Effect of root orientation on the mechanical behavior of sand: Considering coupled shear–volume strain paths

The effect of plant roots on the mechanical improvement of soils and the enhancement of slope stability has been widely evaluated. However, the behavior of root-reinforced sands under coupled shear–volume strain paths remains largely unknown. In this study, due to the inherently unstable nature of sandy soils and the need for precise control over root orientation effects, a novel sample preparation method based on artificial cultivation was developed to produce sand-root composites. The main objective was to evaluate the influence of root orientation angles (0°, 30°, 60°, and 90°) of alfalfa ( Medicago sativa ) on effective stress paths under various drainage conditions. To this end, a series of saturated monotonic triaxial compression and extension tests were conducted on consolidated samples (with and without root reinforcement) under fully drained, undrained, and partially drained conditions, involving both contractive and expansive bilinear strain paths. Results showed that root orientation angle relative to the failure plane plays a decisive role in improving the friction angle at the critical state. Root reinforcement slightly increased the friction angle under compression loading, while the enhancement was more pronounced under extension, particularly in undrained conditions. Contractive partial drainage conditions, through the reduction of pore water pressure, improved the strain-hardening response and enhanced soil-root interaction, thereby enabling more effective mobilization of root tensile strengths under compression. Conversely, under expansive partial drainage conditions, the reinforcing effect of roots diminished significantly; however, under extension loading, rooted samples exhibited higher resistance to static liquefaction. These findings highlight the combined roles of root orientation and drainage conditions, providing a practical framework for designing and optimizing natural plant cultivation to stabilize granular soil slopes.