:

When a helical pier is driven into the ground, the helix pulls itself and the shaft down through the poor soils at the surface to competent soils below.  Once it reaches an appropriate depth, the competent soils support the helix which supports the shaft which supports the load – feel free to sing a round of “the thigh bone’s connected to the hip bone” here.

Before the pier is actually installed, the engineer looks at whatever soil information is available and calculates an estimated installation depth.  He usually bases his calculation on the Individual Bearing Method.  This method assumes that the pier’s bearing capacity is the sum of the bearing capacity of each helix.  The helix’s bearing capacity depends on helix surface area, helix depth, soil type, soil density, and soil consolidation.

During pier installation, the contractor’s gauges can tell him the installation torque necessary to drive the pier.  Years of testing have shown that drive torque correlates quite well with the pier’s installed capacity.  Different shaft shapes and sizes have different factors, but the basic relationship is well established.  For this reason, the engineer will often also specify a minimum installation torque.

Where to use them:

Piers are appropriate in a number of applications.  If a new structure site has poor soils or undocumented fills to a depth that makes excavation and replacement unreasonable, helical piers can often work well.

If an existing structure has experienced a loss of subgrade support, especially if it is an uneven loss, helical piers can provide an economical way to restore support to the existing foundation.

In high wind/light structure situations, helical piers can also provide protection against uplift.  When the highest helix has reached a depth at least five times its diameter into competent soils, the uplift capacity is approximately equal to the compression capacity.  Pier shaft, extension, and bracket connections must obviously be detailed so they will sustain tension.

Please call us if you think your project may benefit from helical piers.  A short (and free!) phone call may save your project a lot of money.

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