Effect of cushion on composite piled raft foundation in layered soil under seismic forces

In order to mobilize shallow soil to participate in the interaction of piled raft foundation sufficiently, the concept of piled raft has been modified to new type of foundation named composite piled raft. In the system of composite piled raft, the short piles made of flexible materials were used to strengthen the shallow soft soil, while the long piles made of relatively rigid materials were used to reduce the settlements and the cushion beneath the raft was used to redistribute and adjust the stress ratio of piles to subsoil. In traditional foundation design, it is customary to consider first the use of shallow foundation such as a raft (possibly after some ground-improvement methodology performed). If it is not adequate, deep foundation such as a fully piled foundation is used instead. In the former, it is assumed that load of superstructure is transmitted to the underlying ground directly by the raft. In the latter, the entire design loads are assumed to be carried by the piles. In recent decades, another alternative intermediate between shallow and deep foundation, what is called piled raft foundation or settlement reducing piles foundation, has been recognized by civil engineers. The concept of piled raft foundation was firstly proposed by Davis and Poulos in 1972, since then it has been described by many authors, including Burland et al. (1977), Cooke (1986), Chow (1987), Randolph (1994), Horikoshi and Randolph (1996), Ta and Small (1996), Kim et al. (2001), Poulos (2001), and many others. Now the piled raft concept has been used extensively in Europe and Asia. In this concept, piles are provided to control settlement rather than carry the entire load. Piled raft foundation has been proved to be an economical way to improve the serviceability of foundation performance by reducing settlement to acceptable levels. The favourable application of piled raft occurs when the raft has adequate loading capacities, but the settlement or differential settlement exceed allowable values. Conversely, the unfavourable situations for piled raft include soil profiles containing soft clays near the surface, soft compressible layers at relatively shallow depths and some others. In the unfavourable cases, Finite element method was applied to study the behaviour of this new type of foundation subjected to seismic forces. This paper focuses on behaviour of various components of foundation system such as long pile, short piles and subsoil under seismic force (Koyna 1967) in layered soil related to Surat city geological condition. A comparative study is done to understand the effect of cushion on axial stresses, shear stresses and shear forces along long pile and short piles