Retaining Structure Effect on Piled Raft Foundation Performance

The effect of retaining structures on the performance of raft foundations enhanced with deep foundation elements (typically piles), simply known as piled rafts, was examined numerically. Illustrative piled rafts were analyzed using a simplified, linear elastic, plane strain finite element model. The effect of the structure was evaluated on the displacement profile, differential displacement, raft bending moments, and pile butt load ratio. For fully piled rafts, the effect is minimal. However, for partially piled rafts, the effect can be significant. Details are given in dimensionless plots. Introduction The concept of raft foundations enhanced with deep foundation elements (typically piles and therefore the name "piled rafts") has received considerable attention in recent years. The raft in this system has adequate beating capacity, so the main objective of these pile elements is to control or minimize the average and/or differential displacements of the piled raft system, rather than to carry the major port,'on of the loads. Therefore, a major design question is how to design the piles optimally to control the displacements. Three different design approaches have evolved to address this question (e.g., Poulos, 1994; Randolph, 1994; Burland, 1995; Horikoshi & Randolph, 1997, 1998; Prakoso & Kulhawy, 2001). The first focuses mainly on reducing the average displacement, the second focuses on reducing the differential displacement, and the third focuses on reducing both average and differential displacements. The solution suggested in the second and third approaches involves the use of piles in some part of the raft (partially piled raft), particularly within the center area of the raft. Piled rafts have been used in basements, in which retaining structures typically are required. The effect of these structures on the performance of partially piled rafts is evaluated, and the displacement profiles, differential displacements, raft bending moments, and pile butt load ratios of these piled rafts are considered. In the evaluation, the retaining structures were considered explicitly. Based on the results, the difference in performance between partially and fully piled rafts will be highlighted. The evaluation was conducted using the geotechnical finite element program PLAXIS (Vermeer & Brinkgreve, 1995). A plane strain finite element model was used, and the modeling procedure will be discussed briefly. The verification results of the model also 236 DEEP FOUNDATIONS 2002 237 will be presented. Plane Strain Finite Element Models for Piled Rafts There are complex interactions among the piles, raft, and soil in a piled raft system, and therefore a sound analytical model is needed to evaluate these interactions. A plane strain finite element model can be used for this purpose. However, this model involves the fundamental simplification of condensing a finite size piled raft into a strip piled raft. Desai, et al. (1974) showed that this type of model can provide good results. In addition, this model can be used to analyze a relatively large piled raft without excessive modeling and computing time. The responses of vertically-loaded pile foundations are controlled mainly by their axial stiffness, as suggested by Desai, et al. (1974). Because the piles are modeled as strips, an in-plane row of piles has to be simplified into an equivalent plane strain pile having the following equivalent pile Young's modulus: Eeq = np .. . . lAp Ep/LrB (1) in which np-row i = number of piles in row i, Ap = area of pile cross-section, Ep = pile Young's modulus, Lr = raft length (in-plane), and B = pile diameter. The plane strain system geometry, other material properties, and load are not changed. The vertical displacements obtained from the analyses were used directly, and a downward vertical displacement is considered as a positive displacement. The definition of differential displacement is given in Figure 1, based on the typical raft displacement profile of downward dish shape, and the differential displacement considered is the centeredge differential displacement, Awc.E. In addition, the stresses obtained at the Gauss points of raft and pile elements were used to analyze the raft bending moments and pile loads, respectively, using small-strain theory. The reliability of the plane strain model was examined by comparing its results with the results of other models. Figure 2 compares the results of the elastic plane strain model and a 3-D finite element model (Wang, 1996), including the maximum and differential Center 88 Point Ec l ~ Undeformed TAw~,.T~ ~Deformed / l e WE[wl,, Iwc Differential Displacement = A W c . E = w C w E Figure I. Definition of Differential Displacement 238 DEEP FOUNDATIONS 2002