Mineralogical and Geochemical Control on Swelling Behaviour of Expansive Soils in Navrongo, Ghana

Soils with expansive behaviour pose a great geotechnical impact on buildings in tropical regions, due to moisture variations causing severe volumetric changes. This study investigates the mineralogical, geochemical, and index property characteristics of expansive soils from Navrongo in the Upper East Region of Ghana to identify the controls on their swelling behaviour. An integrated methodology of X-ray diffraction (XRD), major oxide geochemistry, Atterberg limits, and the Weathering Index of Parker (WIP) was performed on four soil samples. Results reveal a dominant kaolinite-quartz assemblage indicating advanced tropical weathering, yet with spatial variability. One sample (NA0001) was found to contain sodium-rich montmorillonite (smectite), correlating with uniquely elevated geochemical concentrations of Fe ₂ O ₃ (8.83%) and MgO (1.57%) and a very high Plasticity Index (PI = 57.6%), classifying it as having high expansion potential. In contrast, the smectite-free samples exhibited significantly lower plasticity (PI = 20.3–43.2%). The data establish a causal chain of localised geochemical conditions, associated with an intermediate weathering stage, that stabilise expansive smectite clays, which in turn dictate high-index properties and severe swell potential. This study further interprets this variability within a classic tropical regolith profile, identifying the mottled weathering zone as a probable genesis horizon for expansive clays. The findings underscore that the swelling risk in Navrongo is not ubiquitous but confined to specific zones where smectite is present. Consequently, a multi-method approach that combines plasticity tests and mineralogical and geochemical analyses to identify these high-risk, spatially discrete soil units to inform geotechnical design and sustainable infrastructure development in Navrongo.