Below is a list of frequently asked questions about helical piles. This list will be updated periodically with more content.
Q: What is the minimum spacing of helical piles?
A: As a general rule of thumb, space helical piles 3 to 4 diameters on-center. Piles closer than this are not necessarily incorrect but need to be anylized for group efficiency. Piles can be battered slightly away from each other so the pile heads fit into a smaller cap while the helical bearing plates still meet the minimum on-center spacing.
Q: What is the minimum spacing of helical piles in axial load tests?
A: Per ASTM D1143, reaction piles shall be minimum of 5 diameters or 8 feet from the test pile, whichever is greater. Test piles loaded individually do not have a minimum spacing. Tests can be performed individually on sacrificial piles spaced close together using the same load frame.
Q: What is the correct name to use when referencing these foundations?
A: Helical pier, screw pile, helix pier, torque anchor, and helical anchor are some of the names associated with these devices. In 2007, the Deep Foundation Institute committee on helical foundations and tie-backs voted to foreverafter use the name helical pile. Reasons for this terminology are given in Chapter 1 of the book.
Q: Are there any commerical helical design programs out there, and any suggestions where I might purchase a copy?
A: I am currently working on a tool but it won’t be ready until the beginning of next year. There are a few manufacturer’s that have software as described in Chapter 8 of the book. I don’t know of anything else.
Q: Do you have to trim the footing when doing underpinning?
A: Based on my experience, I absolutely agree with minimizing the eccentricity of the wall-to-pile connection as the secret to successful underpinning. Trimming the footing flush with the foundation wall greatly reduces the amount of overturning exerted on the foundation. Another aspect that helps is reducing the eccentricity of the underpinning bracket itself. I frequently use a bracket where the pile shaft is almost directly against the face of the wall so that bracket eccentricity is minimized.
Q: What affect does installation angle have on helical pile performance in underpinning?
A: I prefer vertical piles and round shafts. Round shafts offer increased lateral and buckling resistance. Load testing that I have performed suggests that the vertical round shaft promotes a stronger shear and moment connection to the foundation. The batter angle tends to pry-off the underpinning bracket from the foundation. The reason that some companies use a slight batter angle on their pier is so that the installation contractor can avoid the roof overhand on a home when they install the pier. I do not believe that the angle adds any stability.
Q: How do you account for eccentricity and instability in underpinning designs?
A: The conclusion that I tried to make in my book is that if you run a simple AISC combined buckling and bending analysis on a typical helical pile shaft, you should see that these slender shafts can support very little eccentric loading. In the case of the square shaft, it is almost none. How then do they work? I believe that the simplest and best answer is to treat the helical pile shaft as rotationally flexible (as in free body diagram “b” in Figure 14.4 of my book), place no flexural moment in the shaft, and conservatively work the entire eccentric moment back into the structure/foundation to make sure that the internal stability of the wall and bracing elements, floor slab and main floor framing, are all adequate to support the induced overturning. If the foundation/structure is inadequate, then underpinning has to be done on both sides of the wall to balance the eccentricity (often the case in commercial structures with relatively long, uninterrupted walls). Don't forget to check the moment connection between the pile and the structure.