A new state-dependent constitutive model for cyclic thermo-mechanical behaviour of unsaturated vegetated soil

Climate change has resulted in increasing attention being paid to the effects of temperature and humidity on vegetated soil. However, existing constitutive models rarely investigate the thermo-mechanical behaviour, such as the accumulation of plastic strain under non-isothermal conditions, of unsaturated vegetated soil. To address this issue, this study developed a comprehensive constitutive model by adopting state-dependent dilatancy in conjugation with loading, memory, and bounding surfaces. Moreover, root-induced hardening due to pore occupancy and internal bonding is modelled with dependence on the root volume ratio. The performance of this newly developed constitutive model is then evaluated using previous laboratory element tests. Comparisons between the computed and measured results reveal that the model is capable of capturing variations in soil state at a range of temperatures, suctions, and root volume ratios. The shearing behaviour of vegetated soil, especially its volumetric response, is well predicted by the model, as confirmed by comparing it with triaxial compression test results. Moreover, the model predicts that the accumulated volumetric strain of unsaturated vegetated soil (0.2%) is significantly less than saturated bare soil (1.1%) after four thermal cycles. These computed results imply that vegetation can mitigate the effects of climate change on soil through both transpiration-induced suction and mechanical reinforcement.