Site investigation of municipal solid waste incineration ash in an equatorial offshore landfill

Landfilled municipal solid waste incineration (MSWI) ash exhibits complex heterogeneity in in-situ geotechnical properties due to its spatially varying composition and long-term physicochemical transformation, e.g., stiffness increase induced by pozzolanic reactions. The heterogeneity of in-situ stiffness of landfilled MSWI ash poses challenges for landfill stability analysis, excavation planning, and long-term maintenance; however, field-based studies on this issue remain scarce. This study presents a comprehensive site investigation of an equatorial offshore MSWI ash landfill using apparent shear-wave velocity (AV_s) imaging, standard penetration test, and geochemical analysis. For the first time, apparent shear-wave velocity (AV_s) was employed for landfilled MSWI ash mapping, with its accuracy quantitatively evaluated. The resulting 3D AV_s maps revealed distinct stiffness zones and the presence of naturally formed hard layers. A simplified stiffness classification framework was applied to segment the landfill into zones relevant for engineering planning. The segmented zones show layered patterns. Supporting geochemical analyses identified high levels of pozzolanic elements (e.g., Ca, Si, Al, Fe), elevated pH, and persistent moisture, consistent with pozzolanic conditions that promote in-situ cementation. The study provides a methodological framework for stiffness heterogeneity characterization of MSWI ash landfills, offering valuable insight into applied waste management.