A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering, School of Civil Engineering

Thesis (M.Eng.) - Asian Institute of Technology

This study investigates the raft-pile-soil interaction for vertically loaded flexible raft under layered Bangkok subsoil. Raft flexibility is usually ignored in practice due to complexity of analysis. Further, conventional design of foundation ignores bearing effect of raft and considers entire load to be carried by the piles. In this study, the load capacity of raft as well as pile group is considered for the analysis. The piled raft foundation system is approximated to a plane strain case and analyzed using two-dimensional finite difference method. The soil is discretized into a rectangular finite difference mesh and is modeled as a linear elastic material. The raft is modeled as a beam structure under plane strain condition. The piles are modeled using pile elements, which allow the shear and normal interaction at the pile-soil interface with approprfate scaled stiffness and strength values. The effect of structural connections between raft and piles is also considered. The suitability of piled raft foundation in Bangkok subsoil is assessed and general conclusions are drawn. For the analysis of the piled raft system, the latest version of FLAC program was used. The program introduces pile elements, which can transmit shear and normal forces to surrounding soil through shear and normal coupling springs in addition to flexural characteristics of beam elements. Soil parameters for the analysis were determined from the back analysis of bored pile load testing. The strength and stiffness of shear spring were obtained from the pile load testing data, while the strength and stiffness of normal spring were estimated from the elastic properties of the adjacent soil in contact. Similarly, the shear and normal stiffness of beam-soil interface were estimated from elastic prope1ties of soil in contact. The conversion of problem from 3-D to 2-D plane strain condition was carried out through scaling the pile parameters with spacing in the out-of plane direction.