In today’s world of smartphones, cloud computing, and autonomous vehicles, helical piles and anchors hardly qualify as cutting‑edge technology. Yet this seemingly simple product continues to play a critical and expanding role in both utility and civil construction. Its strength lies not in sophistication, but in simplicity—a low‑tech solution that is versatile, reliable, and based on well understood engineering principles.
Helical screw piles and anchors exist in many forms and types. A quick look through the Chance® Encyclopedia of Anchoring reveals familiar names:
Type PISA®, Tough One®, Square‑Shaft (SS), Round‑Rod (RR), and No‑Wrench anchors. Likewise, the Chance Technical Design Manual – Edition 5 gives Hubbell’s civil construction lineup—Type RS (Round Shaft), Street Light Foundations (SLF), Helical Pulldown® Micropiles (HPM) and GroutForce™ displacement piles. When other manufacturers are considered, it becomes clear that helical piles and anchors have evolved into a broad family of products.
Regardless of type, all helical piles and anchors share three fundamental components:
What differs is how these components are configured to meet specific applications. One of the most common questions—particularly for multi‑helix anchors—is:
how far apart should the helix plates be spaced?
Historically helical piles had just one large diameter helix plate located at the tip (bottom). To understand inter‑helix spacing, we need to look back nearly 60 years to the early development of multi‑helix anchors and piles. In the early 1960s, PISA anchors were already available in single‑ and twin‑helix configurations, typically with helix spacing between 24 and 30 inches. Standard rod lengths were 7 feet, a dimension driven by practical installation constraints.
The benefits of multiple helices were already recognized at the time. As noted in the 1966 Encyclopedia of Anchoring, multi‑helix anchors could replace larger single‑helix designs while delivering higher holding capacities.
Following the logic that if two helices are better than one, then three might be better than two, the A.B. Chance Company introduced extendable Type RR multi‑helix anchors in 1961. These were originally developed as tie‑downs for underground pipelines in weak coastal soils. However, the 1¼‑inch round steel shaft limited torque capacity in firmer soils.
That limitation led to the development of a higher‑torque multi‑helix anchor. In 1964–65, A.B. Chance introduced the Type SS anchor featuring a 1½‑inch square steel shaft. Both RR and SS anchors used 36‑inch inter‑helix spacing. This spacing was not derived from soil mechanics theory, but from practicality: three helices spaced 36 inches apart fit neatly on a 7‑foot shaft, which matched existing tooling and installation methods. Two‑helix versions fit on 5‑foot shafts, and four‑helix versions on 10‑foot shafts.
The three‑helix configuration quickly became the most popular and remains so today.
For nearly two decades, three‑foot helical pile spacing remained standard for Type RR, SS, and later RS products. However, during the 1970s and early 1980s, Chance began incorporating more formal geotechnical engineering principles into helical anchor/pile design.
It became widely recognized that the helix plates transfer load to the soil primarily through end bearing. This load creates a stress zone in the surrounding soil—above the helix for tension loads and below for compression loads. To perform efficiently, helix plates must be spaced far enough apart to prevent these stress zones from overlapping excessively.
This behavior is well described by the Boussinesq equation (circa 1885), which defines stress distribution beneath a loaded plate. The key observation is that soil stress diminishes rapidly with distance from the helix. At one helix diameter away, stress drops to about 28% of its peak value; at three diameters away, it falls to roughly 4%.
Figure 1 – Stress Distribution beneath Deep Bearing Plate
Spacing helix plates too closely can cause overlapping stress zones and unexpected performance issues. Spacing them too far apart, however, results in unnecessarily long shaft lengths and increases the likelihood that helices bear in different soil layers—leading to less predictable capacity and performance.
The challenge, then, is finding an optimum spacing.
Two well‑established analytical models help answer this question:
Bearing Capacity Theory - The total capacity is equal to the sum of the individual capacities of each helix plate. This assumes each helix acts independently in end bearing.
Cylindrical Shear Theory - Capacity is governed by the top helix (in tension) or bottom helix (in compression) plus the shear resistance of the soil along a cylinder formed between the top and bottom helices.
Figure 2 – Plate Bearing Model
Figure 3 – Cylindrical Shear Model
Both theories represent legitimate failure mechanisms. According to the Least Upper‑Bound Theorem, the controlling capacity is determined by whichever mechanism predicts lower resistance. At small spacings, cylindrical shear governs; at large spacings, individual bearing governs.
Field test data from the late 1970s showed the transition between these mechanisms occurs at approximately three helix diameters. This finding aligns with experience from multi‑belled concrete piers (Bassett, 1977) and explains why both analytical methods often produce similar results when three‑diameter spacing is used.
Figure 4 – Capacity as a Function of Helix Spacing
Today, Chance engineers at Hubbell design helical screw piles and anchors using three‑helix‑diameter spacing, measured from one helix to the next based on the lower helix diameter. This spacing maximizes load‑carrying efficiency while minimizing shaft length and improving performance predictability.
High‑strength Type SS products (SS150, SS175, SS200, and SS225) introduced in the late 1970s and early 1980s adopted this spacing from the outset. Type RS helical piles followed in 1986. The standard Chance Type SS5 product retained 36‑inch spacing until 1997, when it too was updated to align with the three‑diameter industry standard.
While 36‑inch spacing can still be specified, the transition to three‑diameter spacing is now accepted by the foundation engineering community.
👉 Download the Chance Technical Design Manual to learn more
Inter‑helix spacing may appear to be a minor design detail, but decades of experience, theory, and testing have shown it to be critical to helical screw pile and anchor performance. The three‑diameter rule represents the optimal balance between soil mechanics, constructability, and reliability—demonstrating that even the simplest products benefit from careful engineering.
For those interested in the deeper technical background or historical details, the Chance Engineering Department at Hubbell remains the definitive resource.