Locust Bean Waste Ash Treatment of Vermi-Remediated, Crude Oil-Contaminated Lateritic Soil for Road Subbase

Abstract: This study evaluates the engineering performance of crude oil-contaminated lateritic soil reclaimed through sequential vermi-remediation and chemical stabilization using Locust Bean Waste Ash (LBWA) for road subbase development. The biological remediation phase achieved a 27% reduction in Total Petroleum Hydrocarbons (TPH), from 4500 mg/kg to 3300 mg/kg. Despite this improvement, the baseline soil remained structurally deficient, displaying high plasticity (PI = 20.71%), high fines content (72.42%), and low specific gravity (Gs = 2.0), classifying as A-7-6 (AASHTO) and CL (USCS). LBWA stabilization triggered significant particle flocculation, reducing fines content sharply to between 2.54% and 5.82%. Compaction testing revealed Maximum Dry Density (MDD) optimized at 1.89 Mg/m³ under British Standard Heavy (BSH) energy with 4% LBWA. Mechanical strength peaked at 12% LBWA with BSH compactive effort, achieving 7-day, 14-day, and 28-day Unconfined Compressive Strength (UCS) values of 701.2 kN/m², 800.6 kN/m², and 889.6 kN/m² respectively, substantially surpassing untreated baseline values. California Bearing Ratio (CBR) values peaked at 55.43% (unsoaked) and 32.56% (soaked). Durability indices confirmed that only the 12% LBWA + BSH configuration exceeded the 80% strength-retention threshold, achieving 81.5%. The findings validate that vermi-remediation mitigates environmental hazards, but subsequent stabilization with 12% LBWA under BSH compaction is technically necessary to fulfil structural and durability criteria for flexible-pavement subbase layers.