Application of a Hydraulic Gradient Technique for
Modeling the Uplift Behavior of Piles in Sand

Dov Leshchinsky¹; Farshid Vahedifard²; Christopher L. Meehan³

¹Professor, University of Delaware, Dept. of Civil and Environmental Engineering, 301 DuPont Hall, Newark, DE 19716, U.S.A.
E-mail: dov@udel.edu
²Senior Project Engineer, Paul C. Rizzo Associates, Inc., Pittsburgh, PA 15235, U.S.A.
(Formerly Graduate Student, University of Delaware, Dept. of Civil and Environmental Engineering, 301 DuPont Hall, Newark, DE 19716, U.S.A.)
E-mail: farshid@udel.edu
³Assistant Professor, University of Delaware, Dept. of Civil and Environmental Engineering, 301 DuPont Hall, Newark, DE 19716, U.S.A.
E-mail: cmeehan@udel.edu

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Geotechnical Testing Journal, 2012, Volume 35, Number 3, pp. 400-408

Abstract
A scale modeling technique is presented for simulating the uplift behavior of piles in sand, which satisfies stress and strain similitude with full-scale prototypes. A hydraulic gradient approach was used to increase the body forces in the scale model tests, until the stresses became representative of reasonable field-scale conditions. The associated model scaling laws that are used with this technique are presented and discussed. Eight pile pullout tests were conducted, at varying magnitudes of hydraulic gradient; one of these tests was conducted on a pile instrumented with strain gages. From the results of this study, it was concluded that the hydraulic gradient technique can be effectively applied to induce reasonable prototype stresses in a scale model, allowing for reasonable simulation of the uplift behavior of piles in sand. The hydraulic gradient technique was found to be particularly sensitive to the distribution of the hydraulic gradient in the soil profile, which is not surprising, given the nature of the scaling laws for this particular modeling technique. Analysis of the results from the pile instrumented with strain gages indicated that (1) very little shear stress was mobilized in the upper 25% of the pile, even at applied uplift loads approaching the ultimate pullout force; (2) the largest amount of shear stress for each applied uplift force was typically mobilized at a point somewhere between 60% and 80% of the length of the pile; and (3) at failure, the mobilized shear stress distribution was clearly not triangular, as is postulated by commonly used pile uplift design approaches. Post-failure investigations conducted after the completion of each pullout test indicated that the failure shear surface in each of the model tests developed along the pile-soil interface.

Keywords
Piles; Uplift resistance; Hydraulic gradient; Granular soils; Similitude; Geotechnical modeling

Reference
Leshchinsky, D., Vahedifard, F., and Meehan, C. L. (2012). “Application of a Hydraulic Gradient Technique for Modeling the Uplift Behavior of Piles in Sand.” Geotechnical Testing Journal, ASTM, 35(3), 400-408. (doi:10.1520/GTJ103850)

DOI
10.1520/GTJ103850