Constitutive Modelling of the Nonlinear Behaviour of Nigerian Laterite-Cement Stabilised Subgrade Under Repeated Traffic Loading

Laterite-cement stabilised subgrade layers are widely used in road pavement construction across Nigeria, yet the constitutive behaviour of these materials under repeated traffic loading, including resilient modulus evolution, permanent deformation accumulation, and fatigue-induced strength degradation, is not captured in the simplified elastic models used in the AASHTO and CBR-based pavement design methods commonly applied in Nigerian practice. This study develops a constitutive model for the nonlinear behaviour of Nigerian laterite-cement stabilised subgrade under repeated loading, building on experimental characterisation data from stabilised laterite specimens collected from road projects in Osun, Kogi, and Niger States representing South West, North Central, and North Central geopolitical zones. Repeated load triaxial tests were conducted over 100,000 load cycles at various deviator stress levels and confining pressures, generating resilient modulus, permanent strain accumulation, and energy dissipation data sets. A nonlinear kinematic hardening constitutive model incorporating creep and stiffness degradation was formulated and calibrated against the experimental data using parameter optimisation in MATLAB. The calibrated model was implemented as a user-defined material in ABAQUS and validated against independent pavement performance data from instrumented road sections. Results demonstrate that the constitutive model captures resilient modulus evolution and permanent deformation accumulation with mean prediction errors below 9 percent, compared to 34 percent for the elastic subgrade assumption. Incorporation of the calibrated model in pavement thickness design analysis yields subgrade layer requirements 8 to 17 percent thicker than current elastic-model-based designs, explaining premature rutting failures documented in laterite-stabilised pavements in Osun and Kogi. Keywords: constitutive modelling, laterite-cement stabilisation, subgrade, repeated loading, pavement design.