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Leshchinsky D.,University of Delaware | Vahedifard F.,University of Delaware | Vahedifard F.,Paul Rizzo Associates Inc.
Journal of Geotechnical and Geoenvironmental Engineering | Year: 2011

Reinforced masonry block retaining walls are comprised of a narrow column of stacked blocks at their exposed end. This column is placed on a nonstructural leveling pad to facilitate the placement of facing units. Theoretically, this column can generate very large toe resistance to sliding. A recent publication indicates that an accepted design methodology implicitly counts on this resistance in assessing the reinforcement load. Although not calculated in this design, it unconditionally considers that over 60% of the resultant horizontal force in a 12-m-high wall is carried by the toe, which is made up of 0.3-m-deep blocks. This paper elucidates this issue by explicitly identifying the magnitude of toe resistance and critically reviews whether such high resistance is universally suitable for design. It shows that high toe resistance may not be feasible for most foundation soils. The high impact of toe resistance on the reinforcement force poses a design dilemma as to the reliability of this resistance, even if attainable. Practically, the leveling pad is not intended to serve as a critical structural element and thus should not be relied on for maintaining the toe resistance in long-term design. Economically, ignoring the toe resistance has little impact on the overall cost. © 2012 American Society of Civil Engineers. Source


Leshchinsky D.,University of Delaware | Vahedifard F.,University of Delaware | Vahedifard F.,Paul Rizzo Associates Inc. | Meehan C.L.,University of Delaware
Geotechnical Testing Journal | Year: 2012

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. Copyright © 2012 by ASTM International. Source

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