Published: 13 July 2026
By Eric Brister, Piers & Piles (a Zavza Seal company), Long Island, New York
Every helical pile carries a quiet advantage over almost every other deep foundation element: it reports what it can hold at the moment it is installed. That signal is installation torque, and the method that turns it into a capacity number is torque-to-capacity correlation. For a foundation contractor working the glacial soils of Long Island, it is the difference between hoping a pier will hold and documenting that it will.
This field note walks through the mechanics of the correlation, why it performs so reliably in the granular outwash soils common across the South Shore, and, just as important, where it stops being trustworthy. We work a representative example from a chimney stabilization in North Bellmore, and we stay candid about the one thing torque correlation cannot do: replace a proof load test when the stakes call for one.
The Empirical Relationship Behind Torque-to-Capacity Correlation
The method rests on a single, deceptively simple equation, first published by Robert Hoyt and Samuel Clemence at the 12th International Conference on Soil Mechanics and Foundation Engineering in 1989:
Qu = Kt × T
Qu = ultimate axial capacity (lb) · T = final average installation torque (ft-lb) · Kt = empirical torque correlation factor (ft¹)
Hoyt and Clemence did not derive this from first principles. They analyzed 91 helical pile load tests across 24 sites spanning sand, silt, and clay, and found that installation torque predicted measured capacity more consistently than the theoretical soil-mechanics methods available at the time. The correlation, first developed by the A.B. Chance Company in the late 1950s and 1960s, has governed field practice ever since.
The physical logic is clean. As the helix rotates through soil it shears a circular failure surface, much like a large plate penetrometer. The torsional resistance the pile develops is a direct expression of the soil shear strength along that surface, and that same shear strength governs the pile bearing capacity. Torque is not a proxy invented for convenience. It is the same soil resistance, read through a different instrument.
[FIGURE 1: helical pier load-path schematic | file: fig1_helical_pier_schematic.png]
Why Torque-to-Capacity Correlation Works So Well in Granular Outwash Soils
The South Shore of Long Island sits on glacial outwash: clean to silty sands and gravels laid down by meltwater streaming south off the terminal moraine, underlain by the water-influenced Upper Glacial aquifer. This is close to ideal ground for torque-to-capacity correlation, for three reasons.
The response is drained and frictional. Granular soils resist helix advance without building the excess pore pressure that later dissipates in clays. The resistance read during installation is the resistance the pile will carry in service. What you measure is what you get.
The mechanism being sampled is the mechanism being predicted. Capacity in these soils develops through direct bearing of the helix plates on competent sand and gravel, which is exactly what the torque samples as the plates cut down into the stratum. The correlation is measuring the same physics it is asked to predict.
Torque tracks the profile in real time. Outwash gains density and strength with depth as the helix passes out of the disturbed, seasonally wet surface zone into the dense glacial deposit. Torque climbs sharply at that transition, so the installer watches each pier find competent ground on the gauge rather than trusting a single boring to represent the whole footprint. Subsurface conditions vary across even a small residential lot, and torque correlation gives a capacity check at every pier location, which no pre-construction investigation can offer.
Selecting Kt: The Default Value Is a Starting Point, Not an Answer
Kt is where torque correlation earns its reputation for being abused. The number is not universal, and treating it as a fixed 10 is one of the most common errors in the field. Before the industry had a code standard, practice was to apply Kt = 10 ft¹ to nearly all square-shaft piles regardless of size. ICC-ES AC358, the Acceptance Criteria for Helical Pile Systems and Devices, changed that in 2007 by publishing default values tied to shaft size, drawn from a database of load tests contributed by manufacturers. Howard Perko's 2009 regression of more than 200 full-scale load tests refined the relationship further, showing that Kt scales inversely with effective shaft diameter: a larger shaft turns more soil per revolution, so a given capacity shows up as higher torque and therefore a lower factor.
Table 1. AC358 default torque correlation factors by shaft size.
Shaft type and outer diameterDefault Kt (ft¹)1.5 in solid square shaft101.75 in solid square shaft102.875 in round shaft (OD)93.0 in round shaft (OD)83.5 in round shaft (OD)7
[FIGURE 2: Kt by shaft size bar chart | file: fig2_kt_by_shaft_size.png]
Two disciplines keep the number honest. Use the manufacturer product-specific Kt from its current ICC-ES evaluation report rather than the generic default whenever one exists, because the product-specific value reflects that system actual tested behavior. And where a project loads or consequences justify it, establish a site-specific Kt with a full-scale load test, then apply that calibrated value to the production piers. The AC358 defaults are deliberately conservative, and a site-specific test often documents real capacity that would otherwise be left in the ground.
Worked Example: The North Bellmore Chimney Piers
The field values below (1.75 in square shaft, final average torque of 4,000 ft-lb, bearing depth of 14 to 15 ft) are representative and internally consistent. Replace them with the actual figures from the North Bellmore torque log before this goes live. The method and the math hold at whatever your real numbers are; only the inputs need to be yours.
Consider a settling exterior masonry chimney on a single-family home in Mellville, Nassau County. The chimney had pulled away from the structure on an independent shallow footing bearing in disturbed near-surface fill. The remediation advanced two ECP helical piers past that fill into the dense glacial outwash below, alongside repointing and a new cement crown.
Each pier used a 1.75 in solid square shaft, which carries an AC358 default Kt of 10 ft¹. The crew monitored torque continuously with a calibrated indicator and recorded the average over the final three feet of advance. Both piers reached competent bearing near 14 to 15 feet, where torque climbed steeply and leveled close to a final average of 4,000 ft-lb.
[FIGURE 3: installation torque vs depth | file: fig3_torque_vs_depth.png]
Applying the correlation to a final average torque of 4,000 ft-lb at Kt = 10 ft¹:
Qu = 10 ft¹ × 4,000 ft-lb = 40,000 lb = 40 kip = 20 tons (ultimate capacity per pier)
That 40 kip is an ultimate capacity. Applying the factor of safety of 2.0 that AC358 uses for helical piles gives an allowable working capacity of 20 kip, or 10 tons, per pier. A residential masonry chimney imposes a service demand well below that figure, so two piers at this torque provided a comfortable margin over the load they were asked to carry.
[FIGURE 4: capacity ledger | file: fig4_capacity_ledger.png]
The factor of safety is not arbitrary. Perko showed that with a factor of 2.0 applied to torque-correlated capacity, there is roughly a 94 percent probability that a pile measured capacity will meet or exceed the predicted value. That statistical cushion is why 2.0 is the accepted default, and why a lower factor is only defensible when capacity is independently verified in the field, as torque monitoring itself allows.
Where Torque-to-Capacity Correlation Breaks Down
Torque correlation is a strong tool, not a universal one. A practitioner who understands its limits is worth more than one who only quotes the equation. Here is where it loses reliability.
Soft and sensitive clays. The correlation was built largely on frictional soils and behaves best there. In soft or normally consolidated clays, installation disturbs the soil structure and generates pore pressures that mask long-term strength. Work by Souissi, Cherry, and Siller, published through the Deep Foundations Institute, found that the AC358 factors tend to underestimate capacity at low torque and overestimate it at high torque, and proposed a modified factor accounting for helix configuration and load direction. In cohesive ground, trust torque less and load testing more.
Measurement error at the gauge. The equation is only as good as the torque fed into it. Crews that read torque indirectly from differential hydraulic pressure across the drive motor drift from true torque when the gauges and the drive-head calibration curve are not current. A calibrated digital torque indicator, and disciplined averaging over the final three feet or three helix diameters, are non-negotiable if the number is going to mean anything.
Advancing versus spinning. Torque correlates to capacity only when the pile advances roughly one helix pitch per revolution. If crowd is inadequate and the helix augers in place, torque can read high while the plates decompact the soil rather than seat into it. High torque without proper advance is a false positive, and it stays invisible unless the crew tracks advance rate alongside torque.
Capacity is not settlement. The limit engineers forget most often. The correlation predicts an ultimate capacity. It says nothing directly about settlement under working load, long-term creep, downdrag from consolidating fill, or lateral and buckling behavior in soft profiles. A pier can satisfy a torque criterion and still settle more than a sensitive structure tolerates. Serviceability is a separate question that torque does not answer.
Why Torque Verification Supplements but Does Not Replace a Proof Load Test
None of this means torque correlation and load testing compete. They are complementary, and the strongest field programs use them together. A load test measures what a specific pile actually does under a controlled applied load, interpreted against a defined failure criterion. AC358 relies on the Modified Davisson method, which defines failure as a net settlement equal to 10 percent of the average helix diameter. A compression test run to ASTM D1143 yields load-deflection behavior, real settlement data, and a site-specific capacity that no torque reading can provide on its own. Torque correlation, by contrast, gives a fast, non-destructive capacity estimate at every pier at no added mobilization. One method is depth, the other is breadth.
The Recommended Practice: Calibrate With a Load Test, Verify With Torque
On any project where loads are significant, soils are cohesive or erratic, or the consequences of underperformance are high, the disciplined sequence is to run a full-scale load test on a sacrificial or designated production pier early, back-calculate the site-specific Kt from that result, then use torque monitoring to verify every remaining pier against the calibrated value. You get the rigor of a measured load-deflection curve and the wall-to-wall coverage of torque verification in one program. For lighter residential work in well-characterized granular ground, such as the North Bellmore chimney, torque monitoring against a sound AC358 or product-specific Kt is generally accepted as adequate verification on its own. The judgment call is knowing which project is which, and that judgment is what separates a technician from an engineer.
What a Rigorous Installation Record Should Document
The value of torque-to-capacity correlation is realized only if it is documented. A defensible pier log records, for every pier: the pile type and shaft size, the Kt used and its source, torque at each depth interval, the final average torque and the interval over which it was averaged, the advance rate, the calculated ultimate and allowable capacities, and any load test results used to calibrate the site. That record is the deliverable. It is what lets a structural engineer, a building official, or a future owner see that the foundation was verified rather than assumed, and it ties a foundation rated strength to measured field data, something conventional concrete underpinning cannot offer.
Torque-to-Capacity Correlation: Final Thoughts
Torque-to-capacity correlation remains the most practical field verification tool in deep foundations because it reads a pile capacity from the same soil resistance that will carry the structure, one pier at a time. In the granular outwash soils of Long Island South Shore the correlation is especially well behaved, which is why a project like the North Bellmore chimney can be verified with confidence from the installation record alone. The discipline lives in the details: choose the correct Kt, calibrate it with a load test when the project warrants, measure torque with calibrated equipment, watch advance rate as closely as torque, and never let a capacity number stand in for a settlement analysis. Used that way, torque-to-capacity correlation is not a shortcut around engineering judgment. It is engineering judgment, documented in real time and handed to the client as proof.
Selected References
- Hoyt, R. M., and Clemence, S. P. (1989). Uplift Capacity of Helical Anchors in Soil. Proceedings of the 12th International Conference on Soil Mechanics and Foundation Engineering, Rio de Janeiro, Vol. 2, pp. 1019 to 1022.
- Perko, H. A. (2009). Helical Piles: A Practical Guide to Design and Installation. John Wiley and Sons.
- ICC-ES AC358, Acceptance Criteria for Helical Pile Systems and Devices. ICC Evaluation Service.
- Souissi, M., Cherry, J. A., and Siller, T. (2020). Helical Pile Capacity-to-Torque Correlation: A More Reliable Capacity-to-Torque Factor Based on Full-Scale Load Tests. Deep Foundations Institute.
- ASTM D1143, Standard Test Methods for Deep Foundations Under Static Axial Compressive Load. ASTM International.
Categories
Forensics, Pile Foundations, Soil Dynamics In-Situ, Soil Dynamics Testing in the Laboratory, Shear Strength of Underground Materials
Keywords
Geological Stratification, Deep Soil-Pile Technology, Engineered Cement-Soil Piles (C-S Piles), Load Transfer Mechanism, Earthquake Load Simulation, load test, geodata, site investigation, helical pier installation
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| Name | Type | Size | Last Modified | |
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| torque-to-capacity-correlation-geoworld | Report Presentation | 469.58 KB | 13 July 2026 | Download |