Abstract
Rainfall and surface water infiltration will affect the safety of loess slope. Ecological slope protection can protect and reinforce the surface layer of the slope by using the anchoring effect of plant root. In this study, the prevention and control technology of grass planting on loess slope in the solidified soil was studied. Heavy metal contents in the soil after addition of the stabilizer met the farmland standard at the end of the growth period. A comparison of the biomass data of different groups of solidified soil showed that the biological characteristics of plants had the best response at the mixture ratio of solidified soil of 0.86%. The shear strength of the root–soil composite increased by about 60% compared with that of plain soil at the mixture ratio of 0.86% stabilizer, and the permeability coefficient can be reduced by two orders of magnitude. Through the field engineering application, the ecological restoration test using loess soil solidified by consolid system with grass planting met the engineering requirements, with clear soil and water conservation benefits. Therefore, planting grass in solidified soil can effectively reduce the erosion of soil and improve the stability of the shallow slope.
1 Introduction
The total area of loess and loess-like in the world is about 13 million square kilometers, and deposits cover approximately 10% of the earth’s surface [1]. The Loess Plateau, which has the largest loess coverage in the world, is located between 34° and 40°N and 103° and 114°E, in northern China [2]. The Loess Plateau is rich in coal, oil, aluminum, and other resources. It is an important energy and chemical base in China. The plateau is covered with deep loessal layer, and the maximum thickness is over 200 m. In the loess area, the main engineering accidents are related to rainfall and surface water infiltration because of water sensitivity of loess [3–5]. The prevention of rainfall and surface water entering deep soil is a major research direction for improved slope accident prevention and control. Traditional slope protection engineering measures include grouted block stone and shotcrete addition [6–8]. These protective measures play a positive role in increasing slope stability and preventing soil erosion at the initial stages of construction. However, their landscaping and aesthetic effects are poor, and they are not favorable for the ecological balance and environmental protection. Over time, the aging of concrete and the weathering of rock also lead to a decrease in their protective effect. Li et al. [9] indicated that traditional slope protection methods are often detrimental to original vegetation and restoration efforts, resulting in large amount of secondary bare land, subsequent rainfall and runoff energy can lead to soil erosion and, in some cases, slope collapse. Vegetation can influence slope stability by improving the hydrological condition of slope soil through transpiration and soil reinforcement (mainly through roots) [10–15]. An and Wang [16] studied the uprightness of the high and steep loess slopes of the Shanxi Expressway, which is subject to a poor soil and arid climate, and proposed the use of three-dimensional nets, geocells, application of organic fertilizers, and selection of suitable vegetation for the growth conditions of the region. Jiang et al. [17] developed a flexible geocell-reinforced ecological retaining wall for protection of high and steep slopes, and applied it to the slope protection project of Xilai Expressway. Through this case study, it is demonstrated that the flexible ecological retaining wall as a slope protection technology can be successfully applied to steep slopes with a height of more than 15 m. Moreover, it brings significant advantages for protecting the ecological environment and improving the highway landscape.
Soil solidified stabilizers can improve soil structure and soil strength and durability, and have the characteristics of high efficiency, economy, ecology, and environmental protection [18,19]. With the rapid development of polymer material technology, especially the enhancement of people’s awareness of environmental protection. It has promoted the research and development of environment-friendly polymer modified materials and their application in soil slope protection and water conservation [20–23]. Wang et al. [24] applied an STW-type polymer stabilizer to reinforce a soil slope. They found that the stabilizer could form a film on the slope, which improved the stability of the slope. The stability of the slope increases with the concentration of the stabilizer and the increase in the reinforcement depth. Xiang et al. [25] used ionic soil stabilizer to improve the shear strength of sliding soil. They found that the stabilizer can improve the plasticity index, void ratio, and free expansion rate of sliding soil. Additionally, it could change the structure of soil surface layer and improve the hydrophobicity of surface soil and effectively improve the slope stability. Li et al. [26] pointed out that with the development of material science and technology, more and more materials are being studied and applied to soil solidification. Especially, polymer materials will promote the further development of soil and water conservation technology due to their advantages of less incorporation, convenient transportation, and simple construction.
Therefore, they have become a major research focus in terms of consolidation materials for soil and water conservation. Soil stabilizer, especially macromolecule stabilizers, can be expected to promote the further development of soil and water conservation technology. It has become a new trend to combine stabilizers with biological measures, engineering measures, and soil and water conservation materials. Moreover, the comprehensive application of chemical consolidation and biological control methods has become an effective means to control soil and water loss.
As a type of chemical reinforcement material, the soil stabilizer of consolid system has been widely used worldwide on road and has shown an improvement effect, but there are few studies on slope protection, especially in this area. Seco et al. [27] found in the improvement of clay in northern Spain that the modified soil with the soil stabilizer of consolid system had high early strength, and the improvement effect of other curing materials with large dosage could be achieved with very small dosage. Eren and Filiz [28] found in the study on the improvement of Italian clay with the soil stabilizer of consolid system that the addition of anti-hydrophobic curing agent can improve the optimal moisture content and California bearing ratio value of the soil, and reduce the maximum dry density, expansion rate, liquid-plastic limit, and relative density of soil. Zhang et al. [29] modified remolding Q3 loess with the soil stabilizer of consolid system, and carried out a water drop infiltration test, flexible wall penetration test, shrinkage test, and pressure membrane dehydration test in the laboratory to explore the influence of the consolid stabilizer content on the infiltration and water loss capacity of loess. Yin et al. [30] studied the compaction characteristics and mechanical properties of loess soil solidified by consolid system. They concluded that the consolid system has an improvement effect on mechanical properties of the base material in the road base layer. Researchers are currently working on its effective application in embankment materials, soft soil stabilization, road sub-base construction, and other geotechnical engineering fields. However, to our knowledge, there are few studies focusing on the use of the consolidated materials on loess slope.
The aim of this article is thus to solve the feasibility of the application of the soil stabilizer of consolid system in loess soil slopes. We went through a series of experiments to collect the growth rules of the vegetation and the contents of organic matter and heavy metals in solidified soil. Also, the shear strength of root–soil composite was tested. At last, the field test was used to compare between untreated slope and treated slope.
2 Research methods
2.1 Overview of the study area
The study area is located in the hinterland of the Loess Plateau, at an altitude of 1,011–1,600 m above sea level, and the terrain is high in the northwest and low in the southeast (Figure 1). The study area is located in Qingyang, China, between 106°20′–108°45′ east longitude and 35°15′–37°10′ north latitude. The terrain is broken into horizontal steep-sloped and deep ravines with a V-shaped cross-section (Figure 2). The gully density is 3.24 km km−2, representing a typical plateau–gully region.
Landform map of Qingyang City, Gansu Province, China.
Status of the studied slope.
The study area is located in the middle latitude zone, in an inland setting with complex terrain and a temperate continental climate (Figure 3). According to data provided by local meteorological offices, the annual average temperature is 8.7°C, the highest temperature is 35.7°C, the lowest temperature is −22.6°C and the average frost-free period is 260 days. The average annual precipitation was 563.0 mm, with great interannual variation, with the maximum annual precipitation of 791.0 mm (1975) and a minimum annual precipitation of 338.3 mm (1997). Precipitation is concentrated from July to September, and it usually falls in the form of heavy rain and rainstorm, which are both limited and intermittent, accounting for 55–70% of the annual precipitation. The annual average evaporation was 1474.3 mm, which is 2.62 times the precipitation. The annual average relative humidity was 65%.
Climate in Qingyang City.
The soil in the study area is mainly loess soil developed from the parent loess and secondary loess. The organic matter content is 0.8%, and the soil fertility is positively correlated with the intensity of soil and water loss. The general soil nutrient status is low nitrogen, low phosphorus, high potassium, and low organic matter content. The project area is located in the northern temperate forest grassland vegetation belt, the vegetation is mainly artificial woodland and grassland, and there is sporadic natural vegetation.
2.2 Research materials and methods
2.2.1 Experimental materials
2.2.1.1 Loess materials
The unsaturated Malan loess in Longdong region was taken as the research soil. Table 1 shows the physical parameters of undisturbed Malan loess obtained from field sampling. According to Table 1, the plasticity index of the selected loess is 10, which belongs to silty clay, and the soil is self-weight collapsible loess.
Physical parameters of loess
| Physical indicator | Value |
|---|---|
| Natural density (g cm−3) | 1.61 |
| Dry density (g cm−3) | 1.44 |
| Water content (%) | 11.32 |
| Saturated water content (%) | 40.39 |
| Specific gravity of soil particles | 2.71 |
| Void ratio | 0.84 |
| Plastic limit (%) | 20.1 |
| Liquid limit (%) | 30.1 |
| Coefficient of compressibility (MPa) | 0.43 |
| Saturation permeability coefficient (cm s−1) | 2.55 × 10−4 |
| Collapsibility coefficient | 0.078 |
2.2.1.2 Soil stabilizer
The selected stabilizer is a soil stabilizer of consolid system. It is an international general soil stabilizer with two parts, namely a liquid stabilizer (consolid 444, referred to as C444) and a powder stabilizer (consolid solidry, referred to as SD). C444, a stabilizer of the electric–ionic solution class, has semi-viscous organic chemistry that is slightly acidic (pH = 6) and consists of monomers and polymers mixed with a catalyst that accelerates penetration. Consolid solidry (powder) is a dry inorganic chemical that can close the capillary to prevent the treated ground from flooding. Generally, the two parts are mixed in a certain proportion when used for solidifying the soil.
2.2.2 Research methods
A preliminary experiment related to solidified soil planting grass was conducted. The physical and mechanical properties of loess are measured in Table 1 by undisturbed loess samples obtained on site. Because the matrix materials and seeds need to be mixed with water and sprayed on the loess slope, the remolding soil is used in planting experiments. The moisture content of the soil on site is 11.32% and the natural density is 1.61 g cm−3. The loess base in the planting box shall be treated according to the moisture content and the density. Loess was sampled from the project area and passed through a 2 mm diameter sieve to remove coarse particles such as shells and small stones. The soil stabilizer of consolid system was purchased from Gansu Ruisi Consolid Technology Engineering Co., Ltd, China. Ryegrass, which is widely used as a grass species for soil and water conversation in the local area, was selected as the study grass, and 50 seeds were planted in each box to facilitate the determination of its germination rate. Twelve groups of different mix ratios were selected for the preliminary experiment. The total amounts mixed with soil (mass ratio) were 0.46, 0.48, 0.86, 0.86, 1.26, 1.28, 1.66, 1.68, 2.06, 2.08, 2.46, and 2.48%, in which liquid content was 0.06 and 0.08%, the power content was 0.4, 0.8, 1.2, 1.6, 2.0, and 2.4%. In the preliminary experiment, two layers are set up, namely seed layer and loess base layer. The seed layer contains loess, soil stabilizer, and grass seeds. The mass of loess is 100 g and the water content is 17% in the seed layer. The content of stabilizers is determined according to the proportion. Mix the loess, stabilizers, and seeds and put them into the incubator. The incubator simulates 12 h of sunshine and 12 h of night. The temperature is maintained at 20°C and the humidity is maintained at 60%. Add 20 mL water to the incubator tray on the first day and 10 mL water to the tray every 3 days. In order to reduce the error of the experiment, each type of loess soil solidified by consolid system with each mixing ratio was repeated four times to carry out the preliminary experiment of planting grass screening, and the germination of ryegrass was recorded after 15 days.
An indoor planting test was then conducted. Combined with the experimental results of the preliminary experiment, the consolid stabilizer proportions of 0, 0.86, 1.26, and 1.66% were selected as the matrix material stabilizer in the indoor planting experiment.
In this experiment, the size of the planting box was 76 cm × 76 cm × 15 cm (Figure 4), in which grass seed weight is about 20 g m−2. At the beginning of the test, the bottom of the box is paved with 6 cm loess base, and then the soil stabilizer, and soil are mixed according to the above mixing ratio to form a 6 cm matrix layer. Finally, there is a 3 cm seed layer formed by mixing soil stabilizer, soil, and seeds. After completion, the mixture was covered with non-woven fabric to retain moisture and watered once a day until uniform germination of seeds. When the height of the grass reaches 2–3 cm, remove the non-woven fabric. Then move the planting box outdoors and take rainfall as supplement to simulate the on-site conditions. The number of plants was counted at 7, 15, 30, 45, 60, and 90 days, and the germination rate was calculated from these. At 7, 15, 30, 45, 60, and 90 days after sowing, the seedling height of ten plants was measured randomly and recorded. After the growth period, take the soil from 1/2 of the box and determine the number of roots in the soil, and the dry weight of the roots was measured after natural air drying.
Germination of ryegrass after 7 days.
A stress–strain test and permeability test of solidified soil–root composite were then conducted from the remaining soil in the box and the roots were not pulled out. The test was carried out according to the “Standard for Geotechnical Test Method” GB/T 50123-2019 [32]. The specimen was removed with a ring cutter with an inner diameter of 61.8 mm and a height of 20 mm. The soil collection method was carried out in accordance with article 3.1.4 of the “Standard for Geotechnical Test Method” GB/T 50123-2019 [32]. Four specimens were taken from each group, and their water content was measured after the soil was taken. Direct shear test was carried out for each group of specimens under the vertical pressure of 50, 100, 150, and 200 kPa.
At the end of the growth period, the organic matter and heavy metal elements of each ratio of the grass box were measured to indicate the engineering application effect of the stabilizer. Nutrients such as organic matter, N, P, and K in the upper mixed layer were determined. Heavy metal elements such as chromium, mercury, arsenic, lead, chromium, and copper were determined in matrix and base soil according to the requirements of “Soil Environmental Quality-Risk Control Standard for Soil Contamination of Agricultural Land” (Trial) GB 15618-2018 [33].
The schematic diagram of the process is shown in Figure 5.
Schematic diagram of the process.
3 Results and analysis
3.1 Preliminary experiment
The germination rate of ryegrass with different consolid stabilizer ratios is shown in Figure 6. All grass seeds germinated under different mixing ratios, and the germination rate of ryegrass was above 50%. The highest germination rate was 83% at a mixture ratio of 0.44%, and the lowest germination rate was 51% at a mixture ratio of 2.46%. With the increase in the stabilizer content, the germination rate decreased considerably. When the mixture ratio exceeded 1.24%, the germination rate decreased to about 60%. When the mixture ratio exceeded 2%, the germination rate decreased to about 50%.
Germination percentage of ryegrass with different ratios of stabilizer.
3.2 Planting test of loess soil solidified by consolid system planting grass
According to the results of the preliminary experiment and the research results in refs [29,31], considering the permeability and the strength properties of loess soil solidified by consolid system, 0, 0.86, 1.26, and 1.66% were selected as the mixing ratios for the planting test.
3.2.1 Grass growth curve in different periods
Figure 7 shows that the plant height of ryegrass in different periods can be divided into two growth stages. The first stage lasted about 60 days, during which the plant height increased rapidly in an approximately linear way. In the second stage, after 60 days, the growth of the plants slowed down. Compared with the plant height of the 0% mixing ratio, the plant height of the 0.86% mixing ratio exceeded the plant height after 45 days, the plant height of the other two mixture ratios decreased and the plant height of 1.26% mixing ratio reduced by 108.77%. This shows that a high dosage of stabilizer will affect the vegetation growth.
Plant height curve of grass planted in different periods.
Figure 8 shows that the change in plant number over time is basically the same as that of the plant height, and the curve of plant number can also be divided into two stages. The first stage lasted for about 30 days. During this stage, compared with the number of plants in the 0% mixing ratio, the number of plants in soils with 0.86, 1.26, and 1.66% mixing ratios all increased, and at 30 days, the number increased by 229.46, 391.51, and 259.82%, respectively. At the end of the growth period, the number of plants in soil with mixing ratios of 0.86 and 1.26% increased by 31.25 and 12.25%, respectively. The number of plants with the mixing ratio 1.66% combination began to decrease, with an overall decrease of 33.13% at the end of the growth period.
Number of plants planted in different periods.
Figure 9 shows the average root length and dry weight of vegetation under various mixing ratios after the end of the growing period. Compared with the vegetation root with 0% mixing ratio, the root lengths of 0.86, 1.26, and 1.66% mixing ratio increased by 19.11, 28.27, and 3.34%, respectively. Compared with the 0% mixing ratio, the root weight of 0.86, 1.26 increased by 89.97, 51.47, and the ratio of 1.66% decreased by 15.83%, respectively. When the stabilizer mixing ratio was 0.86%, the plant root development was better than that of other ratios, which is more conducive to improving plant strength.
Average root length and biomass curves of plants growing in soil with different mixing ratios of stabilizer.
3.2.2 Determination of nutrient elements and heavy metal elements in the matrix layer
The contents of organic matter, nitrogen, phosphorus, and potassium in matrix soil are shown in Table 2. The organic matter content of the upper layer, with mixing ratios of 0.86, 1.26, and 1.66%, increased by 22.9, 21.3, and 21.3%, respectively, compared with that of 0% solidified soil matrix layer. This indicated that the organic material properties of the consolid stabilizer increased the organic matter content of soil. Compared with the 0% matrix layer, the nitrogen content increased by 13.3, 15.6, and 31.1%, respectively. The phosphorus and potassium content did not change significantly.
Contents of organic matter, nitrogen, phosphorus, and potassium in the matrix layer of solidified soil
| Proportion of solidified soil (%) | Organic matter (%) | Nitrogen (%) | Phosphorus (%, as P2O5) | Potassium (%, as K2O) |
|---|---|---|---|---|
| 0 | 0.61 | 0.045 | 0.13 | 1.02 |
| 0.86 | 0.75 | 0.051 | 0.12 | 0.88 |
| 1.26 | 0.74 | 0.052 | 0.14 | 1.01 |
| 1.66 | 0.74 | 0.059 | 0.13 | 1.00 |
The contents of heavy metal elements in matrix materials and base materials are shown in Tables 3 and 4, respectively. The cadmium, mercury, and lead contents were very low. The arsenic, chromium, and copper contents did not exceed the standard requirements. In the base soil (Table 4), arsenic increased by 41.6, 35.3, 12.3, and 6.6% with respect to the matrix of solidified soil. Moreover, chromium increased by 20.4, 3.75, 1.05, and 0.42%, and copper by 58.07, 12.04, 10.24, and 7.14%. Since the planting box is placed outdoors, its water supply mainly comes from rainfall. Some of the heavy metals in the matrix layer flow out with the rainfall, and some infiltrate into the base layer with the rainfall. There was no significant difference in the content of heavy metals in the matrix layer. Some heavy metals in the matrix layer infiltrate into the base soil with water, resulting in an increase in the content of heavy metals in the base soil. As the stabilizer content increased, the content of heavy metals in the soil infiltrated into the underlying soil with surface water decreasing considerably, and the protection of solidified soil plants and grass improved for the deep soil.
Heavy metal content in the matrix of solidified soil
| Solidified soil proportion (%) | Cadmium (mg kg−1) | Mercury (mg kg−1) | Arsenic (mg kg−1) | Lead (mg kg−1) | Chromium (mg kg−1) | Copper (mg kg−1) |
|---|---|---|---|---|---|---|
| 0 | <0.001 | <0.001 | 15.2353 | <0.001 | 26.5331 | 21.4191 |
| 0.86 | <0.001 | <0.001 | 12.4833 | <0.001 | 22.9253 | 18.2034 |
| 1.26 | <0.001 | <0.001 | 14.2355 | <0.001 | 24.3598 | 17.1104 |
| 1.66 | <0.001 | <0.001 | 13.8277 | <0.001 | 26.2204 | 11.6891 |
Heavy metal contents of the solidified soil base
| Solidified soil proportion (%) | Cadmium (mg kg−1) | Mercury (mg kg−1) | Arsenic (mg kg−1) | Lead (mg kg−1) | Chromium (mg kg−1) | Copper (mg kg−1) |
|---|---|---|---|---|---|---|
| 0 | <0.001 | <0.001 | 21.5704 | <0.001 | 31.9687 | 33.8573 |
| 0.86 | <0.001 | <0.001 | 16.8909 | <0.001 | 23.7851 | 20.3969 |
| 1.26 | <0.001 | <0.001 | 15.9960 | <0.001 | 24.6155 | 18.8618 |
| 1.66 | <0.001 | <0.001 | 14.7504 | <0.001 | 27.3261 | 20.0425 |
3.3 Shear resistance and penetration characteristics
3.3.1 Shear characteristics
The shear strength of soil has a clear influence on the stability of a slope, which can reflect the degree of difficulty of shear failure of soil under dead weight or external load. The influencing factors of soil shear strength are mainly determined by the characteristics of friction strength, cohesive strength (soil particle size, particle mineral composition, shape, and gradation), and stress conditions (mechanical properties, size, and loading rate), and the friction strength and cohesive strength play a leading role in many influencing factors. In the root–soil composite, the soil fixation of plant roots should be considered. Through the consolidation fast shear test on the root–soil complex with different solidified soil plants and grass and remolding loess, the relationship curves of soil shear deformation and stress under different pressures of 50, 100, 150 and 200 kPa are shown in Figure 10(a–d). The variation trend of the stress–strain curve of planting grass was similar for all pressures, i.e., the slope of the curve in the earlier stage was relatively large, and the slope of the curve in the later stage was relatively small. This indicates that the soil in the earlier stage needs a large force to cause strain. With the increase of confining pressure, the curve shape of the samples with the same stabilizer content changed from the softening type to the hardening type, and the strength increased continuously. The stress–strain curve was nonlinear.
(a) Stress–strain relationship curve of solidified soil and grass soil when the vertical pressure is 50 kPa. (b) Stress–strain relationship curve of solidified soil and grass soil when the vertical pressure is 100 kPa. (c) Stress–strain relationship curve of solidified soil and grass soil when the vertical pressure is 150 kPa. (d) Stress–strain relationship curve of solidified soil and grass soil when the vertical pressure is 200 kPa.
With increase of the stabilizer content, the failure peak value of the specimen gradually increased, and the stress–strain of the plastic deformation stage was obtained. The curve was approximately a horizontal line, and the soil was in the stage of simple compaction deformation, and finally the soil was destroyed.
Figure 11 shows the relationship between vertical pressure and shear stress of different soils. There was a linear relationship between the vertical pressure and the shear stress of solidified soil. The correlation coefficients of 0, 0.86, 1.26, and 1.66% mixing ratios and remolding loess were 0.97, 0.98, 0.99, 0.94, and 0.99, respectively. The cohesion and internal friction angle of the solidified soil increased with the increase of the content of the solidified soil (Table 5). Compared with remolding loess, the cohesive strength of 0, 0.86, 1.26, and 1.66% mixing ratios increased by 58.19, 156.80, 176.19, and 268.13%, respectively, while the internal fraction angle increased by 42.65, 98.61, 129.73, and 143.15%, respectively. The analysis shows that the root is of positive significance to improve the cohesive force and internal friction angle of loess. The effect of solidified soil–root composite is more obvious than that of each other on the shear strength index. Therefore, the stability of shallow loess slope can be improved by solidified soil–root composite.
Relationship between vertical pressure and shear stress.
Indicators of shear strength of solidified soil
| Stabilizer content (%) | 0 | 0.86 | 1.26 | 1.66 | Remolding loess |
|---|---|---|---|---|---|
| Cohesion (kPa) | 35.34 | 57.37 | 61.70 | 82.24 | 22.34 |
| Internal friction angle (°) | 25.72 | 35.81 | 41.42 | 43.84 | 18.03 |
| Water content (%) | 18.02 | 19.24 | 19.30 | 19.57 | 18.02 |
3.3.2 Properties of permeability
In this part, the permeability coefficient of the soil solidified by different mixing ratios was measured. Table 1 shows that the saturation permeability coefficient of undisturbed loess is 2.55 × 10−4 cm s−1. With the increase in the stabilizer content, the permeability coefficient of loess decreased. The permeability coefficient decreases with the increasing stabilizer content. It is 3.82 × 10−5, 8.25 × 10−6, 4.65 × 10−6, and 1.59 × 10−6 cm s−1 for the stabilizer content of 0, 0.86, 1.26, and 1.66%, respectively. The permeability coefficient of the solidified soil with 0.86% was reduced by 274% compared to that of the 0% solidified soil, while the permeability coefficient of the solidified soil with 1.26 and 1.66% stabilizer was reduced by 337 and 494.7%, respectively. In the study of loess modification by consolid system [29], it was found that the permeability coefficient of loess soil solidified by consolid system can be reduced to 10−7 cm s−1 and the minimum content of soil stabilizer is 0.5%. On the ecological slope protection, due to the existence of roots, the permeability coefficient is about 10−6–10−5 cm s−1. Therefore, the permeability coefficient of solidified soil–root composite increases due to the existence of plant roots.
4 Engineering examples
The on-site slope protection test area is located in Xiaokongtong scenic area, which is for comprehensive disaster management of Xiaokongtong and an H5 slope (Figure 2). The slope is a soil slope with a slope cutting ratio of 1:0.75, slope height of 10 m, and slope length of 12 m.
4.1 Scheme design
The main purpose of this experiment was to study the ecological protection mode of the loess slope. For a stable slope, because water causes soil and water loss on the slope surface, engineering measures or biological measures can usually be adopted for protection. However, the single slope protection mode is not beneficial to the ecological restoration of the slope surface. The ecological mode (the combination of engineering measures and biological measures for slope protection) can be applied to realize the advantages of both engineering slope protection and biological slope protection, and enhance the stability and recovery of loess slopes.
Spray seeding technology is a new technology introduced from Japan, which is suitable for vegetation construction on barren soil and stone slopes. It uses special equipment to mix seeds, rock greening materials, solidified soil, water-retaining materials, aggregates, and stabilizers and spray them on the slope surface through pump and compressed air transportation, eventually forming a plant growth substrate.
In this study, the ecological solidified soil hanging net and spraying sowing approach to ecological slope protection was considered.
4.2 Ecological slope protection construction technology
The mud spraying equipment is needed with related manpower, mixing machinery, transport machinery, mechanical spraying, and construction operations on the slope. The process of ecological slope protection is as follows:
Slope inspection
Pumice and debris on the slope and the construction range should be cleaned up, and the scouring pit on the slope should be filled to ensure that the slope is smooth along the direction of the section.
Slope hanging net
Galvanized iron wire net is hanged on the slope. The diameter of the wire is 1.8 mm, and the mesh is 2.5 cm × 2.5 cm. The net is fixed by steel nail with diameter of 10 mm.
Configure solidified soil substrate materials
Substrate materials include soil stabilizer, humus, water-retaining dosage, and organic fertilizer. The soil stabilizer content is used in the matrix ratio which has been studied. The contents of humus, water-retaining dosage, and organic fertilizer are 3, 0.1, and 0.2%. Well-mixed mud is sprayed evenly on the slope surface from bottom to top with a spraying machine to a thickness of 7–10 cm.
Spraying seed layer and curing
The seeds, soil stabilizer, and loess are mixed with water to form slurry, in which grass seed weight is about 20 g m−2 and prepare for germination before spraying. The mixture was sprayed and attached above the matrix layer to form a seed layer of 2–3 cm. The sprayed slope is covered with non-woven fabric for maintenance.
4.3 Evaluation of slope vegetation restoration effect
In order to analyze the emergence of seedlings with different stabilizers, four quadrats in each slope in the test area were investigated and observed continuously for 45 days after the construction was completed. The seedling emergence shown in Figure 12 is that of all Gramineae and legumes. Compared to 0% mixing ratio of the stabilizer, the emergence of seedlings at 0.86% was better, and the emergence rate was 32.2% higher at 45 days.
Curve of seedling number of planted grass.
Figure 13 shows the slope body coverage. The vegetation coverage value of the slope with 0.86% solidified soil planting and grass in the early stage was slightly higher than that of the slope with 0% solidified soil planting and grass. Both exceeded 75% in 1 month. The survey results on the 45th day show that the vegetation coverage exceeded 90%, meeting the construction requirements. The plants were mainly grasses including perennial ryegrass, tall fescue, alfalfa, and some legumes.
Changes in coverage degree over the studied period.
The construction period of the test was early July. In order to better compare the slope protection effect of soil stabilizer of consolid systems, the effect was compared with that of the slope without stabilizers during the test. The field evaluation of the test slope was evaluated regularly after construction. The modified slope surface with soil stabilizer of consolid system had no obvious erosion, and the vegetation on the slope grew well after the rainy season in the area. However, the slope was not modified by the stabilizer and had obvious gully and the vegetation damage was more serious (Figure 14(a)). After 90 days, following hot summer and rain erosion, the modified slope (Figure 14(b)) was completely covered by vegetation and fully protected. From the above effect of slope protection, it can be concluded that the soil stabilizer of consolid system can improve the anti-erosion property of soil and has good ecological slope protection effects.
(a) Untreated loess slope. (b) Loess slope treated by ecological slope protection measures.
5 Conclusion
The purpose of this article was to study the stability of loess slope. Ecological slope protection of loess slope was carried out by using the soil stabilizer of consolid system. The ecological slope protection effect of soil stabilizer was verified by a preliminary experiment, a laboratory experiment, and an engineering application. The study offers the following conclusions: (1) Soil stabilizer of consolid system is an environment-friendly soil stabilizer and does not pollute the soil. (2) The soil stabilizer of consolid system can greatly improve the strength and anti-erosion ability of the soil because of its good mechanical properties and low permeability. Moreover, it can improve the nutrient content of the soil and promote the growth of vegetation. (3) The engineering application further verifies that the application of soil stabilizer of consolid system in the ecological protection of loess slopes is feasible and provides a new idea for the treatment of loess slopes.
This article proposes an environment-friendly method of strengthening and improving the stability of loess slopes. The soil material and soil stabilizer of consolid system are carried out under certain conditions. There are big differences in the properties of loess, and the properties of loess and the growth of grass are affected by conditions such as rainfall and temperature. In the future, more experiments are needed to promote the method in this article.
Glossary: Consolid system: is a two-product system that to be upgraded to achieve better characteristics necessary in improving soil loading capacity.
Acknowledgments
We thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript.
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Funding information: This research was financially supported by Qingyang Science and Technology Planning Project of Gansu Province (QY2021A-S057) and Key Research and Development Projects of Qingyang city (QY2021B-S004) and the Industrial support planning project of Gansu Provincial Department of Education (2022CYZC-65).
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Author contributions: Yongdong Yang and Shengrui Su designed the experiments. Hai Liu and Hongfei Li carried them out. Yongdong Yang prepared the manuscript with contributions from all co-authors. Wanfeng Liu participated in the proofreading of the manuscript. The authors applied the SDC approach for the sequence of authors.
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Conflict of interest: The authors declare that there is no conflict of interest regarding the publication of this article.
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Data availability statement: The data used to support the findings of this study are available upon request from the corresponding author.
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