Here are some useful references.
Clancy, P; Randolph, MF. 1996. Simple design tools for piled raft foundations. GEOTECHNIQUE 46 (2): 313-328.
Poulos, H.G. (2001a). Piled raft foundations : design and applications. Géotechnique, vol. 51, pp 95-113.
Poulos, H.G. (2001b). Methods of Analysis of Piled Raft Foundations. Report to Technical Committee TC 18 on Piled Foundations. International Society on Soil Mechanics and Geotechnical Engineering, 46 p.