The shrinkage-swelling behavior of the foundation layer in the urban perimeter of the city of Smara (Morocco), results in defects observed in the walls and partitions of public buildings, individual homes and city infrastructures. Since the shrinkage-swelling phenomenon is long-lasting and evolves slowly over time, these defects reappear, even after the damage has been repaired. The present study deals with the lithological, mineralogical and geotechnical characterization of the soil, with the aim of improving our understanding of the origins of its swelling characteristics and thereby adopting the necessary measures to limit the future appearance of defects in constructions. Granulometric analysis and determination of the Atterberg limits in the laboratory revealed that the clay fraction of the mudstone has only a weak potential to swell, which is insufficient to explain the soil's tendency to expand. X-ray diffractometry, together with the examination of several samples under a scanning electron microscope, revealed the presence of sulfate minerals (gypsum and anhydrite), which are known to experience significant changes in volume when hydrated or dehydrated. This soil's tendency to expand thus has its origins mainly in the transformation process of anhydrite into gypsum and vice versa, as a function of variations in its water content. Determination of the pressure modulus, the pressure yield point and the swelling pressure, based on in situ pressuremeter and rigid plate tests, allowed the deformability and state of rupture of the studied soil to be characterized. The measured values of swelling pressure are considerably greater than those transmitted to the ground by the foundations of existing constructions in the city. In the present paper we discuss all of the results obtained and their practical implications.