Editors’ Note

Abstract

We are happy to close out the 2015 with an excellent issue that primarily focuses on unique performance aspects of pile foundations under axial and lateral loading. Issue 2 opens with a Technical Note by Franz providing guidance for drafting specifications for ground improvement. This document is a result of the work performed by DFI’s ground improvement committee. The note can be used as a checklist for selecting the most appropriate and efficient ground improvement technique considering available geotechnical information, operational constraints, desired performance requirements such as target bearing capacity and maximum allowable settlements, the influence of seismicity, and the impact of ground water regimes for the specific project needs. The next two articles discuss a previously published DFI manuscript by Stuedlein et al., entitled “Interpretation of augered cast in place pile capacity using static loading Tests”. Chen and Kulhawy offer a different opinion on the pile load displacement interpretation based on their independent pile database. Stuedlein closes this discussion with additional information on how his research team interpreted their own load test data, pointing out the importance of the pile slenderness ratio and how this ratio can be used as a proxy for axial stiffness in determining the pile failure load. In the fourth article, Bagheri and El Naggar present their analysis of a series of full scale uplift and compression load test data of helical piles and anchors installed in structured clays. Test variations include the numbers of helices and different helix diameters, as well as the embedment depths, and the diversity of natural clay deposits in which the specimens were installed. Findings of this research study indicate that the behavior of the helical piles and anchors is significantly influenced by the degree of soil disturbance introduced by the penetration of the pile shaft and helices. As a result of the disturbance, the undrained shear strength mobilized by the uppermost helix decreases and should be reduced to account for installation effects. The fifth article is the winner of the 2014 DFI student paper competition entitled “Pile pinning and interaction of adjacent foundations during lateral spreading” by Turner, a Ph.D. candidate at the University of California, Los Angeles, and his advisor Brandenberg. They present the results of a PEER funded case study of two parallel, adjacent bridges crossing the Colorado River in Mexico that were subjected to a broad field of laterally-spreading ground during the 2010 M 7.2 El Mayor-Cucapah earthquake. Turner and Brandenberg use equivalent static analysis procedures combined with 2D FEM algorithms to evaluate the lateral spreading demand on the bridges as well as the influence of pinning effects and geometric configurations (e.g. shielding effects of opposite bridges). Findings of this study show that the length and width of the lateral spread feature relative to the size of the foundation zone of influence affect the load imposed on the foundations by the moving soil. Current pinning approaches do not account well for these effects. The analysis method proposed herein can be extended to assess appropriate demands for foundation groups. This issue closes with an experimental study of two drilled shaft foundations installed at a site that experienced soil collapsing during excavation drilling. Race and Coffman from the University of Arkansas compared the response of a shaft installed in collapsed and noncollapsed boreholes and present the effects of the collapse and re-drill on the pile response parameters. They noted larger upward and downward movements, greater movements for the equivalent top-down resistance-movement curve, reduced unit side resistance values, and possible higher end bearing resistance for the foundation that was constructed in the re-drilled excavation. Race and Coffman recommend a reduction in the total unit weight values when predicting the capacity to more accurately model the movement-resistance response for a drilled shaft foundation constructed within a previously collapsed excavation. As we look back at a year of advancing research and case studies received by the Journal from a large variety of investigators and authors across the world, we would like to express our sincere thanks for the privilege of working with so many well established practicing engineers, 2015 DFI Board of Trustees