Abstract: [Objective]The role of plant roots in maintaining slope stability and soil mass has been widely recognized. However, the interaction between soil and roots is complex, so that it is difficult to accurately quantify the effect of root reinforcement. In order to improve the accuracy of root reinforcement quantification, in terms of the different mode of destroying roots in soil, this paper analyzed the tensile and pull-out strength of roots under various growth periods using direct shear tests and root strength tests.[Method]A breakaway type instrument was designed on the basis of traditional direct shear apparatus, which can test the in-situ shear strength of root soil composite of potting plants (as shear box). In this study, 12 well-grown 3-years-old Symplocos saplings were planted in direct shear boxes. The tensile strength, pull-out strength of roots and shear strength of roots-soil composite were measured in one month, 4 months and one year after the plants were transplanted. The relationship between single root diameter and single root tensile strength and pull-out strength was analyzed. On the time scale, the changes in the related parameters of the root system were investigated, the interaction mechanism of root and soil and the destruction of root system were analyzed, and the existing Wu model was optimized, and the effect of root soil consolidation in the dynamic growth process was evaluated.[Result] The results reveal that 1) The root tensile and pull-out strength collectively played an important role in root soil reinforcement. In the process of plant growth, the effect of pull-out strength on soil consolidation was more significant than that of tensile strength; 2) The shear strength of root-soil composite significantly increased after four months, however the increasing rate reduced after 1 year. The percentage of breaking roots, especially for fine roots, increased with the increase of planting time; 3) The root tensile and pull-out strength were added to the calculation of plant root soil consolidation model, the proposed model can estimate the reinforcement effects more accurately with a deviation merely 8.13%.[Conclusion]After one year's growth, the quantity of plant roots did not change greatly, and the root tensile strength, which is determined by its material properties, did not change obviously. However, the bonding between soil and root increased significantly because a number of roots tend to break when the soil was sheared, leading to increased root reinforcement with the plant growth.

Key words: roots, failure mode, tensile strength, pull-out strength, model