Using Waste Marine Clay as an Alternative to Bentonite for Coastal Seepage Cutoff Wall Construction: Engineering and Physicochemical Properties in Seawater

Abstract Amid escalating sea-level rise, seepage cutoff walls have become critical infrastructure for costal protection. However, conventional bentonite used as a sealing material in the walls suffers from degradation in seawater, posing workability and durability concerns. Meanwhile, large volumes of waste marine clay (MC) are excavated in coastal regions, characterized by poor engineering properties but notable fineness and low permeability. Hence, this study investigates the use of MC, which naturally formed in a seawater environment and is inherently stable as a sustainable alternative to bentonite. Experiments were conducted to evaluate engineering and physicochemical properties of MC-based cutoff walls in freshwater and seawater, compared to bentonite-based cutoff walls. The results showed that bentonite mixtures exhibited better flowability control and bleeding resistance in freshwater, but their performance degraded in seawater, becoming comparable to that of the more stable MC mixtures. MC mixtures demonstrated higher strength in freshwater, with further enhancement in seawater; however, bentonite mixtures exposed to seawater exhibited strength degradation. At equivalent dosages, bentonite outperformed MC in impermeability; nonetheless, increasing MC content, which also promoted sustainable waste utilization, reduced its permeability to comparable levels. Physicochemical analyses revealed that MC maintained high stability in seawater because of its strong resistance to changes in water-retention, swelling, and cation exchange capacity. This stability was also attributed to its kaolinite-dominant mineralogy; conversely, bentonite underwent mineralogical transformation from Na-montmorillonite to Ca-montmorillonite, accompanied by disordered smectite and agglomerated microstructure. These findings underscore the potential use of MC as a durable, cost-effective, and sustainable alternative to bentonite in coastal cutoff walls.