Mechanical properties and critical state characteristics of maize root-soil composites at different soil depths

The complex distribution characteristics of root-soil composites pose challenges in understanding their mechanical behaviour during conservation tillage. This study aims to analyse mechanical parameters of root-soil composites at different soil depths, considering root distribution, and establish an empirical critical state model. Three layers were defined based on root density distribution: Shallow Aggregated Root Zone (SARZ: 0-60 mm), Middle Enriched Root Zone (MERZ: 60-150 mm), and Deep Extended Root Zone (DERZ: 150-210 mm). Triaxial tests revealed varying shear strengths, with MERZ exhibiting the highest and SARZ the lowest. The Duncan-Chang model parameters, initial modulus of deformation, and initial Poisson's ratio were significantly influenced by soil depth, mirroring shear strength trends. An empirical formula incorporating soil layer depth into the Duncan-Chang model was proposed. Critical state stress ratios for SARZ and MERZ were determined as 0.93 and 1.11, respectively, quantifying their relationship with soil depth and root distribution. This study provides theoretical and parameter support for understanding the failure mechanism of root-soil composites.