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Combined pile raft foundation (CPRF) system consists of a combination of a raft and piles in transferring the superstructural loads to the founding medium. Providing CPRF system as an effective and economic foundation system is one of the new concept that has emerged in recent decade and are being increasingly used for residential as well as industrial buildings. In general, for such a dual system, piles are generally provided as settlement reducers which also plays the dual role of load resistance. The different load transfer mechanisms viz., bearing with raft, friction between soil and pile shafts and end bearing of piles are a function of the geotechnical parameters which are used for the design of structure supported on CPRF system. Being a comparatively new field, codal stipulations and guidelines on the foundation analysis and design of structure supported on CPRF system are scarce and often design by analysis using rigorous continuum based computational methods are used as a design tool by structural and geotechnical engineers. There exists few simplified and approximate numerical techniques (elastic continuum approach and spring foundation approach) which are often bounded by a range of simplifying assumptions and can introduce uncertainties at times in the prediction of the behaviour of structure supported on CPRF system.
In recent pasts, studies conducted using elastic continuum approach by Clancy and Randolph (1993, 1996), Alnuiam et al. (2013), Srilakshmi and Moudgalya (2013), Singh and Singh (2008, 2014), Joy and Hassan (2014) and Kumar et al. (2015) have led to a better understanding on the complex behaviour of combined pile raft foundation. However, the major drawbacks involved with such continuum based approaches are its limitation for large scale problems due to its large computational requirement.
Studies using the spring foundation approach have been carried out by Naik and Desai (2011), Shukla et al. (2011), Miskin et al. (2014), Kitiyodom and Matsumoto (2003, 2005), Kitiyodom et al. (2011) and Jeong et al. (2014). In these studies, though the combined behaviour of the foundation system has been captured in an approximate manner, most of these studies did not consider the non-linear degrading behaviour of soil which is one of the most important aspects for load sharing between the piles and raft. Moreover, most of the studies on CPRF system were carried out for static loading and only few literatures exist on its dynamic behaviour under seismic loading which has also been attempted in the present study.
Banerjee et al. (2007), Banerjee (2009), and Banerjee et al. (2014) have examined the effect of different level of earthquake on fixed head end bearing piles in soft clay, using centrifuge test and numerical modelling of a 4-piled CPRF system. Numerical analyses were carried out in ABAQUS using 3-D non-linear, stiffness-degrading soil model as per Idriss et al. (1978). The study gives a relationship between active slenderness ratio and maximum bending moment in terms of stiffness, mass and acceleration ratios for thick, soft clay layer having increasing shear modulus with depth.