## 3 Test Results  
### Shear Strength of the Root-Soil Composites during Different Growth Periods  
The shear strength \(\tau\) of the root-soil composites during the different growth periods was measured by a large direct shear test, as presented in Figure 2. It was obvious that the G.s mixture had the highest \(\tau\) under the four normal stresses while bare soil held the lowest. Specifically, the \(\tau\) associated with C.d reached a peak at 90 d and then decreased slightly. M.m had the highest \(\tau\) at 90 d under normal stress conditions of 50 kPa, 100 kPa and 150 kPa. However, under normal stress conditions of 200 kPa, the \(\tau\) was highest at 120 d. However, under the same normal stress, the \(\tau\) first increased and then decreased with the increasing growth cycle. The \(\tau\) of the G.s mixture under a normal stress of 200 kPa for 90 d was slightly lower than that for 60 d. Except for the 30 d, 60 d and 120 d growth periods, the G.s mixture had the highest \(\tau\) at 90 d under the normal stress conditions of 50 kPa, 100 kPa and 150 kPa. The \(\tau\) of the G.s mixture and M.m had the same change trend. As the vegetation growth period increased, the \(\tau\) increased first and then decreased between 90 and 120 days. Obviously, the presence of roots significantly enhanced the shear strength of the soil. The root-soil composite improved the \(\tau\) of the soil through a reinforcement mechanism  
Figure 1: Direct shear test. (**a**) Direct shear device; (**b**) after setting the sample; (**c**) after sample destruction.  
similar to that of conventional reinforcement materials in reinforced soil. Specifically, the roots interacted with the soil to enhance its strength, resulting in an increase in the \(\tau\) of the composite compared to bare soil [15]. Under different normal stresses, the \(\tau\) of the bare soil was less than that of the root-soil composites, which shows that plant roots have a reinforcing effect on soil. The greater the \(\tau\) of the root-soil composite, the greater the benefit to slope stability. Among the plants tested, the root-soil composite containing the G.s mixture generally exhibited a higher \(\tau\) than that of the C.d and M.m under the same normal stresses. Additionally, the reinforcement effect of the different root systems on the soil varied with the growth period. For the 30 d growth period, the \(\tau\) of the root-soil composites in order was G.s \(>\) M.m \(>\) C.d. For the 60 d growth period, the reinforcement effects of the three root systems under different normal stresses had no obvious trends. When the growth period reached 90 and 120 days, the plant growth was stable, and the strengthening effects of all three root systems followed the order G.s \(>\) C.d \(>\) M.m.  
Overall, the presence of roots significantly enhanced the shear strength of the soil, and the reinforcement effect varied with the type of plant and its growth period. The root-soil composite containing the G.s mixture was a more effective way to improve slope stability compared to using single herb or shrub root reinforcement.  
Figure 2: Shear strength under four normal stresses during the different growth periods.