Insights into clay contributions to slope instability in coastal areas: Hydraulic and mechanical properties

Coastal environments give rise to unique and complex conditions due to the influence of clay minerals, salinity, location geology, and other factors, which can lead to potential hazards such as slope instability. Also, coastal areas may even be more prone to hazards due to the influence of marine clays (kaolinite and illite); however, this is still unexplored. This study comprehensively assessed for the first time the coupled hydro-mechanical characteristics of kaolinitic- and/or illitic-rich soil mix and its potential influence on the susceptibility of coastal areas to slope instability. Seventy-five soil samples were studied and analyzed considering the physical, hydraulic, and mechanical properties of these clay soil mixes. The findings from this novel study reveal that in coastal environments, a distinctive soil mixture rich in both kaolinite and illite exhibits mechanical properties primarily governed by the influence of kaolinite content, whereas the hydraulic properties are defined equally by both kaolinite and illite minerals. The comprehensive analysis of the results indicates that generalized assumptions about the shear strength and permeability of soils containing different clay mineral types are incorrect, and the behavior is significantly dependent on the clay mineral type. Furthermore, effects of saltwater in coastal environments lead to drastic changes in soil hydro-mechanical response that can be detrimental to slope stability, although saltwater can increase the shear strength and permeability in isolated instances. In coastal settings, slope stability becomes an intricate process as the shear strength gain in soil is counteracted by the decrease in permeability, with varying clay soil mixes. Further, slope instability in the coastal environment is influenced by complex underlying mechanisms and the interplay of clay mineral type, clay content, local geology, and salinity. • Investigated hydro-mechanical characteristics of soil mix containing kaolinite and illite. • Clay type and clay content primarily control coastal soil strength and permeability. • Kaolinite increases soil cohesion; illite alters clay-rich soil mix in nonlinear ways. • Permeability shifts with clay type and salinity in complex patterns due to inherent geology. • Coastal slope stability needs site-specific analysis of clay type, content, and salinity.