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Installation Torque as a Predictor of Helical Pile Axial Capacity

by contributing author Dr. Howard Perko, Magnum Geo-Solutions

HPW Archive 2007

Introduction

It is generally accepted that installation torque can be used to predict the axial capacity of a helical pier in both tension and compression.  Over the last twenty years, this unique feature of helical piers has helped them gain recognition and popularity.  This article provides a brief overview of the origin, reliability, pit-falls, and applicability of correlations between torque and capacity.

Origins

To the extent of the author's knowledge, the correlation between installation torque and helical pier capacity was originally formalized in a 1989 paper by Sam Clemence (a professor at Syracuse University) and Bob Hoyt (then an employee of the A.B. Chance Company, now an independent engineering consultant).  In this benchmark publication in the Proceedings of the 12th International Conference on Soil Mechanics and Foundation Engineering in Rio de Janeiro, the following elegant equation was suggested that related final installation torque, T, with axial capacity, P.

     P= Kt T

The parameter Kt was termed the capacity to torque ratio.  This parameter did not appear to be a function of the number and size of helical bearing plates; however it was dependent on shaft diameter.  The equation was based on empirical data and experience.

In 2002, the author published a paper in the ASCE Proceedings of the Geo-Denver conference.  In that paper, a theoretical model was derived that correlates installation torque with axial capacity.  The model is based on equivalence between the energy required to overcome friction and penetration of the soil and the energy expended during helical pier insertion into the ground.  The intended benefit of the theoretical model was to explore factors that might affect the capacity to torque ratio.

Empirical Evidence

The paper by Hoyt and Clemence has been referenced by numerous technical articles.  It is cited in the literature of many helical pier manufacturers.  Below is a discussion of some of the assumptions and conclusions in the paper.

Predictions of helical pier capacity based on torque were compared with the results of 91 full-scale load tests.  It is important to note that these tests were in tension.  The industry standard of care is to assume that the capacity of helical piers in compression is at least as great as tension.  Practical studies have revealed that the capacity in compression is typically on the order of 10% greater than tension capacity in relatively homogeneous soils.

Hoyt and Clemence also compared load test data with calculated capacity based on various adaptations of bearing capacity theory.  The standard deviation of the predictions using capacity to torque ratio was less than the standard deviation of calculated capacity.  Hence, it was shown that predictions based on torque were more precise than other calculations.

It is also important to note that all of the tests used to develop the correlation between torque and capacity were on helical piers with 3/8" thick helices, 3" helix pitch, true helix shaped blades, and 8" to 14" helix diameters. Helical pier shafts varied from 1.5" square to 3" nominal pipe (3.5" O.D.).  A capacity to torque ratio of 10 ft-1 was recommended for 1.5" square shaft piers and 7 ft-1 for 3" nominal pipe shaft piers.  Larger, 8" diameter tubular helical piers were tested, but capacity to torque ratios were much lower and more variable.  Larger square shaft piers were not included in the paper.

Theoretical Evidence

The theoretical model presented previously by the author confirmed a number of conclusions drawn from previous empirical data.  The theoretical capacity to torque ratio is generally independent of helix diameter, number of helical bearing plates, and soil type.  It does depend on shaft diameter and other factors discussed below.

The theoretical model suggests that a number of factors may affect the capacity to torque ratio.  Increased helical blade thickness results in higher torque without corresponding increases in theoretical bearing capacity.  Increasing helix pitch also increases torque without theoretically affecting capacity.

Reliability

The standard deviation of the data presented by Hoyt and Clemence suggests that if a factor of safety of 2.0 is used, there is a 94% probability that measured capacity will be equal to or higher than the predicted capacity based on torque correlations.  It is partly based on this fact and the experience of practicing engineers that a factor of safety of 2.0 is used with regard to the capacity of helical piers.  The ASCE special publication on driven piles suggests that a factor of safety as low as 1.5 can be used provided capacity is verified in the field at each pier location using an independent indicator such as driving resistance (or torque).  A factor of safety of 3.0 is typically used with drilled piers, auger cast piles, or other deep foundations where the capacity is not verified in the field.

The load tests used by Hoyt and Clemence to recommend capacity to torque ratios were all taken relative to ultimate bearing capacity (failure) rather than allowable deflection.  It is the author's experience that using a factor of safety of 2.0 and bearing in a suitable bearing material (SPT generally greater than 20), the deflection at the allowable capacity is generally less than 1 inch.  The theoretical model that correlates helical capacity to installation torque is based on a deflection of 1 inch, however, a soil modulus for dense or stiff soils was used in the model derivation.  When deflection is a particular concern, full-scale load tests should be conducted on site to determine probable ranges of deflection.

 Pit-Falls

All of the empirical work that has been published to date is limited to true helix shaped plates, 3" pitch, 3/8" thickness, and 8"-14" diameter helical bearing plates.  It is also limited to 1.5" square to 3.5" diameter shafts.  Products manufactured outside of these tolerances should be approached with caution with regard to using previously published capacity to torque ratios.  In other words, capacity to torque ratios should not be blindly applied to all manufactured products.  

Some engineers believe that the capacity to torque ratio is to a certain extent a function of soil type.  It is believed that previously published capacity to torque ratios are generally conservative.  Some engineers recommend load testing at sites with a particularly large number of helical piers to better assess the capacity to torque ratio at specific sites.  This sometimes allows for better economics through value engineering.

A number of factors affect torque measurements.  The installation speed during all past research has generally been between 10 and 30 rpm.  It is unknown how slower or faster rates of installation affect the capacity to torque ratio.  Augering, defined as rotation of the helix where forward advancement is stalled, causes a significant decrease in torque.  Correlations between torque and capacity will be ultra-conservative when augering occurs.  Augering is most common when the helix advances from a softer stratum into a harder stratum.  Augering is also common with untrue helix blades (blades where the leading and trailing edge are not parallel).  Traditional bearing capacity theory should be used to determine helical pier capacity if augering cannot be avoided.

Applicability

The correlation between installation torque and capacity has been empirically and theoretically proven.  Verification of helical pier capacity by correlations with torque has been used for more than 20 years.  Most engineers agree that helical pier capacity should always be verified in the field through installation torque measurements.  Traditional bearing capacity theory can be used to determine the number and size of helical bearing plates used at a site.  Care should be taken not to compound factors of safety (e.g. a factor of safety applied to the soil strength parameters, allowable bearing capacity, and service loads), otherwise one can end up with a helical pier with too many bearing plates to effectively penetrate the ground (e.g. early refusal).

The following industry standards apply to shafts with blades spaced along the shaft at 2.5 to 3.5 times the average blade diameter on-center and meeting the specifications discussed above.

          1.5" and 1.75" Square Shafts     -      Kt = 10 ft-1

          2.875" O.D. Round Shafts          -      Kt = 9 ft-1

          3.0" O.D. Round Shafts             -       Kt = 8 ft-1

          3.5" O.D. Round Shafts             -       Kt = 7 ft-1

Manufacturers should be consulted to verify that these values apply to their products.  Capacity may be limited by mechanical strength of the helical pier and its components.