Prediction of load-settlement response of helical piles in compression
At the 2014 DFI Helical Piles and Tiebacks seminar in St. Charles, MO, our CEO Mr. Konstantakos presented a new method for predicting the load-settlement response of helical piles. Traditionally, helical piles have been designed on an ultimate load basis with most geotechnical engineers having few clues about their true geotechnical response. On the other hand, ultimate capacity for helical piles is typically realized at greater settlement values that are not reached in most axial load tests.
The lack of understanding of true helical pile behavior and response is a significant barrier to the wider adoption of helical piles!
Did you know? IBC 1810.3.3.1.2 requires that "Ultimate pile capacity shall be determined by a register engineer with consideration given to the total and differential settlement at design loads in accordance with 1810.2.3"
Ultimate capacities are defined from ultimate load criteria that may involve any of the following methods:
1.Davisson Offset Limit
2.Brinch-Hansen 90% Criteria
4.Other Methods Approved by the building official
The settlement response of helical piles theoretically comprises of elastic shortening of the helical pile section, the elastoplastic response of the soil against the shaft, the soil behaving in a cylindrical type mode in between helixes, and the elastoplastic response of the leading helical plate. If the individual plate failure mode is controlling then the cylinder response can be replaced by the individual plate behavior.
Basic Settlement Response Theory
Fleming (1992) presented procedure for estimating axial pile response for rigid piles:
ΔS=MS DS US PS/(US- PS)
MS = Shaft factor, tangent slope of the origin
of hyperbolic function for shaft friction (Randolph 1992)
DS = Shaft diameter (or cylinder diameter)
US = Ultimate shaft resistance
PS = Applied shaft traction
Theory of elasticity settlement under a circular footing
ΔP=π/4 qa/EB DPL (1- v^2 )f
qa = Applied stress on footing (average)
EB = Modulus of elasticity of bearing soils
DPL = Circular footing diameter (in this case, helical plate diameter)
f = 0.85 Shape factor
NOTE: EQUATION IS LINEAR, TYPICAL SOIL RESPONSE NONLINEAR!!!
Using the hyperbolic soil model, a true non-linear approach can be simulated.
Sample load tests used to calibrate method
The following images show a series of load tests used in the calibration of the proposed settlement method.