International Journal of Civil Engineering, Vol. 10, No. 2, June 2012 1. Introduction Pile groups are conventionally designed by adopting a relatively high factor of safety to the piles and the major design criterion is the bearing capacity of the group. The arrangement of these piles in the group is to carry the entire load of the superstructure. Although the connection "cap" often a raft (or mat) is in close contact with the soil, its contribution to the total bearing capacity and general pile group behavior is seldom considered in analysis and design [1]. One of the most effective ways for increasing the bearing capacity of a foundation has been experienced to be the pile enhancement. The system is known as hybrid foundation or piled-raft foundation (PRF) [2,3]. The piled-raft foundation is a recent design concept as one of the effective methods of foundation to reduce settlements of structures [4]. In piled-raft systems, the design procedure differs from traditional foundation design, in which the loads are assumed to be carried either by the raft or by the piles, considering the safety factor in each case. In the design of piled rafts the load sharing between the piles and the raft is taken into account, and the piles are used up to a load level that can be the same order of magnitude as the bearing capacity of a comparable single pile or even greater [2,3]. Therefore, the piled raft foundation allows reduction of settlements and differential settlements in a very economic way compared to traditional foundation concepts. In recent development of piled raft foundation design, the piles are usually required not to ensure the overall stability of the foundation but to act as settlement reducers. The raft in this system has adequate bearing capacity and therefore, the main objective of introducing these pile elements is to control or minimize the average and/or differential displacements of the International Journal of Civil Engineering Study on optimized piled-raft foundations (PRF) performance with connected and non-connected piles- three case histories A. Eslami 1,* , M. Veiskarami 2 , M. M. Eslami 3 Received: June 2010, Revised: October 2010, Accepted: March 2011 Abstract It has been realized that the raft (mat) foundations are capable of bearing very large loads when they are assisted with a pile group. The contribution of both raft and piles to carry the surcharge loads is taken into account, considering the stiffness and strength of involved elements in the system, i.e. piles, raft and surrounding soil. The piles are usually required not to ensure the overall stability of the foundation but to act as settlement reducers. There is an alternative design in which, the piles are non- connected from the raft to reduce the settlement, which are then known to be "settlement reducer non-connected piles" to increase the system stiffness. In this paper, two and three dimensional finite element analysis of connected and non-connected pile-raft systems are performed on three case studies including a 12-storey residential building in Iran, a 39-storey twin towers in Indonesia, and the Messeturm tower, 256m high, in Frankfurt, Germany. The analyses include the investigation of the effect of different parameters, e.g. piles spacing, embedment length, piling configuration and raft thickness to optimize the design. The role of each parameter is also investigated. The parametric study results and comparison to a few field measurements indicate that by concentrating the piles in the central area of the raft foundation the optimum design with the minimum total length of piles is achieved, which is considered as control parameter for optimum design. This can be considered as a criterion for project cost efficiency. On the other hand, non-connected piled-raft systems can significantly reduce the settlements and raft internal bending moments by increasing the subsoil stratum stiffness. Finally, the comparison indicates that simple and faster 2D analysis has almost similar results to the time consuming and complicated 3D analysis. Keywords: Piled-raft foundations (PRF), Optimized design, Connected piles, Non-connected piles, Finite element analysis * Corresponding Author: afeslami@aut.ac.ir 1 Associate Professor, Amir Kabir University of Technology (AUT), Tehran, Iran 2 Assistant Professor, University of Guilan, Rasht, Iran 3 M.Sc. Student, Sharif University of Technology, International Campus, Kish Island, Iran Downloaded from ijce.iust.ac.ir at 18:36 IRDT on Saturday June 17th 2017