Below is a list of frequently asked questions about helical piles. This list will be updated periodically with more content.
A: The pitch affects the capacity to torque ratio. A helical pile is installed to a certain final torque. The torque indicates that there is a good confidence that the pile can hold the required loads. To date, perhaps 99% of all published load test data in the open literature has been for a helix with 3" pitch (I can only think of two papers/reports out of 150 that I read when I wrote my book). Also, current building code document ICC-ES AC358 lists criteria for a proper helix bearing plate construction. One of them is that it has to have a 3" pitch. The fact of the matter is that with 3" pitch, one can assure their clients without load testing that the pile will support the design loads with 99.7% certainty using published capacity:torque ratios from 100's of load tests. This is considered average geotechnical risk. With larger pitch, the data is more limited. It becomes more important to do a comprehensive site specific load test program to prove the capacity:torque ratio for a specific site and helical pitch. I am aware of companies in Canada and in the U.S. using larger pitch on large diameter helical piles, but it has only been in the last 5 to 10 years, whereas the 3" pitch has been used for 70 years in the U.S. Also, there has been quite a bit of litigation surrounding helical screw piles with larger pitch. I am an expert witness in three separate multi-million cases right now in Colorado. In one case several buildings had to be torn down and re-built due to excessive settlement of the piles. The contractor performed several load tests for piles bearing on rock and came up with a site specific capacity to torque ratio that was 4 times higher than the literature would suggest. As a result, many of the piles were not installed to the proper depth and failed.
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.
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.
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.
A: I have developed a beta version of a design tool for foundation layout using helical piles or steel push piers. It can be downloaded FREE by following this link to a popular manufacturer's website. There are a few other manufacturer’s that have software as described in Chapter 8 of the book. I am currently working on some other design software. I don’t know of anything else.
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.
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.
A: Compute the total eccentricity from the point where the loads are applied and where the reaction is provided by the helical pile. This is the total eccentricity. Then use AISC combined buckling and bending calculations to compute the maximum eccentricity that the helical pile shaft can resist. Per IBC Chapter 18, use 5 feet for a depth of fixity in firm ground and 10 feet in soft ground. Subtract this eccentricity from the total. The rest of the overturning must be transfered back into the structure/foundation. Make sure that the foundation wall you are underpinning can resist these rotational forces without tipping over or cracking. A floor slab and main floor framing may provide some bracing. Some engineers also rely on earth pressure to provide bracing. This is not recommended in seisimic areas. If the foundation/structure is inadequate, then underpinning has to be done on both sides of the foundation 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.
A: Round-Shaft helical piers are used primarily for new construction for residential, commercial, and industrial foundations. They can be used with our without concrete pile caps. They provide some lateral capacity and considerably higher buckling capacity. Square-shaft helical piers (also known as square-shaft helical anchors) are used primarily for tie-backs and helical soil nails for support of excavation, permanent earth retention, and slope stabilization. They also are sometimes used for underpinning and new construction foundations, however their low buckling capacity and almost negligeable lateral capacity needs to be taken into consideration. Click here for more discussion from a popular helical pile manufacturing company.