Development of a Theoretical Framework for Predicting Long-Term Creep Behaviour of High-Density Polyethylene Geomembranes Under Nigerian Tropical Environmental Conditions

High-density polyethylene geomembranes are critical containment structures in Nigerian landfill and mining tailings facilities, where long-term creep deformation under sustained stress and thermal loading in tropical environments poses significant containment integrity risks. This research develops a theoretical framework for predicting the long-term creep behaviour of high-density polyethylene geomembranes under conditions representative of South West and North Central Nigerian deployment environments. The framework integrates viscoelastic constitutive modelling based on the Burgers model extension, with temperature-dependent shift factors derived from time-temperature superposition applied to accelerated creep data collected at 23, 40, and 60 degrees Celsius. Experimental master curves of compliance versus time were constructed for three commercial geomembrane grades differing in crystallinity and density. Stress rupture testing established the ductile-to-brittle transition boundary critical for long-term safety factor determination. Finite element implementation of the constitutive model enabled simulation of geomembrane deformation in realistic anchoring and basal liner configurations. Experimental validation was conducted against in-service creep measurements from a lined waste facility in Ogun State over a 36-month monitoring period. Results demonstrate that the proposed framework predicted field-measured creep strains within 11 percent error across measurement locations, significantly outperforming the linear viscoelastic model. Thermal loading analysis revealed that service temperatures in unshaded geomembranes in Northern Nigeria can reach 55 degrees Celsius, necessitating substantial upward revision of conventional creep predictions made at reference temperature. The framework provides design engineers with temperature-corrected service life predictions for geomembranes in Nigerian tropical deployment, filling a critical gap in existing geomembrane design guidance. Keywords: HDPE geomembrane, creep behaviour, tropical environment, constitutive model, service life prediction