Liquefaction-Resistant Backfill Soil Using Slag and Dried Sludge

Liquefaction in urban areas has repeatedly caused severe damage to infrastructure, including manhole uplift, road subsidence, and failure of buried utility lines, as evidenced by reports during major earthquakes such as the 1964 Niigata earthquake and the 2011 Great East Japan Earthquake. Although natural sand has been widely used as backfill, excess pore water pressure leads to rapid loosening. This study evaluates slag–dried sludge mixed soil as a new liquefaction-resistant backfill that improves disaster mitigation while promoting resource recycling. Compaction, cone penetration, and shaking table tests were conducted with sludge mixing ratios of 0–30%, identifying 20% as optimal. Liquefaction in slag-only soil occurred at 1013 s (7 m/s2), whereas the 20% mixture delayed it to 1380 s (11 m/s2), increasing the acceleration threshold by 1.5 times and extending the onset time by 36%. Therefore, the acceleration required for liquefaction to begin was approximately 1.5 times higher, and the occurrence time was extended by approximately 36%. Also, the cone index reached 7750 kPa, exceeding the traffic load requirement of 1200 kN/m2, while still allowing for sufficient permeability and workability compared to the use of natural clay particles. The improved backfill material proposed is promising as a sustainable urban infrastructure technology that simultaneously reduces liquefaction damage, improves the resilience of urban infrastructure, and reduces environmental impact through waste recycling.