Copyright © 2011-17 Helical Pile World, LLC. All Rights Reserved.
by contributing author Gary Seider, Chance Civil Construction
HPW Archive 2008
The Basics of a Helical Deep Foundation
A helical foundation consists of at least one helix-shaped steel bearing plate attached to a central steel shaft. The shaft is typically solid steel bar (12 to 23 inches square) or heavy wall pipe (2f to 8 inches in diameter). The helix plates are high-strength steel (6 to 16 inches in diameter, d or 2 inch thick). Each helix is circular in plan, formed into a screw thread with a defined pitch (typically 3 inches).
Installation is by hydraulic motors mounted to virtually any type of machine. Portable equipment is available for such tight access areas as crawl spaces, basements, and narrow alleyways. Percussion drilling equipment is not used. The five to 25 rpm high-torque motor provides rotational energy and the machine provides crowd (down pressure), necessary for installation. The helical foundation is rotated (screwed) into the ground to advance one pitch distance per revolution. Helical foundations can be fully extendable; so that the helix plates can be installed to any specified bearing depth.
A helical foundation can be used to resist both uplift and compression loads. Installed to proper depth and torque, the helix plates serve as individual bearing elements to support a load. The central shaft, which transmits torque during installation, now transfers axial load to the helix plates. The central steel shaft also provides resistance to axial load via skin friction and to lateral loads via passive earth pressure.
Why Use Helical Foundations?
Low Mobilization Costs: Helical foundations are typically installed with small equipment such as a rubber-tired backhoe. This eliminates the high mobilization costs associated with equipment used to install driven piles, drilled shafts or auger-cast piles. Remote location or difficult access sites also increase mobilization costs, which makes helical foundations a better choice.
Expansive soils: The bearing plates of helical foundations are usually placed below the depth of seasonal moisture variation. The swell force on a shaft is directly proportional to the surface area contact between the soil and shaft. Since helical foundations have smaller shafts than conventional piles, uplift forces are smaller.
Year-round installation: Helical foundations can be installed in any weather because there is no need for concrete or grout. This allows work to proceed without interruption.
Temporary structures: Helical foundations can be removed by reversing the installation process. During the 2002 Winter Olympic Games in Salt Lake City, helical foundations were used to support temporary grandstands and judging booths at various venues, and the huge information signs that informed visitors about events.
Remedial applications: The largest present day market segment for helical foundations is remedial underpinning. They can supplement or replace existing foundations distressed by differential settlement, cracking, heaving, or general foundation failure. Helical foundations are ideal for remedial work since they can be installed in confined, interior spaces. The work is low-impact with minimal damage to landscaping or disruption to building occupants.
Loads: Design compression and tension loads for helical foundations range between 12.5 and 50 tons. The soil is generally the limiting factor as the number and size of helical foundations can be varied to suit the application.
Soils: Helical foundations can be installed into soils with a blow count (N-value) less than 80 blows/foot of the 2-inch OD sampler per ASTM D-1586. A limitation of screw foundations is they cannot be installed into competent rock or very hard, dense soil greater than about 80 blows/foot.
Several methods exist to design helical foundations and to predict their performance under load. Two of those methods are Bearing capacity and Torque correlation.
The Terzaghi general bearing capacity equation suggests that the total capacity of a helical foundation, either in tension or compression, is equal to the sum of the capacities of each individual helix plate. Calculating the unit bearing capacity of the soil and applying it to the individual helix plate areas determine the helix capacity. The bearing capacity method predicts capacity reasonably well when adequate soil data is available. Soil data is typically provided by the geotechnical report. If soil data is lacking or not available, other design methods are required.
The empirical relationship between installation torque and capacity is considered to be the greatest attribute of helical foundations. The relationship is: As a helical foundation is installed (screwed) into increasingly denser/harder soil, the resistance to installation (called installation energy or torque) will increase. Likewise, the higher the installation torque, the higher the axial capacity of the installed helical foundation. The relationship can be described in the following equation:
QU = Kt x T
QU = Ultimate capacity of screw pile
Kt = Empirical torque factor
T = Average installation torque
The value of Kt may range from 3 to 20 ft, depending on the soil conditions and design parameters (principally the shaft size). For square shaft, it typically ranges from 10 to 20. For pipe shaft, it typically ranges from 3 to 10 ft. Torque monitoring tools provide a good method of production control during installation.
The engineer can use the relationship between installation torque and load capacity to establish minimum torque criteria for the installation of production helical foundations. The recommended default values for Kt [10 for square shaft and 7 for 32 -inch OD pipe shaft] will typically provide conservative results. For large projects, a pre-production load test program can be used to establish the appropriate torque correlation factor (Kt) for the existing project soils.
Other Design Issues
Factor of Safety: For compression loads, a factor of safety of 2 has historically been sufficient to account for the inevitable uncertainties in soil, installation, and manufacture. In some instances, as with tiebacks for earth retention, the factor of safety can be less than 1.5.
Helical Foundation Spacing: The recommended center-to-center spacing between adjacent helical foundations is five times the diameter of the largest helix. The absolute minimum spacing is three diameters. Minimum spacing requirements apply only to the helix plate, which means the central shaft can be battered to obtain adequate spacing.
Design Assistance: For design assistance at any step in the design process, including capacity design, helical foundation selection, corrosion, lateral/buckling concerns, and specifications, contact your local helical foundation installer or distributor. They will either assist you directly, or refer your inquiry to the manufacturer. The Design Algorithm flowchart demonstrates the steps involved in the design of a helical foundation.
If satisfactory soils information is known about a particular site, the contractor may lump sum bid the helical foundations or anchors, regardless of length. Lump sum bids are popular with owners because the price is known up-front.
The price per foundation with add/deduct bid is typically used when little to no soil information is available. It is probably the most common type of contract. A pre-determined bid length is used, with an add/deduct amount per lineal foot to accommodate variations in subsurface conditions.