From waste to a watershed-scale solution: Transforming oyster shell and expansive soil into a hydrophobic barrier for intercepting rainfall recharge and stabilizing slopes

Abstract Rainfall-induced cracks easily trigger slope collapse and severe geological disasters, while conventional crack grouting materials are costly and high-carbon emission. Massive waste oyster shells (OSP) and expansive soil (ES) are in urgent need of resource utilization. This paper proposes a sustainable filling material for sandstone fissure anti-seepage, which is prepared by local waste oyster shell from Qinzhou mixed with expansive soil and octadecylamine (ODA) to achieve hydrophobicity. Macro experiments and micro characterization were adopted to investigate the hydraulic-mechanical performance and modification mechanism of the composite material. The results indicate that when OSP content is 30% and octadecylamine (ODA) dosage is 2.4%, the ES+ODA+OSP mixture exhibits durable superhydrophobicity with contact angle over 90°, and the Water Droplet Penetration Time (WDPT) exceeds 3600 s. The desiccation crack ratio of modified soil is reduced by 78.5% compared with untreated expansive soil. Micro-mechanism analysis shows that ODA produces long-term hydrophobicity by melting coating particles, blocking pores and intercalating into montmorillonite interlayers, which increases the basal spacing by 9.5%; OSP acts as inert filler to restrain soil swelling. A 134-day model test was carried out to evaluate its rainfall infiltration interception performance. Monitoring data show that the saturation fluctuation of modified material at different depths is less than 7.2%, much lower than the 34.2%–75.1% of pure expansive soil. This research develops a new green geotechnical engineering technical route to solve slope geological hazards by solid waste recycling.