7.2.2 Load Distribution in a Group and the p-Multiplier Concept Displacement and stress measurements in full-scale and model foundation groups indicate that the load is distributed non-uniformly to the individual elements within the group, with the load demand at anyone deep foundation element depending on the location of the element within the group, as well as the spacing between the foundation elements, and other factors noted in Section 7.2.1. This trend is attributed to the shadowing effect depicted in Figure 7-1. In this case, the foundation elements in the leading or front row (the row closest to the zone of soil resistance) experience the least shadowing effect. Foundation elements in the trailing row (row immediately behind the lead row) experiences the shadowing from the front row. Because the shadowing effects overlap from one row to the next, each additional row experiences a compounding effect, making its lateral load resistance significantly less compared to an individual isolated deep foundation element that would not experience any shadowing effect. In the p-y method of analysis, the shadowing effect is accounted for using a p-multiplier, P m . The p- multiplier is a reduction factor that is applied to the entire p-y curve fora laterally loaded individual pile/shaft to account for the overlapping zones of influence in a group loading condition. The value of P m used in practice is 1.0 or less. Figure 7-2 illustrates the concept of the p-multiplier for an individual p-y curve. Figure 7-3 illustrates the concept of the p-multiplier as it applies to a group of deep foundation elements, with each subsequent trailing row behind the first (leading) row experiencing a greater shadowing effect and therefore having a smaller p-multiplier and lower corresponding p-y curve for analysis. Figure 7-2: Illustration of the p-multiplier concept. p y P m . p
