Effects of SAP and biochar on physicochemical properties and vegetation performance of eco-soil derived from waste slurries

High-water-content dredged sludge and shield tunneling residues remain largely underutilized despite their potential for ecological reuse. This study proposes a direct modification method that transforms such waste slurries into functional eco-soils through the incorporation of superabsorbent polymer (SAP), biochar, and rice husk, thereby eliminating the need for energy-intensive dewatering. The effects of SAP and biochar contents on the physical properties and vegetation performance of the modified eco-soils were systematically investigated, with particular emphasis on the coupling between soil parameters and plant growth responses. Results showed that increasing SAP and biochar contents reduced specific gravity, void ratio, and total porosity, while increasing bulk density and pH. Within the investigated ranges, the modified soils satisfied regulatory requirements for green planting substrates in terms of bulk density, pH, and non-capillary porosity (at SAP contents of 1.5% and 1.7%), demonstrating their applicability for ecological restoration. Vegetation tests indicated that 1.7% SAP yielded the highest germination rate, vegetation cover, plant height, and biomass. Vegetation cover exhibited a non-linear dependence on void ratio and bulk density. At 1.5% SAP, vegetation cover increased with both parameters, whereas at 1.7–2.0% SAP, it increased with void ratio but decreased with bulk density. Plant height showed a comparable trend at higher SAP contents. To characterize the combined structural effects, a Synergistic Loosening Index (SLI) was introduced, integrating void ratio and bulk density. The SLI showed a clear linear correlation with vegetation cover and plant height. These findings establish a quantitative basis for converting high-water-content waste into engineered eco-soils and contribute to sustainable reutilization of waste slurry in geotechnical and ecological engineering practice.