Safety and ecological beautification of highway slope protection?

With the change of people's concept of environmental protection and traffic safety, people pay more and more attention to the design and construction of highway slope protection. In order to ensure the stability, safety and environmental protection of highway slopes, it is necessary to strengthen the comprehensive research on highway slope protection. Highway protection engineering design should firmly grasp the geological, hydrological, climatic and other characteristics of the design object, flexibly adopt different protection types, increase the green area of vegetation under the condition of ensuring the stability and safety of highway slopes, lay a good foundation for the construction of ecological roads, and make the main body of roads both safe and environmentally friendly. Here is a brief introduction to the common protection forms of highway slopes:

First, the main types and mechanisms of slope failure

1. Lower slope of highway: generally, the lower slope of subgrade is filled embankment. The failure of embankment slope with stable stress is mainly manifested in the erosion of slope surface and slope toe. Slope erosion mainly comes from the direct erosion of atmospheric precipitation and the erosion of slope runoff, which makes the subgrade slope form gullies along the direction of slope water flow, and the continuous development of gullies leads to the destruction of subgrade; Dams along the river, dams built in flooded areas and flood detention areas are also threatened by floods. This threat is manifested in the slope failure caused by the erosion of the embankment foot.

Slope failure is also related to the nature of subgrade filler, the height of subgrade slope and the compactness of subgrade. Generally speaking, sandy soil slopes are more easily eroded than cohesive soil slopes, while higher subgrade slopes and lower subgrade slopes are more easily eroded by slope water. The slopes with better compactness have stronger erosion resistance than those with lower compactness.

2. Highway uphill

The upper slope is a manually excavated slope, and its strength should meet the requirements of stable slope. Under the weathering action of rainfall, snow melting and frost heaving, the main failure forms of this stable slope are scouring and collapse.

Erosion damage generally occurs in slow soil slopes, such as sandy soil slopes, loam slopes and loess slopes. Under the action of atmospheric precipitation, many small gullies are formed along the slope runoff direction, which will expand year by year if no protective measures are taken. It often snows at the foot of the slope in winter, which causes the foot of the slope to be wet and soft, reduces its strength, and the upper soil loses its support and is destroyed. At the same time, the rain and snow splashed by high-speed cars also washed away the foot of the slope. In a word, the toe of soil slope is the weakest link of slope.

Slope collapse is generally divided into three types: rockfall type, landslide type and flow type, and sometimes these three forms are combined in one collapse.

Rockfall types generally refer to steep rock slopes. Under the influence of joints, bedding or faults, the rockfall-prone strata will inevitably develop cracks and be divided into weak fault blocks by fracture surfaces with different sizes. These fracture surfaces are wide, smooth and oriented. Falling rocks and rock sliding are easy to occur along the steep fracture surface. The degree of crack opening may not be visible to the naked eye, but it can seep water. Due to repeated freezing and thawing and long-term slight exercise, the cracks gradually expanded. Due to rainfall, the cracks are filled with water, resulting in lateral hydrostatic pressure and collapse. Generally speaking, rock mass develops in cracks, and it is more prone to falling rocks. In addition, this phenomenon can also occur when the soft layer is located under hard rock. This kind of damage must be strictly controlled, and the collapsed rocks are very likely to pose a threat to traffic.

Landslide-type collapse refers to the shearing of rock stratum under the action of external force and sliding along the interlayer soft rock, which mostly occurs in rock mass inclined to subgrade and with weak interlayer between layers. In addition, when the bedrock is covered with loose deposits such as cuttings and piles, these deposits are also easy to collapse along the rock plane, joint plane or fault plane.

Collapse in rainstorm mostly belongs to flowing type. Loose sedimentary soil such as sand, cuttings and shale weathered soil will be affected by water to produce flowing type collapse, and flowing type collapse has no obvious shear sliding surface.

Obviously, when the slope height is high, the above-mentioned type of slope failure is more likely to occur than the low slope.

From the above analysis, we can see that in slope protection design, we should not only do a good job in slope protection design, drainage and waterproof design, but also do a good job in slope design according to geological conditions, rock mass properties, rock occurrence and slope height.

Second, there should be protective measures

At present, expressway has the following protective measures:

1, plant protection

Using plant protection is to protect subgrade slope from atmospheric precipitation and surface runoff by using the covering effect of vegetation and the reinforcement effect of plant roots.

Adopting plant protection, increasing vegetation area and reducing surface runoff can fundamentally reduce soil erosion of subgrade. Plant cover can greatly slow down surface runoff and soil erosion. The leafy canopy can intercept part of the precipitation, and the huge root system can directly absorb and store part of the water and stabilize the topsoil. Where there is no vegetation cover, all the precipitation falls on the surface, forming runoff, causing soil erosion and erosion. The roots of vegetation can be closely combined with the soil layer, and the roots are intertwined, so that the surface soil forms a solid stable layer with different shades, thus effectively stabilizing the soil layer, fixing the gully slope, preventing erosion and collapse, and having a mechanical protection function.

In the warm and rainy south of China, plant protection has been widely used to protect soil slopes, which not only protects the slopes, but also beautifies the environment. In the northern region, plant protection measures are limited to the protection of lower slopes. Uphill is often dry and short of water, which is difficult to maintain, and steep slope is not conducive to plant growth. In the northwest loess area, the slope of loess cutting is often steeper than 1: 0.75, and only when the slope is high will it slow down to 1: 1. In Hebei, the slope of soil slope is generally 1: 1, and it is often difficult to maintain plant growth by natural precipitation on the side slope, because the slope is steep and it is difficult to maintain water and vegetation survival rate.

In recent years, many greening experts have tried to protect expressway on the steep slope in the north, and adopted three-dimensional geotextile and other measures, but they have not been widely promoted in expressway. Therefore, the protection of uphill plants still needs further study, and the bare uphill in the northern region should be covered with green clothes.

Plant protection includes planting grass, turf and trees on slopes. In Hebei, it is more convenient to plant grass and trees because there is no dug turf in general areas. Generally, perennial, cold-resistant, drought-tolerant and well-developed grass species are selected for planting grass, and tree species (including shrubs) that are easy to survive are preferred for planting trees.

Rye and hogweed are cold-tolerant and drought-tolerant plants. The combined planting technology of rye and small crown flower has been successful in some cold and arid areas in the north, which is more suitable for the climatic conditions in the north. Rye grows fast and can grow in the same year, but its roots are shallow and suitable for short-term protection. The small crown flower grows slowly and can only grow up after one year, but it has deep roots and luxuriant leaves, is particularly drought-resistant and has strong propagation and reproduction ability, and is suitable for long-term protection of subgrade slopes. The combination of the two can achieve the purpose of combining short-term protection with long-term protection.

In terms of planting trees, Hebei Province has planted Amorpha fruticosa on many slopes in expressway for many years and gained many valuable experiences, such as expressway in Shi Jing, Tangshan section of Daqin Railway, Shihuang expressway and so on. , have adopted this kind of protective measures, and achieved success, not only protecting the slope, but also beautifying and greening the road.

2. Frame protection

Frame protection is to use concrete, mortar blocks (flaky) and other materials to form a skeleton on the slope, which can effectively prevent the subgrade slope from forming gullies under the scouring of the slope water, improve the surface roughness coefficient of the slope, and slow down the water flow speed. Generally, scouring is limited to local areas in the frame. The combination of frame protection and grass planting protection can improve the protection effect and beautify the environment.

Frame protection is often used in the lower slope of subgrade, which is an auxiliary protection measure. It not only has a certain protective effect on subgrade slope, but also beautifies the road capacity, especially within the scope of interchange. In recent years, people pay more and more attention to expressway's influence on the environment and beautification of road capacity, so this form of protection is often adopted.

According to people's imagination and pursuit of beauty, the shape of the frame can be made into various shapes, such as a large frame with a 45-degree inclination, hexagonal concrete precast block protection, mortar rubble arch protection, wheat ear type made of mortar rubble or precast block, etc.

At present, some sections of the lower slope from Huning and Guiyang to Huangguoshu expressway, which are about to be completed and opened to traffic, are protected by mortar rubble arches, while some sections of the lower slope of expressway in Badaling, Beijing are protected by hexagonal concrete prefabricated small frames, and some sections of Shihuang and expressway in Hebei are wheat ears, which gives people a beautiful enjoyment.

Frame protection measures can also be used to protect the upper slope of soil, which not only increases the aesthetic effect, but also prevents slope erosion. However, because the frame needs to be embedded uphill, the construction is difficult, so it is only used in important scenic spots, and it is generally less used.

3. Slope protection

Laying (mortar or dry masonry) flaky, rubble or precast concrete blocks on a stable slope to prevent surface runoff or slope water from washing the slope is called slope protection. The general paving method is mortar, and dry masonry can be used when the erosion is slight.

Subgrade located in flood plain or flood detention area is often directly threatened by flood, so slope protection measures must be taken, and the protection height should be at least higher than the wave height, backwater height and safety value of 0.5 meters at the design flood level of subgrade. In addition, when the subgrade flows downstream and the subgrade slope invades the river, slope protection measures should also be taken.

When embankment slope protection on soft soil foundation is not washed by water, dry masonry slope protection can be used to adapt to the deformation of embankment slope caused by foundation settlement.

Step 4 cover

Cover covers include plastering, beating, shotcreting, shotcreting and other forms of protection.

1), plastering protection and tamping protection are rarely used on various highways, especially the slopes in expressway, because of their short service life. The author thinks that plastering protection and reasonable mixing of grass seeds when the subgrade is low can not only play a protective role in the initial stage of construction, but also play a protective and greening role in the operation period, which can be tried in future construction.

2), shotcrete protection and shotcrete protection

Shotcrete protection and shotcrete protection are suitable for rock slopes with easy weathering, developed cracks and joints and uneven slope surface. Their main function is to seal the rock cracks on the slope and prevent the intrusion of atmospheric precipitation and running water on the slope, so as to prevent the lateral water pressure and ice crack in the cracks, prevent the rocks on the slope from continuing weathering and protect the slope from falling rocks.

Shotcrete is a widely used protective covering measure in expressway. It is required to set rhombic metal mesh or high-strength polymer geogrid in concrete and fix it on the slope by anchor rod or pier, mainly to prevent concrete from cracking or peeling due to hardening and shrinkage. The treatment of broken rock slopes in Shitai expressway, Hebei Province and Taijiu expressway, Shanxi Province has achieved good results, especially the treatment of altered andesite slopes in Hebei Province, which is very successful. After treatment, rockfall and collapse will no longer occur. However, when sprayed concrete is used for slope protection, spalling will also occur. The rock mass is completely weathered limestone, and the newly sprayed concrete is not well combined with it, with uneven contact and low local strength. In addition, sprayed concrete is not equipped with metal mesh or geogrid, so its integrity is not good, which leads to local peeling under the joint action of internal and external factors.

Therefore, before shotcrete protection, there should be no weathered slag and weathered soil layer on the slope; Sprayed concrete protection measures should not be adopted for completely weathered rocks; Anti-crack metal mesh or high-strength polymer geogrid should be added to prevent sprayed concrete from cracking or peeling due to hardening and shrinkage. When the rock mass has the potential danger of sliding along the strata inclined to the road surface, the anchoring measures of adding shear anchors should also be taken.

5. Face wall

In order to cover all kinds of excavation slopes of soft rocks and broken rocks, as well as soil slopes which are easy to be eroded, and to avoid the influence of the atmosphere, it is called face protection wall.

Retaining walls are mostly used in strongly weathered soft rock layers and broken rock sections such as easily weathered mica schist, chlorite schist, argillaceous limestone and phyllite to prevent further weathering. It can effectively prevent slope erosion, sliding, flowing and rockfall slope collapse, and is the most common type of upper slope protection.

In addition to its own weight, the retaining wall does not bear other loads, nor does it bear the earth pressure behind the wall, so the slope of the excavated slope protected by the retaining wall should meet the requirements of extremely stable slope.

There are solid decorative walls, hole window decorative walls and arched decorative walls. Solid retaining wall is used for general soil and broken rock slopes; The hole window retaining wall is used for the slope with the slope less than 1: 0.75, and the hole window can be used for punching the face (when the slope is dry) or dry paving rubble; Because the lower rock layer of the slope is relatively complete, the arch retaining wall is used to protect the upper slope. The excavated slope protected by retaining wall should not be steeper than 1: 0.5.

In order to enhance the stability of the retaining wall, the retaining wall should be built in stages when it is high. Depending on the quality of bedrock on the section, every 6 ~ 10 meter high should be regarded as Grade I, and a platform ≥ 1 meter should be set at the top of the wall. Behind the wall, an ear wall with a width of 0.5 ~ 1 m is set every 4 ~ 6 meters.

The top of retaining wall should be tamped or paved with original soil to prevent the slope from being washed by water and damaging the wall after infiltration. Before building the retaining wall, loose rocks and loose soil layers on the protected slope should be removed. For slopes with rapidly weathered rock layers, such as mica and chlorite schist, protective walls should be built immediately after fresh rock surfaces are excavated.

In the protection facilities of high-grade highways in mountainous areas of China, face walls are a kind of protection form for upper slopes, and most of them are solid face walls. Generally, according to the height of slope, the weathering degree of rock and the engineering geological characteristics of rock and soil, semi-protective or full-protective measures are taken. In semi-protective measures, slope toe retaining wall is sometimes used. Due to the excavation of cutting, the direction of airflow is changed and a rotating airflow is formed during cutting. In rainy and snowy weather, the airflow carries rain and snow, which is the most destructive to the slope toe, and the rain and snow splashed by cars at high speed also directly washes the slope toe. Natural precipitation flows downward from the top of the slope along the slope surface, and reaches the foot of the slope with the fastest speed and the most serious erosion. Therefore, setting a low wall at the foot of the slope is the minimum protection measure. On the other hand, setting a wall at the foot of the slope also plays a role in inducing the driving sight. For soil slopes, if technical and economic conditions permit, we can also engage in greening and plant some lianas to beautify the environment.

In short, in order to do a good job in highway construction and ensure the stability, safety and environmental protection of highway slopes, it is necessary to deeply understand the types and mechanisms of highway slope damage, flexibly adopt different protection types according to the soil, hydrology and climate characteristics of different engineering objects, strengthen design, strengthen construction management, and build safe roads, ecological roads and beautiful roads.

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