Measurements and Modelling of Hydromechanical Effects of Tall Fescue Roots and Mycorrhizal Symbiosis on Slope Stability

Vegetation and associated mycorrhizal fungi play a crucial role in enhancing soil structure and stability through root reinforcement, aggregate formation, and moisture regulation. Understanding these biotic effects is essential for developing sustainable bioengineering strategies. This study evaluates the potential of Festuca arundinacea (tall fescue), both independently and in symbiosis with mycorrhizal fungi (MF), as a nature-based solution for improving shallow slope stability. A three-phase approach—comprising greenhouse cultivation, geotechnical characterisation, and numerical modelling—was applied to two contrasting soils: silty clay and silty sand. Eighteen soil columns received one of three treatments: (i) control (bare soil), (ii) vegetation only, and (iii) vegetation with MF inoculation. After 16 weeks of growth under controlled conditions, saturated hydraulic conductivity ( k ₛ) and shear strength parameters (effective cohesion, c′ , and friction angle, ϕ ′) were measured at six depth intervals. MF inoculation significantly enhanced shoot length and biomass in silty clay, suggesting a soil texture-dependent response. Across all treatments, kₛ increased, with the greatest gain observed in grass-amended silty sand; however, MF slightly reduced kₛ in sandy soil, possibly due to pore clogging. Direct shear tests showed that the grass–MF treatment notably improved peak shear strength and c′ , particularly in silty clay, where c′ rose from 0 to 12.1 kPa at 30–70 mm depth. Cohesion exhibited a logarithmic decline with depth, consistent with root mass density. Experimental data were integrated into transient seepage and limit equilibrium analyses using PLAXIS LE, simulating seasonal conditions in Southeast England. The grass–MF system improved the slope factor of safety and dampened seasonal instability, demonstrating its potential as a sustainable bioengineering solution for stabilising shallow slopes, especially in fine-grained soils.