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Dec. 3-4th, 2018

Bucharest, Romania

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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
3.Butler-Hoy Criterion
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.

Helical pile settlement response components

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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.

Helical pile load test in sandy soils and large diameter pileHelical pile load test in sandy soils for SS pileHelical pile load test in sandy soils for SS pile 50ft longHelical pile load test in sandy soils for SS pile with local bucklingHelical pile load test in stiff clay soilsHelical pile load test in soft clay soils

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Aknowledgements:

The original DFI presentation has been prepared in collaboration with Mr. Michael Perlow from Engineering Knowledge Management. We would also like to thank the companies that have assisted us in our efforts by sharing load test data with us including: Rockridge Geotechnical, EBS in Toronto Canada, Chance, Pile Dynamics, American Pile Equipment.