A Study on the Planting Effect of Highway Ecological Slope Protection

: An analysis of the current situation of ecological slope protection is conducted to obtain a more optimized ecological slope protection scheme, in which major factors influencing the effect of ecological slope protection constitute the object of analysis. Based on optimum grass seed combination, grass and legume plant matching with better growth effects are chosen, and the mix proportion of plant-growing medium which is the best for plant growth is determined, reflecting the application of plants in ecological slope protection and providing reference for subsequent slope protection projects.


Introduction
Highway construction plays an important role in China's economic development. During highway construction, the effort to ensure the stability of roadbed will result in slopes of different grades, which further give birth to bare side slopes, and the formation of side slopes means the covering of the original vegetation. The covering of vegetation will cause serious damage to the balance of ecological environment. As the restoration of ecological environment and sustainable development are currently hot topics in the world, the research on ecological protection of side slopes has risen to prominence.
Ecological protection of side slopes is a hot topic in the 21st century. Highway construction can improve people's living standard, but high-speed development also results in imbalance of ecological environment. At present, China attaches particular importance to such issues as ecological construction and green transport. President Xi Jinping pointed out, at the 19th National Congress of the CPC, that the construction of ecological civilization is a thousandyear task for the sustainable development of the Chinese nation, thus raising the status of ecological civilization construction and ecological environment protection to an unprecedented height. The 2021 Central Economic Working Conference also stated clearly the resolution to achieve good performance in carbon neutrality, realize synergistic effects of pollution control and carbon reduction, and stick to the path of green and sustainable development.

Analysis of the current situation of ecological slope protection
Along with the development of highway construction and the emphasis on environment resources in the 1990s, China has carried out researches on highway greening. Compared with developed countries, China's research on ecological slope protection technology was started late. Since the beginning of 21st century, relevant scientific research institutions and universities in China have put lots of efforts in the research on ecological slope protection technology, and have achieved certain results, leading to higher maturity of China's ecological control and slope protection technology. In the initial phase, the country has adopted engineering protection as the only means; later, due to the arising of many environment and safety issues, the highway side slope protection has gradually shifted from engineering protection to plant protection, with ecological protection of side slopes turning into the mainstream of the side slope protection technology.
The main functions of ecological slope protection are: (1) stabilize the slope via plant roots; natural geologic disasters common to side slopes include water and soil degradation, soil erosion, collapse, landslide and so on, and ecological slope protection means to ensure the stability of side slope with porous concrete as a frame, meanwhile the root system of the vegetation will transfer shear stress to the root through the soil, and with enhanced force bearing, the root system further stabilize the side slope and also plays the role of reinforcement [1] . (2) Improve slope resistance to erosion; ecological protection of side slopes can reduce the pore water pressure of the side slope, thereby enhancing its stability and reducing chances of slope collapse, water and soil degradation, and other natural disasters; at the same time, the plant surface can also retain some of the rainwater and dew, thereby mitigating the scour effect of rainwater on the slope and reducing rainwater erosion, and improving the resistance of side slope to erosion; meanwhile, the increase of surface water infiltration also provides favorable environment for plant growth [2] . (3) Improve road condition; highway traffic accidents account for a large portion of the traffic accidents each year [3] . According to studies, the wind speed of bare side slope far surpasses that of forest and grassland slope, therefore the slope vegetation contributes to the effective reduction of the sand drift disaster; meanwhile side slope vegetation can alleviate the impact of slope temperature, resulting in effective lowering of road surface temperature, and thereby postponing the aging of asphalt pavement and prolonging the service life of the highway [4] .
3 Selection and analysis of plant seeds for ecological slope protection 3.1 Requirements for plant selection [5] (1) Strong resilience. An excellent character formed during the growing process of vegetation, resilience refers to the capacity of vegetation in resisting various negative factors, mainly reflected in drought resistance, cold resistance, alkali resistance, disease and pest resistance, resistance to sterile soil and so on. Plant seeds with strong resilience can adapt to different slope patterns and climates, with fewer requirements for soil fertility and water, and lower mainteance costs.
(2) Good performance in fast-growing. Fast-growing refers to the accumulation rate and quantity of plant organisms, mainly manifested as high growth rate, early germination, late defoliation, and well-developed root system.
(3) Long green period. Green period refers to the time when the leaves and stems of the vegetation remain green; vegetation of long green period will highlight the landscape effect.
(4) Matching of legume and grass. Herb plants can be divided into legume and grass, with the latter varieties boasting well-developed roots and powerful water-soil retention, while the former can provide basic nutrients for the later growth of vegetation during its own growing process. Diversity of species is favorable to the stability of the ecosystem, and the matched growth of rhizomatous herb and tuft plants is helpful to the utilization of the vertical space.

Experiment for initial plant selection
Based on the research of eco-concrete application in ecological slope protection in North China, the selection of plants suitable for the climate of North China is preferred.
Featuring a warm temperate zone subhumid continental monsoon climate, North China is characterized by cold winter and hot summer, cold and dry in winter, hot and rainy in summer, with annual precipitation of 400mm-800mm and above. In view of the special circumstances of nature and geography in North China, plant seeds for planting porous concrete slope protection should not only boast powerful cold resistance, but also certain capacity in drought resistance and endurance in barren soil, with welldeveloped and robust roots powerful enough to penetrate the soil, and alkaline pores of the ecological concrete; the plant seeds should bear certain resistance to alkali. Plant species after initial screening include: tall fescue, clover, elymus, awnless brome, reed fescue, perennial ryegrass, white clover, winter grass, coronilla varia, among which fives species of grass plants and four legume plants.
The 7-day and 14-day growths of the planting are shown figure 1 and figure 2:

Analysis of vegetation initial selection and experiment results
Direct measurement method is adopted to analyze the planting effect and calculate plant coverage and germination rate. Plant coverage means the ratio of vertical projection area of plant stems and leaves to the planting area, while germination rate is the ratio of the number of germinated seeds to the total number of tested seeds, the calculation formulas are as follows: (1) (2) In the formulas, L 2 stands for plant coverage ratio, %；S for planting area, m 2 ； S 2 for the coverage area of plant stems and leaves, m 2 L 3 for germination rate, %; n 1 the number of germinated seeds, number; n the number of seeds for test, number Vegetation Initial Selection 14-day Effect Analysis in the table 1.These nine kinds of vegetation are under regular observation with their growths recorded, which show that the germination time of clover and white clover---both are members of legume family---is around 3 days, and clover germinates better than the others as most of the seeds have germinated; germination of other seeds needs longer time, generally 7-10 days. In view of the 7-day and 14-day growths of plant seeds, it is discovered that tall fescue, perennial ryegrass, reed fescue, clover and winter grass grow better than the others; among grass plants, the growing of tall fescue, perennial ryegrass and reed fescue is remarkably better than awnless brome and elymus. The comparison of the former three shows that the growth of tall fescue is slightly better than perennial ryegrass and reed fescue, as indicated in Figure 3. In terms of legumes, the growth effect and rate of clover and winter grass are remarkably superior to white clover and coronilla varia. Legume vegetation and grass vegetation differ in growth state and pattern. Grass vegetation boasts long stems and upright growth, while legume vegetation germinates quickly and bears short stems, belonging to tuft vegetation. In plant seed initial selection, tall fescue and clover are chosen as the seeds needed for eco-concrete slope protection.

Design principle of mix proportion
Composed of a variety of materials [6] , plant-growing medium is the base material for plant growth, also regarded as a kind of artificial soil. Meanwhile, it is also the necessary carrier for plant germination, and the direct supplier of water and fertility for plant growth [7] . The filling effect of plant-growing medium in planting concrete is the key factor for plant growth. Ecological concrete is characterized by large and complicated pores, therefore, the selection of plant-growing medium has to abide by the following principles: slightly acidic (principle of alkaline neutrality), light texture, water absorption and water retention principle, resistance to scour, low cost, material easy filling into the pores, free of toxic and harmful components [8] .
The most ideal pH value for plant growth is between 5.7 and 6.4, therefore, in choosing raw materials for plantgrowing medium, the best mix proportion shall be worked out in combination with local climate and geographical conditions, so as to give full play to the plant-growing medium, thereby achieving better effects in slope protection. Thus, the design of mix proportion for plant-growing medium should be more targeted. The mix proportion experiment and analysis are carried out as follows featuring the principle of alkaline neutrality.

Mix proportion experiment for plant-growing medium
(1) Raw materials of plant-growing medium are listed in the table 2: (2) Raw materials of plant-growing medium are used to set the proportion scheme, and 6 schemes are prepared for this experiment; pH value is measured, with respective material mix proportion shown in Table 3. Plant-growing medium is filled in the pores of 30cm×23cm×6cm planting porous concrete, as indicated by Figure 4. (3) The experiment measures the pH value of eco-concrete with pH spear (resolution 0.01); in the initial stage of the experiment, water that has been idle for more than two days (pH 8.2) is used to soak the eco-concrete until reaching saturation, the water consumption at this point is measured to be 1.2L; after being idle for more than 2 hours, add a small amount of water several times, so that the plant-growing medium can fully saturate and solution is dissociated from it; the last step is to measure its pH value [10] . It is indicated in Table 3 that plant-growing medium can effectively adjust the pH value of eco-concrete aqueous solution, and the value can be adjusted from strong alkaline to weak alkaline and then to weak acid, providing environment guarantee for plant growth. Through comparison of the six schemes, it is concluded that decrease of vermiculite addition can reduce the pH value of plantgrowing medium in a quite effective way, and compared with other materials, vermiculite also has the advantage of drainage and breathability; the comparison between perlite and humus shows that under the premise of adding the same amount of vermiculite, humus has more advantages than perlite in terms of acylous action, therefore Scheme 5 is adopted as the mix proportion for plant-growing medium.

Conclusion
By exploring the two factors influencing the effect of slope protection, plant varieties and mix proportions of plant-growing medium, this paper draws the following conclusions: (1)Through planting experiments on 7 kinds of plants, in which their 3-day, 7-day and 14-day growths are observed, and the germination rate and coverage rate are measured, it is clear that the plant varieties suitable for eco-concrete planting are tall fescue and clover.
(2)Six different mix proportions for plant-growing medium are established, and with alkaline neutrality as the evaluation index, the pH values of plant-growing medium of six different mix proportions are measured to get the proportion boasting the strongest alkaline neutrality capacity: natural soil 48%, peat soil 18%, vermiculite 5%, perlite 8%, humus 15%, organic mixed fertilizer 15%, water-retaining agent 1%, plasticizer 1%, and ferrous sulfate 1%.
(3)The above two points can be used as a reference for practical projects. The next step of in-depth research focuses on the carbon and oxygen sequestration capacity of different plants under different ratios of vegetation substrates.