Debris flows are a type of natural disaster induced by vegetation-water-soil coupling under external dynamic conditions. Research on the mechanism by which underground plant roots affect the initiation of gulley debris flows is currently limited. To explore this mechanism, we designed 14 groups of controlled field-based simulation experiments. Through monitoring, analysis, calculation, and simulation of the changes in physical parameters, such as volumetric water content, pore-water pressure, and matric suction, during the debris flow initiation process, we revealed that underground plant roots change the pore structure of soil masses. This affects the response time of pore-water pressure to volumetric water content, as well as hydrological processes within soil masses before the initiation of gully debris flows. Underground plant roots increase the peak volumetric water content of rock and soil masses, reduce the rates of increase of volumetric water content and pore-water pressure, and increase the dissipation rate of pore-water pressure. Our results clarify the influence of underground roots on the initiation of gulley debris flows, and also provide support for the initiation warning of gully debris flow. When the peak value of stable volumetric water content is taken as the early warning value, the early warning time of soil with underground plant roots is delayed by 534 to 1253 s. When the stable peak value of pore-water pressure is taken as the early warning value, the early warning time of soil with underground plant roots is delayed by 193 to 1082 s. This study provides a basis for disaster prevention and early warning of gully debris flows in GLP, and also provides ideas and theoretical basis under different vegetation-cover conditions area similar to GLP.

Under long-term loading, the mechanical properties of geogrids in eco-bag reinforced soil retaining walls (ERSW) gradually weaken due to creep and photo-oxidative aging. In contrast, the continuously growing roots transfer their tensile strength to soil shear strength. However, the reinforcing effect of plant roots within retaining walls is often overlooked in current research and design. This study investigated the reinforcement effect of plant roots and geogrids on ERSW stability through theoretical derivations using the Swedish circle method for roots penetrating potential sliding surface and two-wedge method for shallow roots. The results showed that their synergistic reinforcement effect affords the highest overall stability of ERSW under loading. Additionally, the compensation ability of roots was investigated by vertically loading a scaled ERSW model with palm leaves and designing 16 sets of controlled tests with two geogrid lengths, three geogrid spacings, and four root lengths. The horizontal displacement of the facing, horizontal earth pressure behind the eco-bags, and geogrid tensile strain under various conditions decrease by 5.47%, 3.71%, and 4.17%, respectively, with increases in the unit length of the geogrid, and by 3.625%, 1.411%, and 4.713%, respectively, with increases in the unit length of roots. In the three parameters, the compensation rates of the root for the geogrid length are 0.66, 0.38, and 1.13, respectively. The reduction rates of the three parameters are 34.38%, 45.26%, and 58.62% with the geogrid spacing decreasing from 30 cm to 20 cm under the no-root condition, and the rates change to 56.25%, 21.14%, and 63.22% with a geogrid spacing of 30 cm and the addition of 15 cm long roots, respectively. In the three parameters, the compensation rates of 15 cm length roots for the geogrid spacing are 1.64, 0.47, and 1.08, respectively. Therefore, roots provide a compensatory effect on geogrid strength, enhancing the long-term stability of ERSW.

Context Plant roots can increase soil shear strength and reinforce soil. However, wetting and drying alternation (WD) could lead to soil structure destruction, soil erosion and slope instability.Aims This study tried to explore the effects of wetting and drying alternation on shear mechanical properties of loess reinforced with root system.Methods Direct shear testing was conducted on alfalfa (Medicago sativa L.) root system-loess composites with three soil bulk densities (1.2 gcm-3, 1.3 gcm-3 and 1.4 gcm-3) under 0, 1, 2 and 3 cycles of wetting and drying alternation (WD0, WD1, WD2 and WD3).Key results The morphological integrity of the root-loess composites was obviously better than the non-rooted loess after WD. Under the three soil bulk densities, negative power-law relationships were observed between the shear strength, cohesion and internal friction angle and the cycles of WD. WD deteriorated the soil shear strength. The most obvious decrease in soil shear strength occurred under WD1, which was 13.00-22.86% for the non-rooted loess and 17.33-25.09% for the root-loess composites. The cohesion was decreased more than the internal friction angle by WD.Conclusions The most obvious damage to the soil was under WD1. The roots inhibited the deterioration effect of WD on the shear property of loess, and the inhibition by the roots decreased with the cycles of WD.Implications The results could provide new insights into the mechanical relationship between plant roots and loess under WD, and provide a scientific basis for the ecological construction in the loess areas. Wetting and drying alternation (WD) on the mechanical properties of root-soil composites is not clear at present, or if roots can inhibit the deterioration of soil under WD. This paper investigated the effect of WD on the shear strength of root-loess composites. WD was found to deteriorate soil shear strength and cohesion, while roots inhibited the deterioration of WD on the shear property of loess. The results provide a scientific basis for ecological construction in loess areas.