The properties of clayey alluvial soils in the Erbaa basin on the north bank of the Kelkit River were investigated in the field and in the laboratory. Based on the investigations, soil profiles were constructed and the distribution of clayey soils was identified. Geotechnical properties, mineralogical and chemical compositions of the soils, and their effects on engineering characteristics were established. The dominant minerals are Ca-smectite, kaolinite, and illite, as well as quartz, calcite, and feldspar. Differences in cation exchange capacity values are attributable to the types of clay mineral in the soils and their quantities. The development of heave and swelling pressure in the naturally expansive soils may cause major damage to structures, especially light structures constructed on or in them. The results show that the investigated soils have moderate to high swelling potential. However, because of their high level of consolidation, the soils investigated have high bearing strengths and thus foundation instability problems are not anticipated. This study has special significance for the design and construction of structures, because the Erbaa basin is located near the North Anatolian fault zone.