Q. Can helical piles be installed at an angle to reinforce a retaining wall?
A. Chad Smith, a builder who owns Distinctive Contracting Services in Arnold, Md., responds: My company has used helical piles for a number of applications besides house and deck foundations, including for retaining-wall tie-backs and to provide lateral stability for projects such as walkways and commercial signage. PierTech and Ideal (the brands we typically install), Techno Metal Post, and GoliathTech all publish design values our engineers can use in their designs; these values include allowable compression, tension, and lateral capacity.
On a recent project, for example, we used what are called “batter piles” installed at an angle and in line with the framing for a pedestrian walkway for a museum. While that design would perhaps be overkill for a residential project, the ipe walkway winds through a floodplain and is subject to commercial code. Requirements include higher design loads, and the engineer was concerned about the active loads from people walking on the pathway causing the framing to move back and forth. The batter piles help prevent that movement over the walkway’s straight runs, some of which measure as much as 70 feet in length.
When we use batter piles in a tie-back application such as for a retaining wall, a few different techniques can be used. The design is based upon the application. If a batter pile needs to be serviceable (for tensioning, for example), we can install a sleeve for a threaded rod to pass through the wall, and reinforce the concrete with a rebar grid around the tension location. If the application is fixed, we typically either use a “new construction” pile T cap on the end of the pile, or install a threaded rod and embed a large steel plate or steel washer within the concrete wall, or create a rebar grid around the pile top. In less substantial load applications, a simple hole can be drilled through the end of the pile and one or two pieces of rebar can be inserted through the pile and tied to a rebar grid. The approach we use depends on the height of the retaining wall and the engineered design (required for all retaining walls higher than 4 feet).
Another example of an alternative use for helical piles is a sign pylon foundation we recently built. For this foundation, we installed opposing pairs of batter piles at a 7.5-degree angle from the vertical to anchor the sign’s steel framework. That orientation—rather than a completely vertical one—will provide better resistance to uplift once the piles are encased in a concrete base, which in this project will measure about 15 feet long, 7 feet wide, and 36 to 42 inches deep. The overall size of the mass of concrete not only offers ballast weight to the assembly but also helps increase the side-load shear factor.
Photos by Chad Smith.