Understanding the impact of tree root growth on soil properties is essential for ecological restoration and sustainable land management, particularly in afforestation zones like the Loess Plateau. Decades of restoration have transformed the region from fragile grasslands to extensive forests, reducing erosion but leaving soil structural changes less explored. A controlled laboratory experiment, mimicking natural coarse root growth, was conducted by vertically inserting root analogues with different diameters (1 cm, 2 cm, and 3 cm) into standardized loess soil. The effects of root-induced compaction on soil bulk density and shear strength were evaluated by measuring changes in dry bulk density, cohesion (c), and internal friction angle (phi). Results showed that 3 cm roots induced the highest soil pressure (5.9-6.8 kPa), increased bulk density to 1.49-1.52 g/cm(3), and raised c and phi by 42.95 % and 76.96 %, respectively. Shear behavior shifted from strain-hardening to strain-softening, suggesting increased failure risk near large roots. Structural equation modeling (PLS-SEM) revealed that roots exert pressure on the surrounding soil during growth and penetration, enhancing bulk density and subsequently increasing c and phi. These findings highlight root-induced compaction as a distinct mechanism by which roots enhance soil shear strength, complementary to mechanical reinforcement through root-soil composites and hydrological effects from plant water uptake. This offers new insights into root-soil interactions and emphasizes the need to consider coarse root-induced compaction in vegetation restoration and slope stabilization strategies.