Figure 1. Typical Single-Helix Round-Shaft Helical Anchors/Piles.
Most geotechnical engineers are familiar with the behavior of driven pipe piles loaded in either axial tension or compression.
Load capacity is developed from a combination of:
shaft resistance and end bearing, depending on the direction of loading,
the soil type, and
the geometry of the pile.
Helical piles (and anchors) behave in a similar manner but engineers may not fully appreciate how the addition of just one helix to a plain pipe pile improves the load capacity. The increase in load capacity of a single-helix pile or anchor over a plain pipe pile can be described by the Load Improvement Factor (LIF) which is simply the ratio of the load capacity of the helical pile to the load capacity of the plain pile at the same axial displacement. This discussion is focused on round-shaft pipe piles since it is not applicable to square-shaft helical piles and anchors.
Figure 2 shows some typical results of full-scale axial uplift tests on a straight pipe pile (DS = 2.875 in.) and a single-helix helical pile (DH = 12 in.) of the same length installed to a depth of 10 ft. in stiff clay in Massachusetts (Lutenegger 2018). The pipe pile develops ultimate load capacity at very small displacement and often remains constant, as shown. This is the result of load capacity being developed only from shaft side resistance. By contrast, the helical pile develops load capacity more slowly and continues to develop capacity as the helix becomes engaged, even up to relatively large displacements. In this example, LIF = 4 (using QULT of the helical piles as Q at 0.1DH displacement (10% criterion) for Relative Helix Diameter of DH/DS = 4.2.
Figure 2. Behavior of Plain Pipe Pile and Single-Helix Pile in Stiff Clay.
Figure 3 shows results from a number of field tests at the same site consisting of stiff clay. The LIF will depend on the criterion used to define QULT; in this case the most common method of defining QULT as the load producing a displacement of 10% of the helix diameter (Q10) was used i.e., in this case QULT = Q at Δ = 1.2 in. The solid line shown in Figure 3 represents the theoretical trend, based on a traditional bearing capacity model and soil mechanics.
Figure 3. Variation in LIF as a Function of Relative Helix Diameter (DH/DS) for Single-Helix Field Uplift Tests in Stiff Clay.
Figure 4 shows some typical results of axial uplift tests on a straight pipe pile (DS = 4.5 in.) and a single-helix helical pile (DH = 12 in.) installed to a depth of just 8 ft. in medium dense silty-sand in Massachusetts. The results show similar behavior to the results shown in Figure 2 for stiff clay. In this case, LIF = 4.7 for DH/DS = 2.7. Figure 5 shows a summary of a series of tests performed in medium dense sand on single-helix anchors with different size shafts and helices, giving a range of typical DH/DS.
Figure 4. Behavior of Plain Pipe Pile and Single-Helix Pile in Sand.
These results show that the LIF can be as high as 6 to 7 for a helical anchor with DH/DS on the order of 4 to 6. The large increase in axial capacity would clearly outweigh the cost difference between the added cost of fabrication but lower cost of installation of a helical pile in comparison to a plain pipe pile of the same diameter and length. The solid line shown in Figure 5 represents the theoretical relationship between LIF and DH/DS based on a simple bearing capacity model for a uniform sand with assumed soil characteristics.
Figure 5. Variation in LIF as a Function of Relative Helix Diameter (DH/DS) for Single-Helix Field Uplift Tests in Sand.
Some recent results obtained in sand in Australia on instrumented single-helix piles (Bittar et al. 2024) show that LIF = 4.7 for DH/DS = 2.8 in tension and LIF = 6.3 for DH/DS = 2.8 in compression. Higher values of LIF for compression most likely represent the increase in load capacity developed by the full cross-sectional area of the helix as compared to the net area of the helix in tension, even with the added end bearing of a plain pipe pile in compression.
The difference in axial capacity in tension vs. compression loading of plain pipe piles in most soils is the added end bearing developed in compression which is absent in tension. In uniform soils, the difference in axial capacity between a helical anchor (tension) and a helical pile (compression) with the same size shaft and helix can be attributed to two factors:
1) difference in helix area developing end bearing; and
2) installation disturbance.
Under tension, the helix load is developed from the Effective Helix Area, which is equal to the total cross-sectional area of the helical plate minus the cross-sectional area of the central shaft (i.e., net area = total helix area – shaft area). This depends on the relative sizes of the helix and shaft. Under compression, the load is developed from the total helix area. Even in open pipe shafts, field observations show that in both clay and sand, the shaft becomes plugged with soil after just a short installation distance and during loading the soil plug is stationary.
During installation, the rotation of the helix and shaft disturbs the soil to some degree since the soil must be moved to accommodate the volume of the anchor or pile. “Perfect” installation occurs when the helical blade advances a distance equal to one blade pitch for a full 360° rotation of the central shaft. This is called “Pitch-Matched” installation. Unfortunately, some contractors pay little attention to the advance of the helix during installation. However, a good inspector will work with the contractor to achieve high quality installation. The more rotations with non-pitch-matched installation, the more soil disturbance.
Under tension, the axial load is developed in soil which has experienced the passage of the helical plate. Under compression, the axial load is developed in undisturbed soil beneath the helical plate. The difference between tension and compression will depend on the quality of the installation and in the case of clays, to some degree on the Sensitivity of the clay. Actually, in loose sands, the installation may densify the soil above the helical plate which can lead to tension capacity higher than compression compacity.
Based on available field and laboratory tests on plain pipe piles and single-helix round-shaft anchors and piles:
1. The addition of a single helical plate to a plain straight shaft pipe pile increases the axial load capacity in both tension and compression by a factor of about 3 to 6 and can be quantified by the Load Improvement Factor (LIF).
2. The increase in axial capacity depends on the diameter of the helix relative to the diameter of the pipe shaft (DH/DS), the length of the pipe shaft and the soil type.
3. In uniform soil, the contribution to axial capacity provided by the shaft depends on the length of the shaft in full contact with the adjacent soil.
4. The increase in axial capacity provided by a single helix is similar in compression and tension but generally will be somewhat higher in compression.
5. The increase in axial capacity provided by a single helix is generally larger for sand as compared to clay because of the higher helix capacity developed in sand.
6. In sands, the increase in axial capacity provided by a single helix increases as the depth of embedment increases and vertical effective stress increases.
For many projects where the engineer might be considering using driven piles as a deep foundation system, helical piles are likely to provide a more cost-effective alternative, especially when compared in terms of Cost/Kip of Allowable Capacity ($/QALLOW). The small amount of additional steel and additional fabrication costs needed for a helical anchor or pile are offset by the large increase in axial capacity provided by the helix. The option for the engineer would be to use a larger pipe pile to achieve the same load capacity as the helical pile, however this typically would not be cost-effective.
Bittar, E., et al. 2024. CPT-Based Design Method for Helical Piles in Sand. Canadian Geotechnical Journal, Vol. 61, pp. 102-117.
Lutenegger, A.J., 2017. Uplift Behaviour of Round Shaft Single-Helix Screw-Piles for Elevated Ground Mount Soil Panel Systems. Proceedings of the 19th International Conference on Soil Mechanics and Foundation Engineering, pp. 615-618.
Dr. Alan Lutenegger is Emeritus Professor of Geotechnical Engineering at the University of Massachusetts-Amherst. He has over 45 years of experience in geotechnical engineering and over 20 years of practical experience with helical anchors and piles.