Study on Mechanism and Control Effect of Highway Slope Ecological Protection

Suilun LI

Abstract At present， with the rapid development of social economy， peoples awareness of environmental protection continues to increase， and environmental construction and environmental protection work are put on the agenda. Highway slope greening protection is an important means to achieve social sustainable development. Developed countries carried out research earlier in this area， and Chinas research in this area started later. In engineering construction， ecological protection is to realize slope reinforcement and slope protection depending on the water storage and soil fixation effects of vegetation. Its slope protection can be summarized as the mechanical effect of root system and the hydrological effect of vegetation. As a basic form of slope greening protection， it is necessary for us to understand the mechanism of ecological protection and to enhance the design of ecological protection accordingly. Based on this， the article analyzed the development of slope greening protection technology and ecological protection mechanism from different angles， so as to establish a theoretical framework for future research.

Key words Highway; Slope; Slope protection; Ecological protection; Greening technology; Ecological slope protection mechanism

Highway slopes refer to slope sides with a certain slope made on both sides of roadbeds to ensure the stability of roadbeds （Fig. 1）[1]. They are generally constructed on rock slopes with poor lithology and low strength having a hard rock layer weathered or easy to weather and a weathered and peeled surface layer[2]. Their functions are to prevent rain wash， to fix the soil and to protect slopes， to maintain the balance of water and soil， and to maintain the stability of roadbeds and slopes. Slope protection refers to the act of taking protective measures to stabilize slopes and protect the slopes from erosion and cracking， or the results of the act.

Ecological and environmental protection is one of the themes facing the construction of a well-off society. As people pay more attention to the ecological environment， the protection design and construction of highway slopes have attracted more and more attention[3-5]. The change of peoples consciousness leads to special attention to environmental protection， especially in water conservancy， hydropower and transportation projects that are prone to generate a large number of bare excavated slopes. The traditional slope protection measures are out of step with the surrounding environment， and it is urgent to apply ecological measures such as plants to slope protection projects. The design of highway slopes has a wide range of distribution along highways， and the scope of damage to the natural environment is increasing. If we pay attention to protection and creation of the environment while protecting slopes， and adopt appropriate ecological protection methods， highways will be endowed with the characteristics of safety， comfort， beauty， and harmony with the environment， and considerable economic， social， and ecological benefits will be also produced[6]. In Chinas highway construction， slope construction is often the final project in construction， and it is also the easiest to complete and has the most remaining problems[7]. With the sustained and rapid development of Chinas highway construction， a large number of slopes will be formed. Slope soil erosion prevention and control as well as treatment of slope excavation are extremely important tasks， and they have been valued by the whole society under the environment of increasing environmental protection awareness[8-9]. Slopes will be continuously eroded due to external reasons such as rainfall， which will lead to the instability of slopes. Unstable slopes are usually treated with engineering methods such as retaining walls， anti-slide piles and anchor cables[10]. The unstable factors include mortar block masonry， dry block masonry， spray mixing， ejector anchor， concrete side wall and retaining wall and other slope protection methods. The research on the ecological protection mechanism of highways in China has only begun in recent years.

Analysis of Ecological Protection Mechanism

Ecological protection is to replace the pure engineering protection with fresh plants， so as to stabilize the slope and resist corrosion[11]. The roots of fresh plants penetrate the loose weathered layer on the slope surface and are then anchored to the stable layer， acting as an anchor （Fig. 2）. In addition， the plants can also intercept part of the rainwater during the rain， playing a role in reducing splash erosion on slopes. Moreover， the soil conditioning function of microorganisms also protects highway slopes. The narrow understanding of slope ecological protection is to plant plants on slopes and make the plants cover slopes through artificial means to prevent soil erosion and conserve water. The function system of ecological protection is relatively special. In the subsequent application stage， it is necessary to start from the actual situation， judge the protection index， and implement it in practice according to the specific requirements of the technical form. The mechanism and characteristics of ecological protection were analyzed as below.

The effect of preventing soil erosion is obvious

Ecological protection technology has received extensive attention because it can not only play the role of traditional support measures for slope protection， but also restore the damaged ecological environment[12]. After a slope is formed， plants are planted on it to protect and strengthen the surface of the slope by the interaction of plants and rock soil， so that it can meet the design requirements for slope stability and restore the natural ecological environment destroyed by engineering construction. This kind of slope protection is ecological protection. Vegetation protection mainly uses the reinforcement of plant roots to improve the shear strength of the soil， thereby increasing the stability of subgrade slopes， enhancing the ability of the slope soil to resist erosion， improving the landscape effect of road slopes and improving the driving comfort of roads. Studies have shown that the soil erosion of forests and grassland in heavy rainfall is only 1%-2% of agricultural land， and their effect of preventing soil erosion is very obvious （Table 1， Table 2）. The direct impact of raindrops on bare ground causes the soil particles to shift and causes splash （surface） erosion; and the raindrops hit the tree （grass） leaves， which trap part of raindrops， and even the part that is not trapped is also subjected to energy dissipation by the tree （grass） leaves， causing the impact energy to be greatly reduced and reducing splash （surface） erosion. Meanwhile， the ground with vegetation has a large content of fine-grained soil and organic matter， and the bonding strength of the soil is large. The vegetation root system in the arid area is extremely developed， and thus can store a lot of water during rainfall， strengthen the infiltration of rainwater， and avoid the formation of runoff. Meanwhile， the developed root system plays a similar reinforcement effect， which can stabilize slopes.

Anchoring effect of deep roots

Vegetation restoration is mainly to solve the disaster prevention and control of slopes[13]. The disaster prevention and control of slopes has always been a problem that project builders have paid close attention to but have not completely solved. According to the thickness of the sliding body， the destruction of slopes can be divided into deep sliding and shallow surface sliding （including rain and wind erosion of the surface layer）. How can we determine whether the slopes belong to deep sliding or shallow sliding is not clear at present. Shallow surface sliding refers to the condition that the fracture surface is within the influence range of the deep roots of most trees （generally less than 2 m）. Fig. 3 is a schematic diagram of plant roots anchoring shallow surface slide.

The main root of the vertical root system of woody plants can penetrate into the deep layer of the soil[14]. Through the friction between the main and side roots and the surrounding soil， the root system is connected to the surrounding soil body[15]. Combined with the vertical root distribution characteristics， the root system can be simplified as a full-length bonded anchor with the main root as the axis and the side roots as the branches to analyze its mechanical effect on the surrounding soil. The magnitude of its anchoring force can be obtained by calculating the frictional force of each side root and surrounding soil and the friction force of the main root and surrounding soil. Fig. 4 is the mechanical model of deep anchoring of woody plant roots. The vertical root system of the vegetation passes through the loose weathered layer in the shallow layer of slopes and is anchored to the deeper stable rock and soil layer， which acts as a prestressed anchor. Grasses， legumes， and small shrubs have an obvious soil strengthening effect at the depth of 0.75-1.50 m underground， and the anchoring effect of tree roots can affect deeper rock and soil layers.

Reinforcement effect of shallow roots

The deep and thick roots of plants themselves have a certain strength and rigidity. When they pass through the loose weathering zone of the shallow layer of slopes and penetrate into the deep hard soil layer， the slopes are reinforced through anchoring and side resistance of rock socket， and their role is similar to anchors or anti-slide piles[16-18]. The mechanical effect can be explained by the anchoring theory of geotechnical engineering. The anchoring mechanism focuses on： suspension， composite beam， squeezing and strengthening， and strength harding theory of surrounding rock. The essence of plant anchoring is to improve the mechanical parameters of slopes. The deep roots of plants themselves have a certain strength and rigidity， and when they pass through the loose weathering zone of the shallow layer of slopes and penetrate deep into the hard soil layer， they will play a role of the anchoring system. The root system of planted grass is intertwined in the soil， making the slope soil body a composite material of soil and grass roots. Grass roots can be regarded as three-dimensional reinforcement materials with prestressing force， which improve the strength of the soil.

Suilun LI et al. Study on Mechanism and Control Effect of Highway Slope Ecological Protection

Reducing the pore water pressure of slopes

Plants exert tension on pore water through photosynthesis and transpiration[19]. Most plants can apply 132 MPa tension to the pore water before reaching the withering point. The root system of plants continuously absorbs the water in the soil， so the water in the soil is reduced， and the pore water pressure is reduced， even to a negative value. The reduction of pore water pressure enhances the shear strength of the soil body and is beneficial to maintain the stability of slopes. The instability of a slope is closely related to the water pressure of the slope. Rainfall is one of the important factors that induce landslides. By absorbing and transpiring water in the slope body， plants reduce the pore water pressure of the soil body and improve the shear strength of the soil body， which is beneficial to the stability of the slope body.

Intercepting rainfall， weakening splash erosion

Part of the rain is stored in leaves and trunks， and later re-evaporated into the atmosphere[20]. The rainfall interception effect of vegetation reduces the intensity of rainfall reaching slopes and reduces the rainfall reaching slopes. The vegetation can intercept 100% of light rain and 25% of severe storms， and can intercept 30% of the total rainfall in one year. Vegetation rainfall interception is of great significance for controlling slope erosion and slope stability. The aboveground parts of plants mainly include plant stems and leaves and plant litter. Practice has proved that they have important soil and water conservation effects on slope soil. Part of rainfall is intercepted by plants before reaching slopes， and is later re-evaporated to the atmosphere or falls to slopes. Vegetation can intercept raindrops falling at high speed， reducing the splash of energy and soil particles.

Controlling soil loss

Vegetation reduces the surface runoff velocity and surface runoff by the combined effects of surface roughness increasing， infiltration promotion， interception and evapotranspiration. The surface runoff takes away the soil particles that have been separated by dripping and splashing， and can further cause sheep erosion and gully erosion[21-23]. This capacity increases exponentially with runoff velocity， and the amount of soil loss increases directly with runoff velocity and flow. Therefore， reducing the speed and flow of runoff can effectively control the erosion of runoff. Generally， the amount of soil loss decreases exponentially with the increase of vegetation coverage[24-26]. Meanwhile， the presence of vegetation can accelerate the sedimentation of soil particles carried away by runoff on the surface， reducing the loss of soil within a certain range. The surface runoff takes away the soil particles that have been separated by dripping and splashing， which can further cause sheet erosion and gully erosion. Vegetation can suppress surface runoff and weaken raindrop erosion， and thereby control the loss of soil particles.

Comprehensive benefits are obvious

Vegetation is the primary producer of the ecosystem， the cornerstone for coordination and harmony of the structure and function of the ecosystem， and the prerequisite for the development project to exert good economic， ecological and social benefits. Vegetation is a green protective umbrella for soil， which plays an important role in preventing soil erosion and regulating the hydrological cycle[27]. It is mainly reflected in the interception of rainfall and the regulation of soil hydrological properties， consolidation of soil， enhancement of soil erosion resistance， reduction of flood peaks， and conservation of water. In addition， from an environmental perspective， the functions of the forest and grass vegetation include improving the microclimate， absorbing carbon dioxide， and making oxygen， absorbing harmful gases in the atmosphere and purifying the air， reducing noise and radioactive pollution， regulating light， protecting and enhancing biodiversity， and beautifying urban landscape. The green space also has the functions of adsorbing dust， preventing wind and sand， increasing the circulation of water and soil in the city， increasing rainfall， shading， and alleviating the urban heat island effect. The structure， function and ecosystem service function of vegetation are shown in Fig. 5.

Plant slope protection mainly depends on the functions of the underground root system of plants on slopes and the aboveground stems and leaves protecting slopes from erosion. The effects can be summarized as the mechanical effects of the root system， the hydrological effects of the stems and leaves， and the transpiration and drainage effects of plants[28-29]. The mechanism of plant slope protection is shown in Fig. 6. Plant roots are divided into two types： herb roots and woody plant roots， the mechanical reinforcement effects of which are different. The root system of plants in the vertical direction can penetrate the surface soil on the slope body and be fixed to the deeper stable soil layer. The shallow capillary root system is criss-crossed and staggered， which reinforces the surface soil of the slope body. Moreover， the root system absorbs the water in the soil which is evaporated into the atmosphere， which reduces the rainfall infiltration and reduces the soil water content or pore water pressure. The hydrological effects of vegetation include the interception of raindrops by the stems and leaves to weaken splash erosion and suppress runoff erosion. The deciduous cover layer on the ground can slow down the surface runoff， and can also weaken the impact of streams on slopes.

Ecological protection adopts a combination of arbor， grass and shrub. Among them， grasses and shrubs are the best due to their small plant shape， and they have the best energy dissipation effect on raindrops[30-32]. Moreover， grasses can play their role in the very year of sowing， and shrubs have more developed root systems （generally more than 3 m）， and can usually play their full role in more than 3 years. Trees have a long growth cycle， but their root system is the most developed （up to several tens of meters）， the longest and deepest， which is conducive to stabilizing slopes （Fig. 7-Fig. 9）.

Conclusions

Nowadays， more and more roads are being built， so there are more and more slopes， but the plants on the slope surface have been damaged， increasing the area of soil erosion in our country[33-35]. In places with steep slopes， it is also easy to cause landslides， causing economic and personal harm. Therefore， in order to avoid such tragedies， we can increase vegetation on slopes， to maintain soil and water and prevent erosion. In Chinas highway construction， slope construction is often the final project in construction， and it is also the easiest to complete and has the most remaining problems. With the sustained and rapid development of Chinas highway construction， a large number of slopes will be formed. According to incomplete statistics， since 2000， the highway slope area has increased rapidly at a rate of 200-300 million m2 per year. For every 1 km of highway construction， the area of exposed slopes can reach 50 000-70 000 m2. To effectively prevent erosion， it is advisable to prevent the formation of gully erosion at the initial stage of erosion， i.e.， at the stage of surface erosion， by intercepting rainwater， making it infiltrate in situ. Such measure will play a multiplier role， and combining hydraulic protection structures with soil and water conservation measures， significant economic and environmental benefits can be achieved. The vegetation on slopes and the related mechanical relationship have a significant impact on the stability of soil surface and its erosion. To analyze the stability of slope vegetation to the soil body， the favorable and unfavorable factors should be considered comprehensively. Needless to say， slopes with good vegetation cover tend to be more stable than bare slopes. However， considering that the vegetation system has positive and negative effects on slope stability， it is particularly important to adopt the combination of traditional slope management and ecological protection in actual engineering construction. At the present stage， means including frame beam soil-covering greening， vegetation base spraying and planting bags are mainly adopted， and further research is needed. Slope greening is the key and difficult point of highway greening. Its greening effect not only affects the overall landscape of highways， but also directly relates to the soil erosion of slope surface and the stability of roadbeds. Therefore， slope protection is particularly important in highway ecological construction.

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