Static and cyclic liquefaction instability of reinforced soil with plant roots

Vegetation reinforcement has emerged as an environmentally sustainable measure for enhancing stability of engineered slopes. Although many studies have demonstrated the reinforcing effect provided by roots, few have employed constant shear stress tests to simulate conditions more representative of extreme rainfall scenarios and to identify the static instability onset of reinforced soil with roots. In addition, the linkage degree between static and cyclic instability in root-reinforced soil, as a special type of geomaterial, is still elusive. This study reproduces static and cyclic loading conditions using constant shear drained triaxial tests, constant shear drained direct shear tests, and cyclic triaxial tests. The instability characteristics of root-reinforced soil under specific stress paths are systematically investigated. The results show that vegetation roots effectively enhance soil resistance, delaying static instability and preventing flow liquefaction under cyclic loading. Moreover, the onset of cyclic instability in both root-free and root-reinforced soils is triggered after the cyclic effective stress path crosses the incipient static instability region, typically within zero or one cycle, indicating a high linkage degree between the static and cyclic instability of root-reinforced soils. Therefore, the cyclic instability can be conservatively predicted based on the incipient static instability region of root-reinforced soils.