Analaysis and Desing Of Stiffened Raft Foundation On Highly Expansive

Abstract

Swelling soils are a major geological hazard causing extensive damage to structures world-wide every year. The ultimate aim of this investigation is to study broadly and in depth the stiffened raft foundation as one of the various foundation alternatives that can be adopted to found a simple and cheap residential building on Sudanese expansive soils, and to make better utilization of its advantages; economical, ease to construct and its ability to withstand rigidly against the anticipated large soil differential heave as a floating foundation. For determining stiffened raft foundation design values the computer program SLAB2 was applied based on the concept of the rational analysis for solving soil-structure interaction problem. SLAB2 program necessitated various parameters, structural and soil as a program input data. The structural parameters are mainly from the stiffened raft itself and the load applied on it, while the soil parameters are obtained from the foundation soil. The selected site for this investigation is located at Barakat HQ’s of Elgaziera Scheme in the center of the clay plains of the Sudan. For studying the behaviour of the supporting expansive soil, various laboratory experiments have been carried out that included the tests for soil engineering index properties, oedometer and unconfined triaxial tests and from them the essential soil parameters were obtained such as the maximum differential heave (ym) and soil Young’s modulus (Es). From soil experiments, it was found that; Barakat expansive soil experienced large heave values. In this study the Post Tensioning Institute (PTI) design method have been assessed. It was found that the contribution of the stiffened raft foundation length (L) and the stiffening beam spacing is not influential in determining the stiffened raft foundation design values; moment, shear and deflection. The heavy perimeter load and the large differential heave at Barakat were found to produce large design values when using SLAB2 program, while PTI design method produced irrelevant values to what resulted by SLAB2, particularly when soil differential heave exceeds 100mm. Also it was found that the stiffened raft design values increase systematically with increasing the underneath soil Young’s modulus (Es). From the comparative analysis that has been made in this study, it was generally found that; PTI design method and some of the other previous existing methods will not eventually suit the highly potential Sudanese expansive soils.