Group behavior of piles has always posed a design challenge to practitioners. It is found convenient to relate the behavior of a single pile in a group to that of a single isolated pile. Generally, this is done by utilizing a "P" multiplier. The industry relies of findings from field studies and analytical studies to estimate appropriate value of the multiplier that can be used in design.
Professor Kyle Rollins of BYU (left) received a golden apple from Geotechnical Group Chair Walter Papp at the end of the lecture.
Professor Kyle M. Rollins of Brigham Young University delivered a lecture titled "Lateral Load Analysis of Pile Groups Based on Full Scale Testing" to the Geotechnical Group at the CUNY Graduate Center Recital Hall on October 14, 2009, presenting results from field studies on pile group that he and his students have been conducting for over ten years.
The first part of the presentation involved estimation of the P multiplier based on static load tests conducted on single isolated piles and pile groups (consisting of piles in configurations of 3x3, 3x5, etc.) In each of these cases, the piles were subjected to lateral loads, applied through hydraulic actuators. The results of these field studies were compared with those from popular computer models, such as L-Pile. The results of static tests the study indicated that the load capacity of a pile in a group depends on the row position—the most pronounced difference being that between the leading row (the row farthest from the load) and the remaining rows. The group effect decreased with the pile spacing. Statnamic tests on pile groups in sand indicated that the dynamic resistance of pile groups is 50 to 75 percent higher than static loading. This increase is primarily due to damping.
Dr. Rollins also presented results from tests where the behavior of liquefiable sands was investigated in a site at Treasure Island, near San Francisco, California. In these tests, liquefaction was induced in the shallow, saturated sand deposit and pile load capacities were measured before and after liquefaction. Based on these results, Professor Rollins made recommendations regarding the selection of P multipliers values for sands before and after liquefaction.
Professor Rollins has graciously made available a copy of his PowerPoint presentation.
Photo and summary by Anirban De