A state-of-the-art on the combined use of piles and rafts is presented in the paper "Practical design procedures for piled raft foundations" by H. G. Poulos, Chapter 16 of the book "Design applications of raft foundations", published by Thomas Telford (UK), 2000, Editor J. A. Hemsley, pages 425-467. You may also refer to a similar shorter paper by Poulos, "Piled raft foundations: design and applications", Geotechnique, 2001, Vol. 51, No. 2, March, pages 95-113. These papers include an overview of available methods for the design of piled rafts.