Single-Helix Anchors and Piles Show Load Capacity Gains of up to Seven Times Over Plain Pipe Piles Sponsored

Dr. Alan J. Lutenegger, Emeritus Professor of Geotechnical Engineering at the University of Massachusetts-Amherst, published a technical analysis examining how the addition of a single helical plate to a plain round-shaft pipe pile affects axial load capacity in both tension and compression. The analysis introduces a metric called the Load Improvement Factor (LIF), defined as the ratio of the load capacity of a single-helix helical pile to the load capacity of a plain pipe pile at the same axial displacement. The discussion is explicitly limited to round-shaft pipe piles and does not apply to square-shaft helical piles and anchors.

The axial load capacity of helical anchors and piles is developed from a combination of shaft resistance and helix bearing, analogous to the shaft resistance and end bearing of conventional driven piles. For a fixed anchor or pile length, the geometry is described by the Relative Helix Diameter (DH/DS), where DH is the helix diameter and DS is the shaft diameter. A plain pipe pile has a DH/DS ratio of 1, while typical helical anchors and piles have DH/DS ratios ranging from approximately 2 to 5.

Full-scale axial uplift tests conducted in stiff clay in Massachusetts compared a straight pipe pile with a shaft diameter of 2.875 inches to a single-helix helical pile with a helix diameter of 12 inches, both installed to a depth of 10 feet. The plain pipe pile developed its ultimate load capacity at very small displacement through shaft side resistance alone, while the helical pile developed capacity more gradually as the helix became engaged, continuing to gain capacity at relatively large displacements. Using the 10% helix diameter displacement criterion to define ultimate load capacity, the LIF at a DH/DS ratio of 4.2 was recorded as 4 in that test. A series of field tests at the same stiff clay site confirmed that LIF values vary depending on the criterion used to define ultimate capacity, with results aligned with a theoretical trend based on a traditional bearing capacity model.

Axial uplift tests in medium dense silty sand in Massachusetts compared a plain pipe pile with a shaft diameter of 4.5 inches to a single-helix helical pile with a helix diameter of 12 inches, installed to a depth of 8 feet. The behavior was similar to that observed in clay, with a LIF of 4.7 recorded at a DH/DS ratio of 2.7. A summary of multiple tests across single-helix anchors with different shaft and helix sizes in medium dense sand showed that the LIF can reach as high as 6 to 7 for DH/DS ratios on the order of 4 to 6. A theoretical relationship between LIF and DH/DS based on a simple bearing capacity model for uniform sand with assumed soil characteristics was also presented.

Instrumented single-helix pile tests conducted in sand in Australia, referenced from Bittar et al. (2024), recorded a LIF of 4.7 for a DH/DS ratio of 2.8 in tension and a LIF of 6.3 for the same DH/DS ratio in compression. The higher LIF in compression is attributed to the load capacity being developed from the full cross-sectional area of the helix under compression, compared to the net helix area, which excludes the shaft cross-section, used under tension, even after accounting for the end bearing contribution of a plain pipe pile in compression.

The analysis identifies two factors responsible for differences in axial capacity between helical anchors loaded in tension and helical piles loaded in compression in uniform soils. The first is the difference in effective helix area. Under tension, the load is developed from the net helix area, calculated as the total cross-sectional area of the helical plate minus the cross-sectional area of the central shaft. Under compression, the full total helix area develops the load. Field observations in both clay and sand indicate that open pipe shafts become plugged with soil during installation, and the soil plug remains stationary during loading. The second factor is installation disturbance. During installation, soil is displaced to accommodate the volume of the anchor or pile. Pitch-matched installation, in which the helical blade advances a distance equal to one blade pitch per full 360-degree rotation, is described as the ideal condition. Deviations from pitch-matched installation increase soil disturbance. Under tension loading, the axial load is developed in soil that has experienced the passage of the helical plate, whereas under compression, the load is developed in undisturbed soil beneath the helical plate. In clays, the degree of disturbance-related difference between tension and compression capacity is also influenced by the sensitivity of the clay. In loose sands, installation may densify the soil above the helical plate, which can result in tension capacity exceeding compression capacity.

The analysis concludes that adding a single helical plate to a plain pipe pile increases axial load capacity by a factor of approximately 3 to 6 in both tension and compression, as quantified by the LIF. The increase depends on the DH/DS ratio, shaft length, and soil type. Compression loading generally produces somewhat higher LIF values than tension. The capacity increase provided by a single helix is generally larger in sand than in clay due to higher helix bearing capacity in sand, and in sand the increase grows with depth as vertical effective stress increases. The analysis notes that for projects where driven piles are under consideration, helical piles may offer a more cost-effective alternative when evaluated on the basis of cost per unit of allowable capacity, with the additional steel and fabrication costs offset by the large increase in axial capacity.