Commercial Push Pier Systems

Commercial push piers are the primary components of a heavy steel support system designed to stabilize commercial building foundations. Unlike helical piles, which require a compact excavator to provide the power to advance them through the soil, push piers require no heavy equipment to install. For this reason, they are ideal for use in difficult-to-reach situations. 

Foundation problems occur in commercial applications for many of the same reasons they occur in residential applications: the soil beneath the structure is unable to bear its weight. This can be due to one or more of several factors, several of which relate to an excess or lack of moisture.

When there is too much moisture in the soil, it can soften or even wash away. When there is too little moisture, the soil can shrink, leaving voids into which a foundation can sink. 

A third possible reason for foundation settlement is that it may have been built upon soil that was poorly compacted to begin with. As part of the initial construction process, the location was graded and landscaped — soil from hilltops was moved into valleys to create a level surface. If that fill soil isn’t compacted properly, it can compress over time under the weight of the newly constructed building. 

Regardless of the case, the solution to foundation settlement is to connect the structure to bedrock or load-bearing competent soils. One of the best ways to do this is via the installation of a push pier system that can push past the poor soil and find these load-bearing strata.

Commercial Push Pier Detail

The Importance of Foundation Stabilization

There is a common misconception that it is normal for a building to “settle” over time. Unfortunately, when people refer to a building “settling,” what they typically mean is that the structure’s foundation is sinking into the soil beneath it. Unfortunately, when a foundation sinks, it rarely does so in a uniform fashion.

This uneven settlement results in a structure that is itself uneven — which means that everything that is related to it is affected by this unevenness. Floors sag, ceilings gap, doors and windows stick. Therefore, before a building is completed or before renovations can be made, its foundation must be stabilized to prevent further settlement. Again, push piers are a simple, reliable and cost-effective solution to this problem.

Design

The Supportworks Push Pier System utilizes high-strength round steel tubes and a load-transfer bracket (retrofit foundation repair bracket) to stabilize and/or lift sinking or settling foundations. The foundation bracket is secured against the existing footing and pier sections are driven hydraulically through the foundation bracket and into the soil below using the combined structural weight and any contributory soil load as resistance. Pier sections are continuously driven until a suitable load-bearing stratum is encountered. 

At that point, the structure either begins to lift or the target pressure/load is achieved. The weight of the structure is then transferred from the unstable soil to the foundation brackets, through the piers, and to firm load-bearing soil or bedrock.

The Supportworks Push Pier System develops a factor of safety against pier settlement by the pier installation methods used and the sequence with which multiple piers are driven and then re-loaded. Piers are first driven individually using the maximum weight of the structure and any contributory soil load. After all the piers are driven, the piers are re-loaded simultaneously, and the total reaction load is distributed over the multiple pier locations.

Since the average load on each pier during the load transfer operation is less than the load during pier installation/driving, a factor of safety against settlement is achieved. Typical factors of safety against pier settlement range from about 1.5 to 3.0, with higher values, generally achieved for structures with greater rigidity. These factors of safety conservatively ignore any additional long-term frictional component to the pier's capacity (see below for more information).

Supportworks Model 288 Pier System Specifications

  • Bracket: Weldment manufactured from 0.25", 0.375", and 0.50"-thick steel plate. Yield strength = 36 ksi (min.), tensile strength = 58 ksi (min.).
  • External Sleeve: 3.50" OD x 0.216" wall x 30" or 48" long with sleeve collar welded to one end. Yield strength = 50 ksi (min.), tensile strength = 62 ksi (min.).
  • Pier Starter Tube: 2.875" OD x 0.165" wall x 50" long, triple-coated in-line galvanized. Yield strength = 50 ksi (min.), tensile strength = 55 ksi (min.). 3.375" OD x 0.188" wall x 1" long friction reducing collar welded to one end.
  • Pier Tube: 2.875" OD x 0.165" wall x 36" long, triple-coated in-line galvanized. Yield strength = 50 ksi (min.), tensile strength = 55 ksi (min.). 2.50" OD x 0.180" wall x 6" long internal coupler at one end with 3" extending out of pier tube.
  • Pier Cap: 5.0" wide x 9.0" long x 1" thick plate with confining ring welded to one side. Yield strength = 50 ksi (min.), tensile strength = 65 ksi (min.).
  • All-Thread Rod: 0.75" diameter x 16" long, zinc plated, Grade B7, tensile strength = 125 ksi [min.].
Push Pier Capacity Chart

Model 288 Capacity Chart

Design Considerations

Push piers are installed directly adjacent to the existing structure utilizing side-load brackets. This introduces eccentricity into the system. The Model 288 Push Pier System incorporates an external sleeve at the top of the pier to aid in resisting the bending forces generated by this loading condition. This helps preserve the axial compressive capacity of the pier shaft. The external sleeve extends through and below the foundation bracket to essentially create a bracket that is 48 inches tall.

The moment or bending force is localized within a relatively short distance below the bracket. Although the bending force is dissipated quickly by the pier bearing against the confining soil, it is significant and cannot be ignored. The depth or length of sleeve and pier over which the bending force dissipates is a function of the soil stiffness near the surface. The depth is greater in soft clay and loose sand, and less in stiff clay and dense sand. In soft or loose soils, a small portion of the bending force may be transferred to the pier below the sleeve, thereby reducing the pier's allowable axial compressive capacity. A modified, lower capacity system is also available with a shorter, 30-inch long sleeve for low headroom applications.

Push Pier Tech

Friction Reducing Collar

The first pier section advanced into the ground includes a larger-diameter "friction reducing collar" welded to the lead end. This collar, being larger in diameter than the pier tube, effectively creates annular space around the pier as it is advanced through most clayey soils. In soft clay or clean sand and gravel, an annular space may only temporarily be created.

However, the larger diameter collar causes soil disturbance or remolding to occur, which also significantly reduces frictional resistance on the outside surface of the pier during driving. The result is a driven pier that generates most of its capacity in end bearing.

Over time, the soils surrounding the pier relax back into the annular space and against the pier shaft. This provides an additional frictional component to the pier's capacity. Even though this frictional capacity may be significant, it is conservatively ignored in the determination of the pier's factor of safety against pier settlement.

Installation

First, the soil is removed from around the foundation's footing, where the pier will be installed.

Then, a specially engineered steel bracket is placed under the footing. As mentioned above, steel pier sections are hydraulically driven through the bracket to bedrock or a stable soil layer. Next, the weight of the structure is transferred from the concrete foundation, through the steel bracket and piers, to the deep load-bearing soil or bedrock.

A lift can then be attempted to bring the structure back toward its desired position. Once the stabilization and lift are completed, the soil is backfilled and the structure is permanently stabilized in its new position. Because push piering is a below-grade system, once it is installed, it is completely hidden from view. 

Advantages at a Glance

  • This system reaches much greater depths than other options. The specially designed starter tube is fitted with a friction-reducing collar. This allows the piers to advance further to reach bedrock or other stable soil layers, which ensures permanent stability for the foundation.
  • Features a patented bracket and external sleeve. The external sleeve strengthens the pier system directly below the bracket to prevent kinking and bending and ensure a vertical installation all the way to competent soils.
  • Does not require the use of heavy equipment, and installation is less invasive to the area around the structure.
  • Galvanized pier sections will not rust.
  • As a below-grade solution, it’s concealed from sight.
  • Can often lift a foundation back toward its original position.
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