Liquefaction Susceptibility of Saturated Sandy Interbeds in Low-to-Moderate Seismic Zones: A Case Study of the Guinea Coastal Wetland System

This study evaluates the liquefaction susceptibility and geotechnical stability of saturated sandy interbeds within the Guinea Coastal Wetland System (Sonfonia site), a low-to-moderate seismic zone. Integrating data from four deep boreholes (BH-01 to BH-04), a stratigraphic cross-section was developed illustrating extreme lateral heterogeneity. Findings reveal that while the eastern sector (BH-03) is dominated by competent Well-Graded Gravel (GW-GM) with a high safe bearing capacity of 893.8 kPa, the western sector (BH-04) contains a thick (approx. 7m) deposit of highly compressible Fat Clay (CH). Liquefaction triggering analysis, conducted at a design Peak Ground Acceleration (PGA) of 0.10g (475-year return period) and a fully submerged groundwater table (0.0m–0.22m), yielded a minimum Factor of Safety (FOS) of 1.45. Despite the high saturation, the risk of liquefaction is deemed low due to the dense nature of the sandy interbeds (N1 60 values up to 41.5). However, significant settlement risks in the western sector, calculated between 1,075 mm and 1,286 mm, necessitate the use of deep-bored pile foundations to ensure structural integrity. These results provide a critical technical framework for seismic-resilient infrastructure design in tropical coastal wetlands.