Since 2002 Granite Foundation Repair has been installing Helical Piers for residential home foundation repair. How are Helical Piers installed? What makes Helical Piers so good? These questions are good to know when you are in need of a foundation repair or trying to determine which method to use.
This article is written for the layman, and will not go into the mathematical formulas which form the basis for Helical Pier installation guidelines. Helical Piers are also known as Helical Pilings and Screw Pilings.
Helical Piers are a proven technology. They were developed over 150 years ago in England and used to stabilize lighthouses. A later variant was an application of the Helical Pier referred to as the Anchor Piling. Anchor pilings are constructed with a square tube and round plate. An anchor piling relies upon soil overburden to prevent pulling the piling from the soil. Typical uses for anchor pilings include tie downs for tents, retainer walls, mobile home tie downs, basement wall tie backs and telephone poles. Anchor pilings with square rods should not be used by foundation support.
Helical Pier Construction
A Helical Pier consists of one or more round plates welded to a square or round shaft. The plates are cut and twisted into a helix shape, resembling a single screw thread. Hence, the alternate name for a Helical Pier is a “Screw Pile”. The name for the plate is a “ Helix”. Additional shafts are added to obtain depth. When multiple Helix’s (plates) are used, they are typically spaced 3 times their diameter along the piling shaft. Two 10 inch plates will be spaced about 30 inches apart. The first plate is installed near the leading edge of the shaft. The leading edge of the shaft is referred to as a pile toe, and is slanted to auger into the ground. When used for foundation support, the shaft is normally a round shape, resembling a pipe. Square shafts are great for anchors but offer poor support for foundations. Square shafts do not have the ability to resist bending under load. Round shafts can resist bending under load.
Helical Pier Installation
Helical Piers are literally screwed into the earth using a rotating hydraulic head. Unlike pressed concrete pilings, Helical Piers do not use the resistance of the foundation for installation. There is no pressure or vibration of the foundation during Helical Pier installation. Installation is gentle, yet very effective. The success of the installation is ultimately based upon the pier’s ability to support the foundation, which is directly measurable.
To assure reliable support, Helical Piers are installed to a minimum depth (or rock) and a minimum rotational torque. Why are both Helical Pier depth as well as torque so important?
The minimum installation depth of a Helical Pier for clay soil is that depth at which there is not a seasonal moisture change. Seasonal moisture change might cause the pier to move with changes in clay soil moisture. It is generally accepted that the depth where no seasonal moisture change occurs is around 12 feet in the Dallas Ft Worth area.
Rotational torque is used as a measure of the soil’s load-bearing capacity. As the helical plate rests within the soil, torque resistance is a useful measure of the ability of the helical pier to support a load. For Dallas Fort Worth there are tables, which show plate diameter and clay soil load bearing capacity as they relate to rotational torque.
6 reasons why Helical Piers are a good method for underpinning a residential home foundation
- There is no mechanical stress placed upon the foundation during the installation. The screwing method uses soil overburden to obtain depth. Contrast this with pressed concrete pilings, which directly use the foundation as an opposing force during the drive of the piling system.
- Installation can be performed in a small space using a hand held hydraulic torque head. The torque head is held in position with a long lever arm.
- Helical Piers cause minimal disturbance of vegetation. Less than half of the surface soil is disturbed during installation when compared with a pressed concrete piling. A helical pier requires about 2 square feet for installation of the foundation support bracket. A pressed concrete piling installation requires an excavation of 5 square feet.
- Quick installation, which means less foot traffic upon the lawn during installation of the foundation support system.
- Can be installed in all types of weather.
- Installation of Helical Piers can be achieved to a known load bearing capacity. Rotational torque provides a direct measurement of the soil load bearing capacity . Contrast this with pressed concrete, which is installed until “refusal”. Refusal means “that depth at which the structure can no longer exert sufficient reaction to the end bearing force of the clay soil”, which must be overcome to drive the concrete pilings to further depth. Pressed concrete piling depth to refusal is dependent upon the weight of the structure, while Helical Pier installation depth does not use the structure. Depth is only dependent upon the required soil load bearing capacity
For further information on Helical Piers, or to schedule a free foundation evaluation (for homeowners) give Granite Foundation Repair a call at 972-412-2171.