ISSN: 2319-5967 ISO 9001:2008 Certified International Journal of Engineering Science and Innovative Technology (IJESIT) Volume 3, Issue 6, November 2014 477 Abstract— This study investigated the behaviour of single rammed stone columns. Prior to this assessment, the load – settlement and bulging responses of the columns were evaluated by conducting laboratory tests using a specially designed bench scale tank. The latter was a mild steel cylindrical tank which represented unit cells reinforced with single stone columns. The testing technique enabled the evaluation of the influence of variation of column diameter on stone column deformation responses and also provided data that was compared with the numerical results. In the numerical analysis, Mohr-Coulomb and modified Drucker-Prager models were used in the idealization of the behaviour of the column and soil materials respectively. Results revealed that the bigger the column diameter, the higher the loading capacity of the columns. Increasing the column diameter,D by 1.5, 2 and 3 times its initial size generally led to improved load carrying capacity by approximately 2, 4 and 10 times the initial strength respectively. The radial expansion of the columns was prominent in their upper parts with the highest value being experienced at a depth of about 0.5D from the ground surface. Index Terms—South African, soft soil, stone columns, unit cell, load carrying capacity. I. INTRODUCTION According to a recent report on the “State of African Cities” published by the UN Habitat [20], urbanization is occurring at a furious rate in most African cities. This is mainly due to human migration from rural areas to urban cities which is rapidly happening on the continent; and the development booms linked to current foreign investment and increased trade. Consequently, areas exhibiting weak soil strata such as swamps which were previously considered unsuitable for construction are being developed to meet the high demand in housing and service facilities. In addition, areas along the coast which usually consist of soft marine clay are also under development since they present environments with high cultural interest and rich biodiversity which greatly attract tourists. Although pile foundations may be deployed in those areas to meet the design requirements for stability, integrity and serviceability of the infrastructures, they are costly. Hence other ground improvement techniques may be preferable because they are cheaper. Amongst the wide range of ground improvement techniques are stone columns, which are often considered as cost-effective and environmentally friendly. Stone columns are installed in pre-bored openings in marginal soils such as cohesive soils and silty sands [1],[17]; and consist of coarse materials such as gravels or crushed aggregates with better properties. This technique was apparently first implemented in France in 1830[3]. Since then, it has been extensively used in numerous construction projects realized in marginal areas throughout the world especially in America, Asia and Europe for increasing the bearing capacity of marginal soil and reducing total and differential settlements. Stone columns are also used to facilitate the consolidation process and to improve the resistance to liquefaction [1], [18], [21]. Given the many advantages associated with the use of stone columns, the performance of single stone columns installed in a South African soft soil was investigated in this study. Using the approach of a unit cell comprised of a single column whose tributary area is surrounded by soft soil, the behaviour of single columns was studied through a numerical analysis. Numerical analysis enables the resolution of complex problems where analytical solutions and experimental measurements are sometimes cumbersome to be established. Prior to the extensive use of numerical models developed with the finite element analysis software, Abaqus 6.10, laboratory tests were performed for the calibration and validation of the models. Experimental and Numerical Analyses of the Behavior of Rammed Stone Columns Installed in a South African Soft Soil Ruben Aza-Gnandji 1,2 , Denis Kalumba 1 1 Department of Civil Engineering, University of Cape Town, South Africa. 2 Department of Civil Engineering, University of Abomey-Calavi, Benin.