Simplified Design Method for Piled Raft Foundations

Abstract

Raft foundations are the favorite choice for most of the designers; but nowadays, due to constraints of cost increment decreasing availability of land because of rapid industrialization and urbanization, there is growth in the vertical direction resulting in many high-rise buildings going up. This has resulted in heavy load, complicated stress conditions, limitation of bearing capacity of soil, and settlement of high-rise buildings. As a solution to the settlement problem of high-rise buildings, a number of piles are used and a new type of foundation - called a piled raft foundation - is coming up in a big way. In some designs, approach piles are used for reducing the settlement and a load is carried by raft only. Another design method still concentrates on providing adequate axial capacity from the piles to carry the structural load and the bearing capacity of the raft is neglected. In both the design approaches, the piled raft foundation becomes uneconomical as bearing capacity of raft and pile are not utilized in the single design approach. This shows that design rules and standards for piled-raft foundations are not well established. The interesting observation in the Poulos-Davis-Randolph (PDR) design methodology for piled raft, but using a simple stiffness formula, the piled raft foundation can be designed and analyzed. It is observed that an increase in pile length leads to increase in the settlement, which is contrary to the practical observation. It is because the stiffness of any structural member in this case pile, decreases with increase in length for a given diameter. This can be avoided by using a length of pile up to depth of fixity. The length of pile below fixity, due to surrounding soil, can be neglected. This length of pile above depth of fixity is also called free-standing length of pile. This free-standing length can be calculated using Indian standard on pile. By using simple stiffness formulae in PDR methodology, the piled raft design can be simplified.