Multi-Helix Piles: Load Capacity, Helical Improvement Factor & Efficiency

In an earlier post, ("Going Solo"), the increase in axial load capacity achieved by adding a single helical plate to a plain pipe shaft was discussed in terms of the Load Improvement Factor (LIF). It was shown that the Load Improvement Factor (LIF) could vary from about 2 to 6 depending on the size of the central shaft and the size of the helical plate.

But what happens when one or more additional helices are added to a single-helix anchor or pile?

For example, a 10 in. or 12 in. helix could be added to a single 10 in. helix to give a 10/10 or 10/12 configuration. Obviously, the addition of a second or third helix should produce an increase in axial capacity, but how much? How much additional load capacity is developed by adding additional colinear helices to a single-helix anchor or pile? Figure 1 shows a typical installation of a multi-helix anchor at a solar farm site in Massachusetts.

Figure 1. Installation of Multi-Helix Round Shaft Anchor.

Based on the results of available full-scale field tests and model-scale laboratory tests on both sands and clays, it is clear that the use of additional helices increases the axial load capacity of a single-helix anchor or pile in both tension and compression. We can look at some recent field tests to evaluate how much of an increase can be expected for both “tapered” and “cylindrical” multi-helix anchors and piles.

Double-Helix and Triple-Helix Anchors and Piles

Helical anchors and piles are available with the helix diameter increasing along the shaft (referred to as “tapered”) as well as having the same diameter helices along the shaft (referred to as “cylindrical”). Comparing the behavior of a 10/10 to a 10/12 is not really fair since the surface area of the helical plates is not the same. It isn’t until a third helix is added that a more direct comparison can be made; e.g. 10/10/10 vs. 8/10/12. Both of these configurations have almost the same surface area.

Tapered Multi-Helix

The increase in load capacity obtained by adding a second helix might be considered in terms of the additional plate area provided by the second helix. It might be expected that the increase in load capacity would be directly proportional to the added area. Recall that the area of a circular plate increases as the square of the radius. For example, a single 8 in. helix has an area of 50.3 in.² while a double 8/10 has a combined area of 128.8 in.²; the ratio being 128.8 in.²/50.3 in.² = 2.56. It might be expected that the increase in capacity would also then be increased by a factor of 2.56. But actually, this isn’t the case.

Table 1 summarizes some recent tests at two sites in Massachusetts. Figure 2 gives the helix bearing area ratio, i.e., total area of all helices relative to the lead helix and the resulting load capacity ratio, relative to the load capacity of the single-helix. Table 2 gives the same results but in terms of the % load capacity increase. The relationship between HIF and Area Ratio for these tests is shown in Figure 2.

Table 1. Summary of Field Tests on Tapered Single-Helix and Multi-Helix Anchors. 

Figure 2. Relationship Between Helical Improvement Factor at Helix Area Ratio from Field Tests.

Table 2. Summary of Field Tests on Tapered Single-Helix and Multi-Helix Anchors.

For example, in the stiff clay, adding a 10 in. helix to an SS5-8 to make an 8/10 configuration increases the capacity by a factor of 2 (102% increase); now further adding a 12 in. helix to make an 8/10/12 increases the capacity over the 8/10 by a factor of just 1.35 (34.8%). A similar comparison can be made using the results of multi-helix cylindrical anchors at the same sites.

Cylindrical Multi-Helix

Some field tests from two sites in Massachusetts using “cylindrical” anchors are summarized in Table 3. While the load capacity is obviously increased with additional helices, these results suggest that, as with “tapered” anchors and piles, progressive disturbance may produce reduced capacity from individual helices following the path of the lead helix, i.e. the increase in load capacity is not “1 for 1”.

Table 3. Summary of Field Tests on Cylindrical Single-Helix and Multi-Helix Anchors.

Helical Improvement Factor (HIF)

In the previous post ("Going Solo") the relative increase in axial capacity achieved by a single-helix anchor over a straight shaft pipe pile of the same diameter and length was described using the Load Improvement Factor (LIF). For multi-helix anchors and piles it is more appropriate to define the Helical Improvement Factor (HIF) as the relative increase in load capacity of a multi-helix anchor or pile of the same diameter and length central shaft and the same helix diameter as a single-helix. This generally applies to multi-helix anchors and piles with an interhelix spacing of 3D_H.

In the late 1970s, Chance Civil Construction performed a large number of field tests on square-shaft (5 in x 1.5 in. (38 mm x 38 mm)) single-helix and multi-helix anchors in stiff clay in central Missouri. Multiple tests were performed at the same depth for different diameter helices ranging from 8 in. (203 mm) to 15 in. (381 mm) but all with interhelix spacing of 2.5. Figure 3 shows the average calculated Helical Improvement Factors for multiple tests performed for each geometry which indicate that in this stiff clay there is no influence of helix diameter on HIF. Note that in this clay, three helices are needed to obtain double the load capacity.

Figure 3. Calculated Helical Improvement Factors for Square-Shaft Helical Anchors in Stiff Clay.

Results from other more recent full-scale field tests on multi-helix anchors show similar results. So, even though a multi-helix anchor has higher axial capacity compared to the same diameter single-helix anchor, the second helix develops less capacity than the lead helix and so on. Figure 4 shows the calculated values of HIF for the set of tests previously given in Table 2.

Figure 4. Calculated Helical Improvement Factors for Square-Shaft Helical Anchors in Clay and Sand from Table 2.

Field tests were performed in sand in at a site in Missouri by Clemence et al. (1994) using SS200 (2.0 in. x 2.0 in.) square shaft cylindrical anchors with 12 in. diameter helices. Single-helix, double-helix and triple-helix anchors installed to a depth of 20 ft. were performed. The calculated Helical Improvement Factors are shown in Figure 5. Additional tests presented by Lutenegger (2011) for similar tests performed in Massachusetts are also shown. In this case SS5 (1.5 in. x 1.5 in. anchors with a helix diameter of 10 in. were used. In both cases the interhelix spacing was 3.

Figure 5. Calculated Helical Improvement Factors for Square-Shaft Helical Anchors in Sand.

Efficiency

Another way to look at the behavior of co-linear multi-helix anchor and piles is the consider the Efficiency. Although for deep foundations Efficiency is used to relate the behavior of a pile group to a single pile, in this case, the behavior of a multi-helix anchor or pile can be related to the behavior of a single-helix anchor or pile of the same geometry. How efficient is a second or third helix relative to the first helix? Efficiency of co-linear multi-helix anchors could also be defined in terms of ultimate capacity of a single-helix anchor with the same size helical plates as:

E = [Q_MH/(N x Q_SH)] x 100%

where:

E = efficiency of ultimate capacity

Q_M = ultimate load capacity of multi-helix anchor

Q_S = ultimate capacity of single-helix anchor

N = number of co-linear helical plates

Efficiency factors for the set of tests previously shown in Figure 2 are given in Figure 6 which shows that Efficiency progressively decreases as additional helices are added. The reduction in Efficiency suggests progressively increased disturbance of the clay during installation. Since all tests were performed using square shafts, the results predominantly reflect only the plate resistance.

Figure 6. Calculated Efficiency for Square-Shaft Helical Anchors in Stiff Clay.

These results indicate that in tension, the addition of a second helix results in an average Efficiency of 62% capacity as compared to a single-helix. The addition of a third helix results in an average efficiency of 52% and the addition of a fourth helix results in an average Efficiency of about 45%, independent of helix diameter. Field tests performed in sand by Clemence et al. (1994) previously shown in Figure 5 are shown in Figure 7. Results from additional field tests in both clay and sand are shown in Figures 8 and 9.

Figure 7. Calculated Efficiency for Square-Shaft Helical Anchors in Stiff Clay.

Figure 8. Calculated Efficiency for Square-Shaft Helical Anchors in Stiff Clay.

Figure 9. Calculated Efficiency for Square-Shaft Helical Anchors in Stiff Clay.

Summary

• • The addition of one or more helical plates to a single-helix anchor or pile increases the axial load capacity in both clay and sand under both tension and compression loading.

  • The increase in load capacity is related to the number and size of additional plates, but is not directly proportional to either the number of plates or the additional helix bearing area.

  • The increase in load capacity ranges from about 2 to 5 times the capacity of a single-helix for both double-helix and triple-helix anchors and piles in comparison to a single-helix. This increase can be described by the Helical Improvement Factor.

  • The increase in load capacity obtained by additional helices may be somewhat greater under compression than under tension because of the greater soil installation disturbance under tension in some soils, although this difference appears to be relatively small.

  • The increase in load capacity obtained by additional helices for both tension and compression may depend on the shape of the central shaft, i.e., square-shaft vs. round-shaft.

  • For multi-helix anchors and piles, additional helices are not as efficient at adding additional capacity as the lead helix in both clay and sand. Typically, a second helix is about 70 to 80% as efficient as the lead helix; a third helix is about 50 to 60% efficient.

As an alternative to using multi-helix anchors or piles, the engineer could consider using a single-helix anchor or pile with the same helix bearing area as a double-helix or triple-helix. For example, the combined total helix area of a 10/10 configuration is 127 in.²; the closest common size single-helix that gives similar area is a 12 in. (A_H = 113 in.²). This is slightly less, but a 14 in. helix gives 196 in.² and could be used. The final selection might be based on cost, availability and other site or project variables.

👉 Read Dr. Lutenegger's article "Going Solo: Load Improvement of Single-Helix Anchors and Piles"

References

Clemence, S., Crouch, L. and Stephenson, R., 1994. Uplift Capacity of Helical Anchors in Sand. Proceedings of the 2^nd Geotechnical Engineering Conference – Cairo, Vol. 1, pp. 332-343.

Lutenegger, A., 2011. Behavior of Multi-Helix Screw Anchors in Sand. Proceedings of the 14^th Pan-American Conference on Soil Mechanics and Geotechnical Engineering, 6 pp.

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.