Induced Soil Liquefaction and Critical Hydraulic Gradient Variations in Estuarine Mudflats Subjected to Hydrophobic Hydrocarbon Infusion in Opu-Nembe, Nigeria

This study investigates the mechanisms driving widespread riverbank slumping and monotonic soil liquefaction in the intertidal zones of the Nembe estuarine network following the 38-day, uncontained, high-pressure crude-oil blowout at Santa Barbara South Well-1 (OML 29).

Undisturbed organic-rich silty clay cores (Chikoko soils) were collected along the CC-01 core axis across 47 impacted communities under macrotidal conditions.

Geotechnical index profiling and fluid-interaction screening followed ASTM D4318 and ASTM D2216, while high-pressure cyclic triaxial shear testing (ASTM D5311) and automated pneumatic shear-cell systems quantified multiphase soil behaviour and fluid-dependent strain anomalies.

The dataset calibrated an extended Bishop multiphase effective- stress tensor model incorporating nonlinear log-likelihood change-point analysis to identify structural degradation thresholds during 2.0-m semidiurnal tidal oscillations.

Crude-oil infusion induced severe hydrophobicity, increasing Water Drop Penetration Time beyond 3,600 s and forming an impermeable surface crust.

During rapid low-tide drawdown, this crust acted as a hydraulic seal, restricting pore-fluid dissipation and causing excess pore-oil pressure to accumulate behind slope faces.

Effective vertical confining stress approached zero, triggering sudden monotonic fluidization.

H ydrocarbon lubrication of clay platelets reduced cohesion (c ' ) from 12.50 to 4.20 kPa and friction angle (ö ' ) from 31.8° to 2.3°.

The critical hydraulic gradient declined by 38.2%, enabling normal tidal seepage forces to exceed slope-yield resistance and i nitiate catastrophic rotational bank collapse.

These failures displaced contaminated topsoil into anoxic horizons 0.5–1.5 m deep, burying weathered polycyclic aromatic hydrocarbons (PAHs) and trace heavy metals.

This geotechnical entombment limits oxygen-dependent biodegradation and atmospheric weathering, creating persistent pollutant reservoirs that slowly leach into groundwater and the coastal food web.

Conventional shoreline washing is therefore inadequate; recovery requires non-destructive bank stabilization combined with targeted subsurface biostimulation to treat concealed contaminant sinks safely.

The findings establish a process-based basis for predicting delayed slope failure and designing safer, site-specific remediation across similar oil-impacted estuarine environments.