Effect of shrub root diameter classes on shear strength of soil in Benggang collapsing walls

A Benggang is a typical soil erosion landform in the red-soil region of southern China. Its collapsing-wall stability is closely related to the soil shear performance, which can be mechanically reinforced by plant roots. Shrub-root reinforcement mechanisms and model optimization have not been studied systematically. This study considers Melastoma candidum, a dominant shrub species in southern Benggang areas. Using remolded soil direct shear tests, the effects of different moisture content levels and root diameter classes on the shear characteristics of root–soil composites are explored. A shear strength equation for root-soil composites, based on the Wu–Waldron model (WWM), was established, incorporating soil moisture content and root diameter (Equations (12) and (13)). The key findings are as follows: (1) Roots significantly enhance soil shear strength and cohesion. At 15% moisture content, 1.50–3.00-mm roots provide optimal shear strength improvement; at 25% moisture content, 0.00–1.50-mm roots perform best, but achieve maximum cohesion enhancement under both moisture conditions. (2) Increased moisture content significantly reduces root–soil composite shear strength and cohesion. (3) Root diameter variation minimally affects the soil internal friction angle, which decreases with increasing moisture content. (4) The WWM overestimated the measured shear strength by ≥5.60 times. Incorporating the moisture content and root diameter, the WWM correction coefficient was 0.02–0.18, and the newly established shear strength prediction equations based on the Mohr-Coulomb criterion and the WWM model demonstrated significantly improved accuracy (R², Nash–Sutcliffe efficiency [NSE] ≥ 0.92). These findings elucidate shrub-root regulatory mechanisms on collapsing-wall stability and provide theoretical support for vegetation allocation strategies and soil-reinforcement model optimization in red-soil erosion areas.