## 3 Feasibility Test for Greening of Retaining Wall Face
### Test Results and Analyses  
After the specimens were made, they were kept in a sunny and ventilated place. In case of insufficient light, fluorescent lamps were used to compensate for part of the light time and the specimens were watered every 3 days. Throughout the test process, every 7 days, pictures were taken of each group of specimens and the corresponding records were made (see Figures 8-11).  
The development of each group of grass seeds under different periods is shown in the figures above.  
1. Compared with the grass seeds of A and B, the grass seeds of species C exhibited earlier germination and faster growth across all four planting substrates. These species  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **No. 1** & **No. 2** & **No. 3** & **No. 4** \\ \hline Grass seed A & loess & loess slurry & cement-modified loess (5\%) & cement-modified loess (10\%) \\ (Tall rescue) & loess & loess slurry & cement-modified loess (5\%) & cement-modified loess (10\%) \\ Grass seed C & loess & loess slurry & cement-modified loess (5\%) & cement-modified loess (10\%) \\ (Annual bluegrass) & loess & loess slurry & cement-modified loess (5\%) & cement-modified loess (10\%) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Test scheme.  
Figure 6: Grass seed.  
are considered to be fast-growing and adaptable to various environments. They are particularly well suited for situations where quick plant coverage is needed on the surface to establish a protective layer against rainfall erosion, contributing to soil and water conservation. Additionally, they create favourable conditions for the subsequent planting of other plant species.
2. Planting substrate No. 2 showed poor growth and germination for all three types of grass seeds compared to the other substrates. The reason is that planting substrate No. 2 is essentially a soil slurry. Due to the small particle size of soil particles used in the test and the high water-solid ratio, the soil slurry lacks the necessary air for seed germination, resulting in a low germination rate in later stages. In contrast, the cement-modified loess slurry allows for adequate air supply, as the cement does not completely encase all aggregates (soil particles), leaving gaps between them.
3. Planting substrates No. 3 and No. 4 are both composed of cement-modified loess. The grass seed germination rates in these substrates were lower than in planting substrate No. 1, which used vegetal loess. However, species C of the grasses exhibited relatively robust growth in these substrates, indicating that selecting suitable plant species and using cement-modified loess as a planting substrate is a viable option.
4. The seeds' germination and growth rate in planting substrate No. 4 (with a higher cement content) were lower than that in planting substrate No. 3 (with a lower cement content). This is because, as the cement mixing ratio increases, the strength of cement-modified loess gradually rises after hardening, which can hinder the germination and breakthrough of plant seeds.  
In summary, when using cement-modified loess as a planting substrate, it is advisable to limit the cement mixing ratio to 10% or less. Excessive cement mixing not only becomes uneconomical, but also results in a cemented material with greater strength and hardness, which may not be conducive to the long-term growth of plants.  
Figure 7: Prepared specimens.  
Figure 8: Growth of each group of specimens after 7 days of maintenance.  
Figure 9: Growth of each group of specimens after 14 days of maintenance.