Influence of the roots of mixed-planting species on the shear strength of saline loess soil

In order to improve our knowledge of the mechanical effect of the roots of mixed-plantings on soil reinforcement and slope protection, the influence of roots of a mixed-planting with four herb species (Medicago sativa L., Elymus nutans Griseb., Puccinellia distanx (L.), and Poa pratensis L.) and one shrub species (Caragana korshinskii Kom.) were investigated on the shear strength characteristics of saline loess soil. The root distribution characteristics were assessed via a survey when the plants grew for one year. The effects of the root biomass density, the root mass ratio (RMR) of the fine roots to the coarse roots, the moisture content, and the salt content on the shear strength index of the rooted soil were analyzed via a triaxial compression test, and the mechanism of these effects was discussed. The results indicate that the biomass density decreased linearly with increasing depth. The RMR initially decreased with depth and then increased, exhibiting in a quadratic relationship. The cohesion of the rooted soil increased linearly as the biomass density increased. The cohesion of the rooted soil initially increased with increasing RMR and salt content, and then it decreased. The turning point of the cohesion occurred when the RMR was 0.6 and the salt content was 1.18%. The internal friction angle of the rooted soil initially increased with biomass density and then decreased, and the turning point of the internal friction angle occurred when the biomass density was 0.015 g/cm3. The relationships between the internal friction angle of the rooted soil and the RMR and salt content were exponential incremental and linear subtractive relationship, respectively. Both the cohesion and the internal friction angle of the rooted soil linearly decreased with increasing moisture content.