Helical Piles: Equation Methods vs Settlement Based Approach
Helical pile design is governed by simple geotechnical engineering principles. Helical pile foundation design is controlled primarily by the bearing resistance on each helical plate. In some cases, the shaft resistance can also be included. Since the helical pile is in compression the pile might have to be examined in buckling. In principle the ultimate capacity of a helical pile has traditionally been determined as the most critical of the individual bearing mode or the cylinder method when more than one plates are employed.
The disadvantage of both methods is that they give no clue to the axial force - settlement response of a helical pile. In practice, the design capacity of a helical pile is given from a load test by applying settlement criteria such as Davisson or ICC-AC358. We are now working in collaboration with Mr. Michael Perlow, P.E. and we are in the final stages of concluding a settlement based approach with early results been very satisfactory.
Traditional helical pile capacity methods:
a) Individual bearing method: This is the sum of the bearing capacity at each helical plate. In this mode the total ultimate resistance of a helical pile is determined as:
Qtotal = sum {Ahelix (9 c + gamma' x Depth x Nq)}
The geotechnical helical anchor capacity should be smaller than the individual structural capacity of each plate.
b) Cylinder strength method: In this mode the capacity is calculated as the shear resistance of a cylinder of soil contained by the helical plates, plus the bearing resistance of the helical plate closer to the excavation. The cylinder method can be applied only in cases where the helical anchor has atleast two plates.
Qcylinder = AhelixPlate1 (9 c + gamma' x D x Nq) + side resistance
In practice the individual bearing method and the cylinder strength method have generally produced smaller geotechnical capacities than actually experienced by pullout tests. For this reason, a number of researchers have proposed the torque installation method that relates the pullout resistance to the installation torque which can be measured during each helical anchor installation.
c) Torque correlation method: The Torque Correlation Method is an empirical method that distinguishes the relationship between helical pile capacity and installation torque and has been widely used since the 1960's. The process of a helical plate shearing through the soil or weathered bedrock in a circular motion is equivalent to a plate penetrometer test. The method gained notoriety based on the study performed by Hoyt and Clemence (1989). Their study analyzed 91 helical pile load tests at 24 different sites within various soil types ranging from sand, silt and clay soils. They demonstrated the direct correlation of the installation torque of a helical pile to its ultimate capacity in compression or tension. The common denominator discovered from the study was a parameter referred to as the torque correlation factor, Kt.
The equation is: Pu = Kt T Where: Pu is the ultimate capacity of the helical pile or anchor [lb (kN)]. Kt is the empirical torque factor of the central shaft of the pile [ft-1 (m-1)]. T is the final installation torque [ft-lb (m-kN)].
It's important to point out that the tests analyzed by Hoyt and Clemence (1989) were in tension. It was shown in sub-sequential studies that the tension capacity of helical piles was 16 to 33 percent less than the measured compression capacity. The difference is attributed to the fact that the lead helical plate is bearing on relatively undisturbed soil in compression applications. In tension applications, the leading and trailing helical plates are bearing on soil affected by the installation of the helical plates. It has become common practice to use the same torque correlation factor for a helical pile of the same size for tension and compression and ignore the slight increase in compression capacity. This creates a more conservative compression capacity for helical piles when compared to the Individual Bearing Method. Also unlike the Individual Bearing Method, the number of helical plates on a pile is completely independent of the piles capacity based on the Torque Correlation Method.
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Estimation of Helical pile response with settlement based method
The basic concept behind the settlement based approach for helical piles has been illustrated by M. Perlow at the recent Helical Pile conference at U. Mass. Amherst. Basic settlement is divided into an elastic component, the shaft response, the cylinder response, and the bottom helix.
We have incorporated this together with aa hyperbolic soil model formulation and we are now able to estimate reasonable helical pile responses:
