Six expansive clays from the city of Madinah, Saudi Arabia have been investigated. The expansive nature of these clays is due primarily to the presence of smectite. The basaltic lava around Madinah is considered to be the source of this mineral. Kaolinite and illite also present in these clays are thought to be derived by the alteration of smectite. Calcite occurs only in the ‘white clayey silt’ and in the ‘upper brown clay’. Qualitative estimates using plasticity index and clay fraction data suggest ‘low’ to ‘very high’ swell potentials of these clays. Laboratory swell-consolidation tests indicate that for initial liquidity index in the range of -0.91 to +0.32, the percent swell for these clays is 1.02% to 13.2% and the swell pressure range is from 58 kPa to 770 kPa, the higher values of these two parameters being observed for grey-green clays with liquid limit less than 120% and particularly at low moisture contents. The swelling characteristics of expansive clays depends on numerous factors making the prediction of volume change and swell pressure difficult. This paper discusses the use of multi-variate analysis to aid the prediction of these parameters for the Madinah grey-green clays. The progressive lowering of the groundwater table in the Madinah area is likely to promote more severe desiccation of Madinah clays in the future. In view of the high swell potential of Madinah clays, there are important implications in relation to foundation design in Madinah.

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