| With the increasing development of ecological protection and the construction of ecological civilization of China,vegetation slope protection can not only protect the slope effectively,but play a role in ecological restoration and environmental protection.In this paper,a series of experimental studies were carried out on the mechanical properties of the Hainan red clay,the Leucaena leucocephala’s single root and the reinforced function of root system of the Leucaena leucocephala on soil shear strength.In order to better simulate the stress of the root-soil composite in practice,our research group developed the large simple shear test device which was applied to root-soil composite.Explored the strengthening effect of the root system on the root-soil composite under different conditions with water content,root diameter,root number and relative position of roots as variables.The following conclusions were obtained:The experiments on Hainan red clay with different water content with large simple shear test device and indoor standard direct shear test device were show that: The shear strength of red clay decreases with the increase of water content,and the shear strength of simple shear is 4%-6% lower than that of direct shear.Moreover,the stress-strain curves are both stain hardening type,the simple shear line tends to fold line,and the direct shear line tends to hyperbolic line.The water content is negatively correlated with the shearing strength index of red clay,and the relationship can be fitted by quadratic function.The red clay’s angle of internal friction of simple shear is 25% lower than that of direct shear,and the difference value of cohesion is less than 6%.In the simple shear test,the shear zone of red clay changes with the change of water content,which is also the reason for the difference of the red clay’s shearing strength in simple shear and the direct shear tests.The large simple shear experiments on the Leucaena leucocephala’s rootsoil composite under different conditions with root diameter,root number and relative position of roots as variables,it’s shows that: Because of the addition of root system,the shearing strength of root-soil composite can improve effectively.And with the increasing of root number and diameter,the shear capacity of root-soil composite is also improved.This indicates that within a certain range,the increasing of root number and diameter can improve the resist ability of sliding and stability of slope.With the increase of water content,the shear resistance of root-soil composite decreases.The change law is consistent with that of plain soil.But the degree of improvement of root-soil composite shear strength by roots did not decrease significantly,indicating that roots could play a certain role in the resistance to slip even the water content of soil changed.The change of relative position of roots system also had a certain effect on the shear strength of the root-soil composite.And the increase of soil cohesion of root system to root-soil composite is that the vertical roots is better than the diagonal roots better than the parallel roots,but the difference among the three is not significant.Using the root-to-area ratio(RAR)and the Waldron model to analyze and verify the Leucaena leucocephala’s root-soil composite,the results shows that:When RAR is not much different,the resistance slide effect of 15 mm three roots is better than that of 25 mm single root.That’s because that the strengthening effect of root system on root-soil composite is mainly caused by the friction between root and soil.The larger contact area between the root and soil,the larger friction and the larger resist slide force it can produced.The increased shear strength of root system ΔS calculated by the Waldron model is large than the cohesion increment Δc obtained by linear fitting.The reason may be that cause the addition of root system,cracks appear between root and soil during the shearing process,resulting in slight changes in the friction angle of rootsoil composite,resulting in deviation between the two.But the deviation is within 20%,it also indicates that even the Waldron model ignores the small changes in the friction angle of the root-soil composite,it is of positive significance to evaluate the slope fixation of the root system.And the increment of shear strength of the root system in the root-soil composite calculated value obtained by the Waldron model is larger than the actual value,which overestimates the reinforcement effect of the root system on the slope.Therefore,when calculating the reinforcement effect of the root system,the constant value should be reduced appropriately to improve the safety factor of vegetation slope protection. |
