Recycled Soil for Reservoir Embankments: Effects of Paper Sludge Ash Treatment and Compaction Energy

Abstract The depletion of natural resources and overuse of landfills have intensified interest in recycling construction surplus soil generated during excavation and pile-driving activities. Although these materials have traditionally been used for backfilling, their advanced utilization remains limited. Recent efforts have been directed toward reusing construction sludge for road base layers and high embankments using onsite or centralized soil improvement facilities. However, most studies have primarily emphasized bearing capacity and strength, with comparatively less attention on permeability. This study examines the use of recycled soil from the Tohoku region of Japan in reservoir embankments, where permeability is equally critical as strength. The effects of compaction energy and the incorporation of a paper sludge ash-based stabilizer (PSAS) on the geotechnical properties of the soil were evaluated. Cone index and falling head permeability tests demonstrated that increased compaction energy enhanced soil strength; however, untreated recycled soil did not meet the hydraulic conductivity criteria for impervious materials. To address this limitation, PSAS was incorporated, which significantly improved both strength and impermeability—even at lower dry densities—attributed to ettringite formation. An optimal PSAS content of 25% achieved a hydraulic conductivity below 5 × 10⁻ 8 m/s in laboratory tests. The compaction behavior of the treated soil in response to variations in water content and compaction energy resembled that of natural soils, allowing for effective compaction control during field construction. Moreover, compacting the treated soil to a degree of compaction of 95% or higher, within a specific wet-side moisture range, ensured sufficient strength and low permeability for use in impervious zones. The evaluation of CO 2 fixation based on calcium carbonate content revealed that PSAS addition increased CO 2 sequestration by approximately 15 kg per ton of dry soil. As paper sludge ash is an industrial byproduct with negligible associated CO 2 emissions, its use in converting construction sludge into high-performance embankment material offers a promising approach for reducing overall carbon emissions.