Stabilisation of Vermi-Remediated Crude Oil-Contaminated Lateritic Soil with Portland Limestone Cement for Road Subgrade Application in the Niger Delta Region, Nigeria

Crude oil contamination of lateritic soils is a pervasive geotechnical and environmental crisis in Nigeria's Niger Delta, rendering vast land areas unsuitable for road construction. This study investigates a two-stage treatment strategy combining vermi-remediation using the African Nightcrawler (Eudrilus eugeniae) followed by Portland Limestone Cement (PLC) stabilisation for rehabilitating crude oil-contaminated lateritic soil for road subgrade application. Approximately 250 kg of lateritic soil collected from Shika, Zaria, Kaduna State was contaminated with Bonny Light crude oil at 8% by dry weight and subjected to vermi-remediation at a density of 50 worms/kg for 30 days, achieving a Total Petroleum Hydrocarbon (TPH) removal efficiency of 26.67%, reducing average TPH from 4,467 mg/kg to 3,300 mg/kg. The vermi-remediated crude oil-contaminated (VRCOC) soil was subsequently stabilised with 0%, 2%, 4%, 6%, and 8% PLC under three compaction standards: British Standard Light (BSL), West African Standard (WAS), and British Standard Heavy (BSH). Key geotechnical parameters evaluated include index properties, compaction characteristics (Maximum Dry Density, MDD; Optimum Moisture Content, OMC), Unconfined Compressive Strength (UCS) at 7, 14, 21, and 28 days, California Bearing Ratio (CBR) in soaked and unsoaked conditions, durability under wetting-drying cycles, and microstructural analysis by Scanning Electron Microscopy (SEM). Results demonstrate that 6% PLC under BSH compaction produced the optimal combination, achieving 28-day UCS of 715.84 kN/m², unsoaked CBR of 124.58%, soaked CBR of 71.30% at 2.5 mm penetration, and MDD of 1.83 Mg/m³—all exceeding the Federal Ministry of Works and Housing (FMWH, 1997) and NIS 6:2020 thresholds for road subgrade materials. SEM analysis confirmed progressive development of interlocking calcium silicate hydrate (C–S–H) and calcium aluminate hydrate (C–A–H) gels over the curing period. The combined vermi-remediation and cement stabilisation strategy offers a sustainable, eco-friendly, and technically viable pathway for transforming petroleum-contaminated lateritic soils into high-performance road subgrade materials.