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Article

Research on the Strength Characteristics of Red Soil Amended by Biochar

by
Ke Li
1,
Yu Xu
1,
Yang Tan
1,
Xianxiong Cai
1,
Zhikui Liu
1 and
Qinxue Xu
2,3,*
1
Guangxi Key Laboratory of Geomechanics and Geotechnical Engineering, Guilin University of Technology, Guilin 541004, China
2
Guangxi Key Laboratory of Environmental Pollution Control Theory and Technology, Guilin University of Technology, Guilin 541006, China
3
Collaborative Innovation Center for Water Pollution Control and Water Safety in Karst Area, Guilin University of Technology, Guilin 541006, China
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(3), 1174; https://doi.org/10.3390/su17031174
Submission received: 30 December 2024 / Revised: 23 January 2025 / Accepted: 30 January 2025 / Published: 31 January 2025
(This article belongs to the Topic Sustainable Building Materials)
Browse Figures
Versions Notes

Abstract

:
In order to investigate the effect of biochar on the strength characteristics of red loam, a number of experiments were carried out in the red loam of northern Guangxi Province, including a direct shear test, a scanning electron microscopy (SEM) test, and an X-ray diffraction (XRD) test of red loam, bagasse biochar–red loam, and rice platycodon biochar–red loam. The aim of this study was to determine the effects of different biochar contents, types, and particle size ranges on the shear strength of the improved soils. The results showed that both bagasse biochar and rice platycodon biochar could effectively improve the shear strength of soil, and the shear strength increased first and then decreased with the increase in biochar content and reached the peak value when the content of biochar reached 6%. Under vertical pressures of 100 kPa, 200 kPa, 300 kPa, and 400 kPa, the shear strength of the two improved soils increased by 53.39%, 52.52%, 43.43%, and 47.08%, respectively, and 54.56%, 23.89%, 33.71%, and 47.78%, respectively, compared with that of plain soil. In addition, the grain size was negatively correlated with the shear strength, and the shear strength of the sample increased with a decrease in the grain size, in which the biochar in the range of 0~0.5 mm had the best effect on the strength improvement of the red loam. The results of this study provide theoretical and technical support for revealing the evolutionary mechanism of red loam strength and coping with soil erosion in red soil areas.

1. Introduction

Red loam is mainly distributed in the southern part of China, with obvious regional and special characteristics. It has the characteristics of high natural water content, low density, swelling, and shrinkage. Under the action of long-term water erosion, the red loam is prone to soil erosion, which leads to the destruction of river bank slopes. Soil and water loss not only lead to a decline in soil fertility and affects agricultural production, but also cause river siltation, ecological environment deterioration, and other problems. The main reason is that the strength of the red loam is low under the action of erosion, so it is particularly important to study the strength characteristics of the red loam and its ability to resist erosion [1,2].
Shear strength is a key parameter in soil properties, which directly affects the stability, erosivity, and soil erosion degree of soil. An increase in shear strength means that the soil is more resistant to external shear forces, which has a significant impact on soil erosion [3]. Specifically, improved shear strength not only fortifies the soil against external shear forces and diminishes the risk of soil particles being dislodged by water flow, but also helps to preserve soil integrity, reduces topsoil loss, and prevents landslides triggered by rainfall or water flow. Moreover, enhanced shear strength often correlates with improved water retention capacity, further mitigating erosion factors such as water flow and infiltration, thereby reducing the risk of soil erosion [4,5]. In order to reduce soil erosion, a variety of measures have been taken in agricultural and engineering practices to enhance the shear strength of soil. These include improving soil structure by planting native plants and using cover crops; the use of soil amendments, such as lime and organic fertilizers, to improve soil cohesion and stability; and infrastructure such as terraces, reservoirs, and ditches to reduce the speed and flow of runoff, thereby reducing erosion and erosion of the soil [6,7,8]. In summary, increasing the shear strength of soil has a significant positive impact on reducing soil erosion at all levels. By enhancing the shear strength, it can not only improve soil stability and reduce erosion, but also improve water retention ability, thus playing an important role in agricultural production and ecological protection [9]. Therefore, for areas facing soil erosion problems, improving soil to improve soil shear strength is an effective way to achieve sustainable development and ecological balance.
Biochar is a solid carbonoid material generated by the pyrolysis of organic matter in an anoxic environment, with a high specific surface area and complex pore structure, enabling it to effectively adsorb water, and these physicochemical properties are affected by the type of raw material, pyrolysis temperature, and pyrolysis time [10]. Because of these properties, biochar has received a lot of attention in fields such as soil improvement and environmental remediation, especially in agriculture, where its ability to improve soil is highly valued. Biochar can not only increase soil porosity, improve aeration, and water retention, but also enhance soil stability and promote plant growth [11]. Moreover, biochar has a good cation exchange ability, which can effectively improve soil fertility and promote the absorption of nutrients by plants. After adding biochar, nutrients such as nitrogen, phosphorus, and potassium in the soil can be better fixed and slowly released, reducing the use of fertilizer and reducing agricultural production costs [12]. With the emphasis on environmental protection and resource conservation, researchers have explored a variety of new materials and technologies to reduce the environmental impact of construction and infrastructure projects [13]. Studies have shown that biochar and biocomposites made from agricultural waste (such as rice husks, bagasse, etc.) exhibit good mechanical properties in geoengineering applications [14]. At present, commonly used biomass raw materials include kitchen waste, rice straw, rice husk, and bagasse. As a sustainable soil amendment, biochar has a significant positive effect on improving soil strength. By improving soil structure and water properties, as well as promoting soil microbial activity, biochar offers new ways to enhance soil mechanical properties. These properties make biochar promising for a wide range of applications in both agriculture and engineering [15,16,17].
In the field of soil improvement, biochar, as an important additive, has attracted widespread attention. Many studies have shown that the incorporation of biochar can optimize soil structure, improve moisture retention, and enhance tensile strength. When the biochar incorporation exceeds 2%, the cohesion strength of the soil will decrease, while the internal friction angle will increase [18]. In addition, there is a positive correlation between biochar addition and soil strength, but a negative correlation with initial water content. The addition of biochar effectively fills the pores in the clay, improves the compactness of the soil, and thus enhances the strength of the soil [19]. Further studies have shown that the effects of biochar on the water retention capacity, strength, and aggregate stability of different soils are significantly affected by the amount of biochar added and the soil characteristics [20]. For example, in paddy soil, with the increase in biochar content, the shear strength gradually increases, while the permeability coefficient significantly decreases [21]. In the study of fine particle size, it was found that when the particle size of biochar was less than 0.15 mm and the content was 1%, the tensile strength of the soil was significantly improved [22]. At the same time, the difference in biochar particle size will also affect the physical and chemical properties of soil. A small biochar particle size can fill the soil pores more effectively, thereby reducing the total porosity of the soil and increasing the bulk density. Such changes usually help to improve the strength and shear resistance of soils, thus improving the overall stability of soils [23,24]. Therefore, it is very necessary to study particle size in exploring the influence of biochar on soil strength.
At present, the research on the effect of biochar on soil strength mainly focuses on dry density and water content, and the influence of particle size has not been fully considered. In order to investigate the effect of different biochar contents, types, and particle sizes on the strength of red loam soil, the shear strength of plain soil, bagasse biochar-improved soil, and rice platycodon biochar-improved soil were studied by a direct shear test, and the microstructure of the soils were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). By exploring the influence of biochar on the strength characteristics of red loam soil, we can provide new data and theoretical support for soil improvement technologies. The introduction of biochar can effectively reduce soil and water loss, protect and improve the ecological environment, and, especially in the southern red soil region, promote sustainable development, soil health, and sustainable use. It will help to reveal the mechanism of the evolution of red loam soil strength and to provide theoretical and technical support for coping with soil erosion and soil erosion in the southern red soil region.

2. Materials and Methods

2.1. Experimental Materials

The experimental soil was taken from the red loam of Yanguan Town, Xing’an County, Guilin City, Guangxi Zhuang Autonomous Region, and the geographical coordinates were roughly 24°55′ N 110°47′ E. The climate of Xing’an County is of the subtropical monsoon type. The average annual temperature is about 20 °C, and the annual precipitation is between 1600 mm and 2200 mm. The seasonal distribution of rainfall and heat is consistent. Xing’an County is located in the karst landform area, where the surface features are significant and the overall terrain is mainly mountainous and hilly. The soil samples were collected in layers by the S-shaped sampling method with a sampling depth of 0~20 cm. The impurities were removed from the soil samples, and the soil samples were air-dried for 3~5 days and then screened by 2 mm for use. The soil samples were tested by compaction, specific gravity, particle size distribution, and free expansion rate, and the index characteristics of the red loam were obtained. The characteristics are shown in Table 1, and the soil particle grading curve is shown in Figure 1.
The two types of biochar used in the experiment were bagasse biochar and rice platycodon biochar. The initial water content of the biochar was 9.78%. The biochar was naturally air dried, dried in a 105 °C oven for 12 h, and then crushed. Subsequently, the biochar was pyrolyzed in a box muffle furnace at 300 °C for 2 h under oxygen-limited conditions, and air dried for 48 h. Finally, the biochar was sifted by 2 mm, 1 mm, and 0.5 mm screens to obtain three different particle sizes of biochar particles, which were then added to the soil for testing. The biochar preparation process is shown in Figure 2 and the physicochemical indexes of biochar are shown in Table 2.

2.2. Experimental Methods

2.2.1. Sample Preparation

In order to explore the influence of the strength and microscopic properties of the red loam mixed with biochar, bagasse biochar (ZZC) and rice platycodon biochar (SDC) with three particle sizes (0~0.5 mm, 0.5~1 mm, 1~2 mm) were added to the red loam according to the mass ratio of 0%, 2%, 4%, 6%, and 8% (the percentage of biochar mass to dry soil mass). The simple soil (CK), bagasse biochar improved soil (ZZT), and rice platycodon biochar improved soil (SDT) were designed, respectively.
The red loam was naturally air dried and crushed, and then placed in a sealed plastic bucket after 2 mm screening. Then, the biochar was dispersed evenly into the dry soil according to different ratios, and a certain amount of distilled water was added. After fully mixing, an appropriate amount of distilled water was sprayed onto each mixture until all the distilled water was sprayed. The soil sample was configured according to the optimal moisture content of 22%. After the soil sample was made, the material was blanked in a sealed environment for 48 h to ensure the uniform distribution of soil moisture.

2.2.2. Direct Shear Test Protocol

In this study, a conventional direct shear test was used to explore the effects of different biochar contents, biochar types, and particle size ranges on the shear strength of biochar red loam mixed soil. The optimum moisture content of 22% was selected to prepare the soil samples, and the dry density of all the soil samples was 1.85 g/cm3. In the test, the soil samples prepared under the same carbon content, water content, and dry density were installed on the ZJ strain-controlled quadruple direct shear instrument. According to the “Standard for Testing Methods of Engineering Geotechnical Works” (GB/T50123-2019) [25], vertical pressures (σn) of 100 kPa, 200 kPa, 300 kPa, and 400 kPa were applied, respectively, to perform fast shear tests. The shear rate was set to 0.8 mm/min, and the stop displacement of the test was 6 mm.

2.2.3. Microscopic Experiment Scheme

In order to study the effect of biochar on the microscopic characteristics of soil, scanning electron microscopy (SEM) was used to conduct experiments. First, the dry sample was cut, coated with conductive adhesive, and sprayed with gold to prepare the SEM sample. Then, the samples were put into the SEM scanning chamber, and the soil samples with different carbon contents were analyzed by SEM using a Zeiss Sigma 360 field emission scanning electron microscope (Jena, Germany), and the mechanism of biochar improving the strength of red loam soil was qualitatively discussed from the microscopic level. In addition, X-ray diffraction (XRD) analysis was also performed in this study. First, the different samples were cut and ground and processed through a 0.075 mm screen, followed by a tablet preparation process, and finally tested using an X’pert-Pro X-ray diffractometer. In the X-ray diffraction experiment, the incident radiation was Cu Kα (1.5406 A), the acceleration voltage was 40 kV, the scanning speed was 0.1°/min, the analysis interval was 0.01°, and the measuring angle range was limited to 5°~80° (2θ). Finally, the MDI Jade 9 software and PDF-4+2009 powder diffraction database were used to qualitatively analyze the mineral composition of soil samples.

3. Results

3.1. Influence of Bagasse Biochar on Strength

Figure 3 shows the shear stress and shear displacement curves of bagasse biochar-improved soil with different dosages under the same vertical pressure. It can be seen from the figure that in the initial stage of shear (0–1 mm), the shear stress increases very fast, and the deformation of the sample is of the strain hardening type. When the shear displacement reaches about 1.5 mm, both the plain soil and the improved soil reach the yield point, and the peak value of the shear stress appears significant. This is because the addition of biochar fills the pores in the red loam, increases the compactness of the soil, and thus enhances the strength of the soil. In the late shear period (2–6 mm), the shear stress decreases significantly with the continuous increase in shear displacement, and the deformation mode of the sample changes from strain hardening to strain softening. This change indicates that the strength of the soil is gradually weakened with the destruction of the soil and the development of the strain, showing that the resistance to the shear force is reduced.
Figure 4 shows the relationship between the shear strength and vertical pressure of bagasse biochar-improved soil under different dosages of bagasse biochar. It can be seen from the figure that the shear strength of the modified soil is significantly higher than that of the plain soil under vertical pressures of 100 kPa, 200 kPa, 300 kPa, and 400 kPa. Under 100 kPa vertical pressure, the shear strength of the modified soil with four kinds of additives increased by 25.07%, 48.36%, 53.39%, and 51.77%, respectively. This indicates that the addition of bagasse biochar can effectively improve the shear strength of red loam soil.
Through the comparison of the four kinds of additives, it is obvious that the shear strength of the sample increases first and then decreases with the increase in the amount added, and reaches the peak value at 6%. Under the vertical pressure of 100 kPa, 200 kPa, 300 kPa, and 400 kPa, the shear strength of composite soil with a 6% content is increased by 53.39%, 52.52%, 43.43%, and 47.08%, respectively.
Figure 5a shows the relationship between shear strength and vertical pressure under different bagasse biochar contents. It can be seen from Figure 5a that the cohesion of the red loam soil changed significantly after the addition of bagasse biochar. With the increase in biochar content, the cohesion first increased and then decreased, and reached a peak value when the biochar content reached 6%. This phenomenon indicates that an appropriate amount of biochar can effectively enhance the cohesiveness of soil, but after a certain amount of biochar, the cohesiveness may decrease due to the weakening of the dispersion and interaction of biochar particles. Figure 5b also shows that the internal friction angle first increases and then decreases with the increase in the content of bagasse biochar, which is consistent with the change in the shear strength of red loam with the content of biochar in Figure 4 and Figure 5. The results showed that the appropriate amount of biochar not only increased the cohesion of soil, but also enhanced the sliding resistance of soil, and thus improved the overall shear strength of soil.

3.2. Influence of Biochar Type on Strength

Figure 6 shows the relationship between the shear strength and vertical pressure of rice platycodon biochar-improved soil under different contents of rice platycodon biochar. The results showed that the shear strength of the improved soil was significantly higher than that of the plain soil under vertical pressures of 100 kPa, 200 kPa, 300 kPa, and 400 kPa. Among them, the improvement effect of 6% biochar content is the most significant, and the shear strength of the soil sample is increased by 54.56%, 23.89%, 33.7%, and 47.78%, respectively.
Figure 7 shows the relationship between the shear strength and vertical pressure of plain soil, bagasse biochar-improved soil, and rice platycodon biochar-improved soil at 6% content. It can be clearly seen that the strength of the two improved soils is higher than that of the plain soil. Under vertical pressures of 100 kPa, 200 kPa, 300 kPa, and 400 kPa, the shear strength of the bagasse biochar-improved soil was higher than that of rice platycodon biochar-improved soil and higher than that of plain soil. From the data comparison, the shear strength of bagasse biochar-improved soil increased by 53.39%, 52.52%, 43.43%, and 47.08%, respectively. The shear strength of rice platycodon biochar-improved soil was increased by 54.56%, 23.89%, 33.71%, and 47.78%, respectively. Both bagasse biochar and rice platycodon biochar can enhance the shear strength of red loam soil, but the improvement effect of bagasse biochar is slightly better than that of rice platycodon biochar.

3.3. Influence of Biochar Particle Size on Strength

Figure 8 shows the relationship between the shear strength and vertical pressure of bagasse biochar-improved soil and rice platycodon biochar-improved soil when 6% of different particle sizes of bagasse biochar and rice platycodon biochar were added. It can be seen from the figure that different particle sizes of biochar have different effects on the shear strength of red loam soil under vertical pressures of 100 kPa, 200 kPa, 300 kPa, and 400 kPa. The shear strength of the improved soil in the range of (0~0.5 mm) is higher than that in the range of (0.5~1 mm), the shear strength of the improved soil in the range of (0.5~1 mm) is higher than that in the range of (1~2 mm), and the shear strength of the plain soil is the lowest. This indicates that there is a negative correlation between the biochar particle size and the soil strength, and the shear strength of the sample increases with the decrease in the biochar particle size.

4. Discussion

4.1. Mechanism of Action of Biochar to Improve Red Loam Strength

Figure 9 shows the scanning electron microscope (SEM) images of red loam samples with different biochar additions at 22% water content, and a magnification of 500 times and 2000 times. It can be seen that there are obvious differences in pore structure between the plain soil and the soil samples with biochar added. As shown in Figure 9a, the soil particles in the plain soil are stacked with each other, resulting in a loose structure, low shear strength, and many obvious large pores can be seen. As shown in Figure 9b, when the content of biochar is 2%, a small number of biochar particles begin to appear in the soil sample. These particles fill the pores, enhance the structural stability of the soil, and thus improve the shear strength [26]. As shown in Figure 9c, with the increase in biochar content to 6%, the number of carbon particles further increases, resulting in a significant reduction in the number of macropores. As shown in Figure 9d, when the content of biochar reaches 8%, several obvious macropores begin to appear in the figure, indicating that excessive biochar may inhibit the formation of small pores and thus reduce the shear strength of soil. The addition of biochar can significantly improve the pore structure of red loam soil. The interaction between carbon particles and soil particles not only filled the large pores and enhanced the compactness of the soil, but also resulted in the decrease in the number of large pores, the increase in the number of small pores, and the decrease in the pore size, thus improving the shear strength of the soil. However, when the amount of biochar is too large, the formation of small pores may be inhibited, and the strength of the soil mass may be reduced [27,28]. In general, proper biochar addition can effectively improve the pore structure and shear strength of red loam soil, while excessive biochar addition may have negative effects.

4.2. Influencing Mechanism of Particle Size on Strength

There are significant differences in the specific surface area and pore characteristics of biochar with different particle sizes. Biochar with a smaller particle size usually has a larger specific surface area and can stably exist in deeper soil layers or larger soil aggregates, providing necessary structural support [29,30]. This property effectively improves the water holding capacity and nutrient retention capacity of the soil, promotes the activity of microorganisms and the development of plant roots, and thus enhances the mechanical properties of the soil. These modifications improve the adhesion between particles, and small biochar particles can fill the tiny pores in the soil, improve the shear strength of the soil, and enhance its ability to resist shear forces. In addition, the smaller particle size of biochar can increase the number of small- and medium-sized pores in the soil, help the soil to retain more water, increase the hydraulic conductivity of the soil, and reduce the compressibility of the soil. However, larger biochar particles may cause changes in pore structure and reduce the formation of small pores, resulting in a decrease in strength [31,32,33].

4.3. Mechanism of Influence of Biochar Type on Strength

Both bagasse biochar and rice platycodon biochar can enhance the shear strength of red loam soil, but bagasse biochar has a slightly better improvement effect than rice platycodon biochar. This is because different types of biochar have differences in specific surface area, pore structure, chemical functional groups, and cation exchange capacity [34]. Bagasse biochar usually contains higher carbon elements and lower ash content, so it may have better cation exchange capacity and be more suitable for soil improvement [35,36]. The surface structure and porosity of biochar will directly affect its ability to improve strength. In general, biochar with fewer pores (denser) enhances the shear strength of soil more, while biochar with higher porosity may result in reduced strength due to compression [37]. Bagasse biochar has a higher specific surface area and porosity, which helps to improve its stability and water retention in soil, while rice platycodon biochar is relatively less stable in terms of physical properties, especially in terms of compressive strength and water holding capacity. In addition, physical properties such as the density and hardness of biochar can also affect its strength. Higher density biochar is generally more stable under stress and can form better intergranular connections with soil, thereby strengthening the overall structure of the soil and improving its shear strength. In general, biochar with higher density is more stable under stress, and can bind to soil particles more effectively, improving the overall strength of soil [38,39].

4.4. X-Ray Diffraction Test Analysis

Figure 10 shows the X-ray diffraction (XRD) patterns of the plain soil, the bagasse biochar-improved soil with the best content, and the rice platycodon biochar-improved soil. It can be seen from Figure 10 that the peak positions of the diffraction patterns of the three soils are roughly the same, but compared with the plain soil, new diffraction peaks appear in the bagasse biochar-improved soil and the rice platycodon biochar-improved soil. This indicates that after the addition of bagasse biochar and rice platycodon biochar to the red loam soil, a new mineral composition was formed in the soil. It can be seen from the figure that CaCO3 appeared in both soils after the biochar was added to the red loam. This is due to the lime potential of biochar, which contains a lot of carbonates, organic anions, inorganic bases, and other components [40]. When biochar is added to soil, it reacts with the free water in the soil and then combines with CO2 to form CaCO3, which fills the pores between soil particles and enhances the cohesion of the soil. These newly formed mineral components directly or indirectly increase the strength of the soil and improve its properties.

5. Conclusions

In this experiment, bagasse biochar and rice platycodon biochar were used to improve the red loam in northern Guangxi. The shear strength of biochar and red loam mixed soils with different biochar contents, biochar types, and particle size ranges was determined, and microscopic analysis was carried out in combination with SEM and XRD tests. The main conclusions are as follows:
(1)
Adding bagasse biochar and rice platycodon biochar to red loam can effectively improve the shear strength of the soil, but the improvement effect of bagasse biochar is better than that of rice platycodon biochar. The shear strength of red loam soil first increased and then decreased with the increase in the blending amount, and the optimal blending amount was 6% for both. Under different vertical pressure conditions (100 kPa, 200 kPa, 300 kPa, and 400 kPa), the shear strength of the two improved soils was increased by 53.39%, 52.52%, 43.43%, and 47.08%, respectively, and 54.56%, 23.89%, 33.71%, and 47.78%, respectively, compared with that of plain soil.
(2)
The particle size of biochar is negatively correlated with the strength of soil, and the shear strength of the sample increases with the decrease in particle size. Among them, the biochar with a particle size in the range of 0~0.5 mm has the best effect on the strength improvement of red loam soil. The shear strength of bagasse biochar-improved soil increased by 90.72%, 64.63%, 68.95%, and 62.64% compared with plain soil when the diameter of biochar was (0.5~1 mm). Compared with plain soil, the improved soil with rice platycodon grandiflorum increased by 54.56%, 23.89%, 33.71%, and 47.78%.
(3)
The addition of biochar changes the soil microstructure. Biochar particles fill the large pores in the red loam, resulting in a decrease in the number of large pores, an increase in the number of small pores, and a decrease in the pore size, thereby improving the compactness and shear strength of the soil.
(4)
The study of the microscopic pore structure in this paper was conducted only through SEM for local pore analysis, so the overall pore changes can be deeply and comprehensively analyzed in the future. In addition, this paper only discussed the influence of two single biochar improvements on the strength characteristics of red loam soil, and the improvement effect of composite biochar on the strength of red loam soil can be studied in the future.

Author Contributions

K.L.: conceptualization, methodology, software, validation, writing—review and editing, project administration. Y.X.: formal analysis, investigation, resources, writing—original draft. Y.T.: investigation, data curation, writing—original draft. X.C.: methodology, investigation, resources, data curation, visualization. Z.L.: methodology, validation, writing—review and editing. Q.X.: investigation, writing—review and editing, project administration. All authors have read and agreed to the published version of the manuscript.

Funding

This project has been funded by the Guangxi Key R&D Program with grant numbers Guike-AB22035075.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Particle grading curve.
Figure 1. Particle grading curve.
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Figure 2. The preparation process of biochar.
Figure 2. The preparation process of biochar.
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Figure 3. The relationship between shear stress and shear displacement of composite soil under different bagasse biochar contents. (a) 100 kPa; (b) 200 kPa; (c) 300 kPa; and (d) 400 kPa.
Figure 3. The relationship between shear stress and shear displacement of composite soil under different bagasse biochar contents. (a) 100 kPa; (b) 200 kPa; (c) 300 kPa; and (d) 400 kPa.
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Figure 4. The relationship between shear strength and vertical pressure under different bagasse biochar contents.
Figure 4. The relationship between shear strength and vertical pressure under different bagasse biochar contents.
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Figure 5. The relationship between the shear strength index and biochar content. (a) Peak stress and (b) internal friction angle.
Figure 5. The relationship between the shear strength index and biochar content. (a) Peak stress and (b) internal friction angle.
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Figure 6. The relationship between the shear strength and vertical pressure of composite soil under different rice platycodon biochar contents.
Figure 6. The relationship between the shear strength and vertical pressure of composite soil under different rice platycodon biochar contents.
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Figure 7. The relationship between shear strength and vertical pressure of different soils.
Figure 7. The relationship between shear strength and vertical pressure of different soils.
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Figure 8. The relationship between the shearing strength and vertical pressure of biochar with different particle sizes. (a) Bagasse biochar and (b) rice platycodon biochar.
Figure 8. The relationship between the shearing strength and vertical pressure of biochar with different particle sizes. (a) Bagasse biochar and (b) rice platycodon biochar.
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Figure 9. SEM images of soil samples. (a) 0%; (b) 2%; (c) 6%; and (d) 8%.
Figure 9. SEM images of soil samples. (a) 0%; (b) 2%; (c) 6%; and (d) 8%.
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Figure 10. X-ray diffraction patterns.
Figure 10. X-ray diffraction patterns.
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Table 1. Physical properties of red loam.
Table 1. Physical properties of red loam.
Air Drying Moisture Content/%Plastic Limit/%Liquid Limit/%Plasticity IndexOptimal Moisture Content/%Maximum Dry Density/(g·cm−3)Specific GravityFree Expansion Rate/%
3.162544.119.1221.852.5845
Table 2. Physical and chemical properties of biochar.
Table 2. Physical and chemical properties of biochar.
SamplesDensity
ρ/(g·cm−3)		Specific Surface Area (m2·g−1)Relative Particle
Density		Ash Content
%		pH
Bagasse biochar0.5637.060.7315.787.36
Rice platycodon biochar0.4532.040.6515.644.20
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Li, K.; Xu, Y.; Tan, Y.; Cai, X.; Liu, Z.; Xu, Q. Research on the Strength Characteristics of Red Soil Amended by Biochar. Sustainability 2025, 17, 1174. https://doi.org/10.3390/su17031174

AMA Style

Li K, Xu Y, Tan Y, Cai X, Liu Z, Xu Q. Research on the Strength Characteristics of Red Soil Amended by Biochar. Sustainability. 2025; 17(3):1174. https://doi.org/10.3390/su17031174

Chicago/Turabian Style

Li, Ke, Yu Xu, Yang Tan, Xianxiong Cai, Zhikui Liu, and Qinxue Xu. 2025. "Research on the Strength Characteristics of Red Soil Amended by Biochar" Sustainability 17, no. 3: 1174. https://doi.org/10.3390/su17031174

APA Style

Li, K., Xu, Y., Tan, Y., Cai, X., Liu, Z., & Xu, Q. (2025). Research on the Strength Characteristics of Red Soil Amended by Biochar. Sustainability, 17(3), 1174. https://doi.org/10.3390/su17031174

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