The spacing of helical piles has many definitions across manufacturers, acceptance criteria and codes. The only similarity is all are based on the Diameter of Largest Helix Plate, (DLH as defined in this report). Based on this similarity, we can conclude that helical piles as an end bearing system would naturally cause spacing criteria to be at the bearing depth. Please refer to the figure for pictorial spacing definitions.
Case 1: Chance Helical Pile Tech Manual 3rd Edition 2014
Summary:
- 5 x DLH , Where Spacing is defined as the distance between helix plates. See Spacing B.
- Minimum = 3’It is recommended that the center-to- center spacing between adjacent anchors/piles be no less than five times the diameter of the largest helix. The minimum spacing is three feet. Minimum spacing requirements apply only to the helix bearing plate(s), i.e., the pile/anchor shaft can be battered to achieve minimum spacing. Group effect, or the reduction of capacity due to close spacing, has never been accurately measured with helical piles. However, bearing capacity theory would indicate that capacity reduction due to group effect is possible, so it’s considered good practice to install helical piles into dense bearing stratum when center-to center spacing is less than 4 feet.
Case 2: AC358 – Acceptance Criteria for Helical Pile Systems and Devices
Summary:
- 3 x DLH , Where Spacing is defined as the distance between helix plate edges. See Spacing B. • OR
- 4 x DLH , Where Spacing is defined as center-to-center of adjacent helix plates. See Spacing C.
The results of this analysis and the structural capacities shall be used to select a helical pile system. The evaluation report shall state that to avoid group efficiency effects an analysis prepared by a registered design professional shall be submitted where the center-to-center spacing of axially loaded helical piles is less than three times the diameter of the largest helix plate at the depth of bearing. The spacing between helical plates shall not be less than 3D, where D is the diameter of the largest helical plate measured from the edge of the helical plate to the edge of the adjacent helical pile plate, or 4D, where the spacing is measured from the center to the center of the adjacent helical pile plates.
Case 3: 2014 New York City Building Code
Summary:
- 4 x DLH , Where Spacing is defined as center-to-center of pile shaft. See Spacing A.
1812.10 Minimum Spacing of Piles. Minimum spacing between the center lines of helical piles shall be four times the largest helix plate diameter.
By Steve Gencorelli, P.E. (Steve@danbro.com)
Original article can be found here:
http://www.danbro.com/spacing-of-compressiontension-helical-piles-based-on-3-definitions/
