Helical Pier, Piles & Pull Down Micropiles

Helical Pier, Piles & Pull Down Micropiles Technology

Sinking foundations, cracked and buckled walls and uneven floor are problems commonly faced annually by some quarter million homeowners as well as commercial buildings. Structures situated on unstable soils settle when their foundations are subjected to extreme moisture conditions. A shifting foundation may result in structural damage to your biggest investment.

Abby Construction has been installing Chance Helical Piers since 2003. The unique advantage of Chance Piers is they not only stabilize a foundation, but also raise the structure back up to its original location. The pier is attached to the footer and a screw anchor is bored down to stable soil.

Helical piers

A helical anchor/pile is a segmented deep foundation system with helical bearing plates welded to a central steel shaft. Load is transferred from the shaft to the soil through these bearing plates. Central steel shafts are available in either Type SS (Square Shaft) series or Type RS (Round Shaft) series. The Type SS series are available in 1-1/4″ to 2-1/4″ square sizes. The Type RS series are available in 2-7/8″ to 4-1/2″ diameter sizes. Type SS-RS combinations are also available for compression applications in soil conditions where dense/hard soils must be penetrated with softer/loose soils above the bearing strata. The Helical Pulldown® Micropile series is also used in applications similar to those requiring the use of the Type SS-RS combinations.

Segments or sections are joined with bolted couplings. Installation depth is limited only by soil density and practicality based on economics. A helical bearing plate or helix is one pitch of a screw thread. All helices, regardless of their diameter, have a standard 3″ pitch. Being a true helical shape, the helices do not auger into the soil but rather screw into it with minimal soil disturbance. Helical plates are spaced at distances far enough apart that they function independently as individual bearing elements; consequently, the capacity of a particular helix on a helical anchor/pile shaft is not influenced by the helix above or below it.

Square Shaft (SS)

The first section or lead section contains the helical plates. This lead section can consist of a single helix or up to four helices. Additional helices can be added, if required, with the use of helical extensions. Standard helix sizes are shown in the table to the right. The helices are arranged on the shaft such that their diameters increase as they get farther from the pilot point. The practical limit on the number of helices per anchor/pile is four to five if placed in a cohesive soil and six if placed in a cohesionless or granular soil.

Plain extensions are then added in standard lengths of 3, 5, 7 and 10 feet until the lead section penetrates into the bearing strata.

Round Shaft (RS)

The first section or lead section contains the helical plates. This lead section can consist of a single helix or up to four helices. Additional helices can be added, if required, with the use of helical extensions. Standard helix sizes are shown in the table to the right. The helices are arranged on the shaft such that their diameters increase as they get farther from the pilot point. The practical limit on the number of helices per anchor/pile is four to five if placed in a cohesive soil and six if placed in a cohesionless or granular soil.

Plain extensions are then added in standard lengths of 3, 5, 7 and 10 feet until the lead section penetrates into the bearing strata.

Combination (SS-RS)

Chance® Helical Transition Coupler

Adapts Type SS to Type RS Pile Shafts

The Type SS/RS Combination Pile is used mainly in compression applications in areas where soft/loose soils are located above the bearing strata (hard/dense soils) for the helices. The Type RS material with a much greater section modulus will resist columnar buckling in the soft/loose soil. The Type SS material will allow adequate penetration of the helices into the hard/dense material without “spin-out”, i.e., loss of thrust of the helices in the soft/loose material.

The design should be prepared by a Registered Professional Engineer with suitable soil information.

Helical Pulldown® Micropiles (HPM)

The Helical Pulldown® Micropile (HPM) is a method used to form a grout column around the shaft of a standard helical anchor/pile. The installation process can employ grout only (see Uncased Helical Pulldown® Micropile) or grout in combination with either steel or PVC casing (see Cased Helical Pulldown® Micropile).

To begin the process, a helical anchor/pile is placed into the soil by applying torque to the shaft. The helical shape of the bearing plates creates a significant downward force that keeps the foundation advancing into the soil. After the Lead Section with the helical plates penetrates the soil, a Lead Displacement Plate and Extension are placed onto the shaft. Resuming torque on the assembly advances the helical plates and pulls the displacement plate downward, forcing soil outward to create a cylindrical void around the shaft. From a reservoir at the surface, a flowable grout immediately fills this void surrounding the shaft. Additional extensions and displacement plates are added until the helical bearing plates reach the minimum depth required or competent load-bearing soil. This displacement pile system does not require removing spoils from the site.

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