Abstract: The alluvial or diluvial silty clay around Benguela GRASA in Angola has the properties of swelling. Because of the very low water content, it is too dry to fetch the undisturbed sample, but the laboratory index from disturbed samples such as free swell and atterberg limits may distort the real facts. So it is necessary and helpful to adopt field water immersion test for ascertaining the swelling properties. The results of the field test indicated that the laboratory results have limitations and mistakes to determine the swelling properties; the silty clay has a high potential of swelling because of low water content; while increasing the water content, the soil has high swelling deformation and expansive force; the deformation curve of the plate shows subsoil heave is sensitive with small fluctuation of moisture content; but, the strength would fall fast if the water content increase more, the Benguela silty clay has obvious sensitivity to water with the phenomena swelling and weakness during water content increasing.