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Article

Effects of Biochar on the Mechanical Properties of Bermuda-Grass-Vegetated Soil in China

by
Bo Wang
1,
Feng Wang
2,
Hongwei Liu
3 and
Hui Xu
4,*
1
South Zhejiang Comprehensive Engineering Institute Co., Ltd. of Investigation and Mapping, Hangzhou 310012, China
2
Nuclear Industry Southwest Geotechnical Investigation & Design Institute Co., Ltd., Chengdu 610000, China
3
Zijin School of Geology and Mining, Fuzhou University, Fuzhou 350025, China
4
School of Civil Engineering and Architecture, Zhejiang Sci-Tech University, Hangzhou 314423, China
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(17), 7596; https://doi.org/10.3390/su17177596
Submission received: 16 July 2025 / Revised: 18 August 2025 / Accepted: 20 August 2025 / Published: 22 August 2025
(This article belongs to the Section Environmental Sustainability and Applications)
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Versions Notes

Abstract

The effects of biochar on Bermuda grass growth and mechanical properties of vegetated soil were investigated in this study. Six groups of soil column tests were conducted, including two degrees of compaction (DOC) (70% and 90%) and two types of biochar content (5% and 10% by soil dry weight), with two groups of bare soil serving as a reference (soil used in the test was classified as silty sand with gravel, i.e., SM). It was found that biochar increased the effective cohesion by up to 70% and slightly enhanced the effective internal friction angle while mitigating the detrimental effects of wetting–drying cycles, with the effective cohesion and friction angle retaining up to 73% and 99% of their initial values, respectively. Root biomass initially increased and then decreased as biochar content increased, particularly at a low degree of compaction of soil (i.e., 70% DOC was two times that of 90% DOC). The effective cohesion of intact biochar–root–soil initially increased up to 23% (at the biochar content of 5%, 90% DOC) and then decreased as biochar content increased, regardless of DOC. At the optimal biochar content (5%), the effective cohesion and internal friction angle of rooted soil were 1.4 and 1.1 times greater at low DOC (70%). For the remolded biochar–root–soil composite, at a high degree of compaction (90% DOC), the effective cohesion increased with the increase in root and biochar content. For a given root content, the shear strength of the remolded biochar–root–soil mixture was higher than that of intact biochar–root–soil (i.e., the shear strength of intact soil at 5% of biochar content was 87% of remolded soil), suggesting that the remolded soil mixture overestimated the biochar–root–soil strength. Generally, the present study demonstrates that a 5% biochar addition is optimal for enhancing plant root growth and soil strength, particularly under low compaction. Biochar significantly improves the mechanical performance of root–soil composites and mitigates the degradation of soil strength under wetting–drying cycles.

1. Introduction

Biochar is an organic material produced through the pyrolysis of biomass under oxygen-limited conditions of plants or animals [1,2], characterized by high porosity, excellent adsorption capabilities, and environmental friendliness [3,4,5]. Consequently, biochar can be utilized to treat leachate, cover landfills, and construct permeable reactive barriers to reduce the environmental impact of landfills [6,7,8,9]. Meanwhile, biochar is widely applied as a soil amendment to enhance physicochemical properties. Previous studies have shown that biochar can enhance soil fertility, improve hydraulic conductivity, and increase water retention capacity, thereby contributing to both agricultural productivity and environmental sustainability [10,11]. Most studies have focused on the effects of biochar on soil physicochemical properties, indicating that biochar improves soil compression resistance and bearing capacity [12,13,14]. Additionally, biochar positively impacts soil shear strength by enhancing interparticle adhesion and increasing frictional resistance, reducing the risk of shear failure. Biochar has also been proven to enhance soil water retention, nutrient availability, soil structure, and aeration, thereby promoting plant growth, although some studies report limited effects on plant growth [15,16,17].
Soil compaction is critical in geotechnical engineering and significantly influences soil properties and plant growth. Increased compaction reduces soil porosity and impedes air and water flow, thereby limiting root water absorption [18]. High compaction restricts root expansion, confining roots to shallow soil layers and preventing access to nutrient-rich layers [19]. However, excessively loose soil can also harm plant growth by providing insufficient support and causing lodging or by having large pores that restrict root growth despite adequate water and nutrient availability [20,21]. Overall, current studies predominantly focus on the effects of biochar on plant growth and soil hydro-mechanical properties under a certain degree of soil compaction. However, agriculture and geotechnical engineering have different requirements for the degree of soil compaction, necessitating further exploration of biochar–root–soil composites under varying compaction levels. Understanding these effects is essential for optimizing landfill cover design and management. Additionally, the influence of biochar on plant growth under different compaction levels remains unclear. In addition, the vegetation selected for this study (Bermuda grass) has been widely used in soil ecological restoration and reinforcement due to its rapid growth, extensive root system, and characteristics such as high root tensile strength and wide adaptability. This research aims to fill these gaps and expand knowledge on biochar’s role in plant protection, slope protection, and landfill design in the world.
Overall, previous studies have not explored changes in vegetated soil induced by compaction (including soil shear strength and plant characteristics). Furthermore, there are significant differences between the agricultural and geotechnical engineering fields affected by compaction and biochar content, both in terms of plant characteristics and soil mechanical properties. In addition, this study would be beneficial not only for slope protection but also for landfill cover design. Therefore, the study seeks to achieve the following objectives through a series of systematic column tests and element tests: (i) investigating the effects of biochar content and degree of compaction on soil mechanical properties under wetting–drying cycles; (ii) exploring the effects of degree of compaction and biochar content on plant growth; and (iii) revealing the mechanical mechanisms influenced by biochar content and soil compaction.

2. Experimental Materials and Methods

2.1. Experimental Soil, Biochar, and Plants

The test soil was collected from Minhou County, Fuzhou City, Fujian Province (S 119°20′38″, N 26°7′46″). The soil was first air-dried, crushed, and sieved through a 2 mm mesh to remove larger particles and impurities. After air-drying, the soil was uniformly mixed with deionized water to achieve the optimal gravimetric water content (GWC) of 18.4%. Then, the moist soil was sieved through a 2 mm mesh and stored in an airtight container for at least 24 h. The index properties of the test soil are presented in Table 1.
The biochar was produced through high-temperature pyrolysis of coconut shells at 600 °C under oxygen-limited conditions, which has strong structural stability and porosity, making it suitable for environmental remediation and soil improvement [29]. It was sieved through a 2 mm mesh to remove impurities and then dried for use. The properties of the biochar are shown in Table 2, and the surface functional groups, depicted in Figure 1, are analyzed in the 400–4000 cm−1 wavenumber range using an FTIR spectrophotometer (FTIR, Nicolet IS50 FTIR Spectrophotometer, ThermoFisher, Waltham, MA, USA). Due to its rich oxygen functional groups, it is inferred that the biochar is a hydrophilic material [30].
Bermuda grass (Cynodon dactylon), a perennial herbaceous plant from the family and the Cynodon genus, was used in this test [31], which exhibited commendable drought resistance and waterlogging tolerance [32]. Before transplantation, a healthy and minimally disturbed patch of Bermuda grass (about 25 cm2) was selected to assess plant characteristics, including shoot length and green coverage. Shoot length was measured by a vernier caliper, while green coverage was assessed through image analysis (GreenCover, Version 1.0), in which the percentage of vegetation pixels in the potted sample images was quantified.

2.2. Experimental Setup

The experimental setup is shown in Figure 2. The column had an inner diameter of 300 mm and a height of 300 mm. A drainage hole, 10 mm in diameter, was positioned in the center of the column base. A 20 mm thick gravel layer was placed at the bottom, with a geotextile layer installed directly above the gravel to prevent soil migration.

2.3. Experimental Design and Text Procedures

Six sets of soil column tests were conducted to investigate the mechanical properties of biochar–root–soil composites. The degree of compaction (DOC, %) was quantified as the quotient of dry density and the maximum dry density determined by a laboratory test obtained from the standard procedure [25], expressed as the ratio of dry density to maximum dry density. These included two degrees of compaction (DOC = 70% and 90%) and three biochar addition levels (0%, 5%, and 10% by soil dry weight, w/w).
Before compacting the biochar–soil mixture, a thin layer of Vaseline was applied to the inner layer of each column. The soil columns were compacted in five layers, each 50 mm thick, to ensure uniformity. Bermuda grass was then transplanted into each column and watered daily (50 g) to promote growth. After 37 days, five wetting–drying cycles were applied over a total test period of 117 days. Rainfall intensity and duration are provided in Table 3.
Upon completion of the tests, intact rooted soil samples were collected at three depths (0–40 mm, 40–80 mm, and 80–120 mm), and direct shear tests were conducted to determine soil mechanical properties [34]. Then, soil samples were carefully washed, and the root content (Rc: ratio of root fresh weight to dry soil mass per unit volume, w/w) was measured using an electronic balance with an accuracy of 0.001 g (Shanghai Shunyu Hengping (Shanghai, China), JA5003J), following the method described by Freschet et al. [35]. The root volumetric ratio (Rv: a dimensionless parameter defined as the volume of plant fresh roots per unit of soil within a certain depth) of Bermuda grass was quantified using the water-displacement method based on Archimedes’ principle [36].
Additionally, remolded rooted soil samples were prepared at 70% and 90% degrees of compaction (DOCs) for subsequent analysis. In these samples, the biochar content (BC) was adjusted to 0%, 5%, and 10%, respectively, while the root content (the mass ratio of fresh roots and dry soil) ranged from 0.05% to 0.2%. Direct shear tests were conducted at a controlled shear rate of 0.02 mm/min (under the slow shear rate, full drainage of soil was ensured to directly determine the effective cohesion (c′) and internal friction angle (φ′), respectively) to assess the mechanical properties of the remolded soils (determined by the Mohr–Coulomb model).

3. Results and Discussion

3.1. Effects of Degree of Compaction and Biochar Content on Plant Characteristics

Figure 3a,b depict the changes in green coverage and shoot length of Bermuda grass during the test. The 70% DOC groups displayed higher green coverage compared to the 90% DOC groups, attributed to larger soil pores in the less compacted soil, which provided more oxygen and root growth space. Specifically, the 70% DOC-5% BC group exhibited a 43% increase in green coverage, while the 90% DOC group showed the lowest increase at 34%. After 103 days, shoot length in the 70% DOC group was also more significant than in the 90% DOC group. However, in the 70% DOC group and the 90% DOC group, adding biochar did not significantly affect green coverage and shoot length of Bermuda grass, indicating that at a high degree of compaction, the addition of biochar has limited effects on plant characteristics, and the effectiveness of biochar is less pronounced compared to soils with a lower degree of compaction.
Figure 3c shows that root content initially increased with biochar addition, peaking at 5% BC, regardless of DOC. At a depth of 60 mm, root content in the 70% DOC-5% BC and 90% DOC-5% BC groups was double that of the 70% DOC-0% BC group and the 90% DOC-0% BC group, respectively. Adding moderate amounts of biochar enhances soil structure and improves aeration and permeability, facilitating plant roots in acquiring water, nutrients, and oxygen [37]. Moreover, biochar contains essential plant nutrients and can serve as an additional source of nutrients for plants, improving the physical and chemical properties of soil such as increasing porosity, adjusting pH, and enhancing water retention [38]. It also contains a certain amount of inorganic minerals, such as potassium, calcium, magnesium, and phosphorus, which can be absorbed and utilized by plants under slow release [39]. However, biochar is alkaline (pH = 9.36), and when added to soil, water dissolves soluble organic and mineral compounds on biochar’s external and internal surfaces. These solutes increased the dissolved organic carbon and anion levels in the soil [40], consequently increasing soil pH (refer to Table 1). The optimal pH range for Bermuda grass growth is between 6.0 and 7.5 [41]. Adding 10% biochar increased the pH to 8.57 which would affect the growth and development of Bermuda grass, resulting in a decrease in root biomass. Combined with the distribution of root volume ratio (Rv, in Figure 3d), the effect of biochar on root content and volume was more pronounced at 70% DOC-5% BC.

3.2. Effects of Biochar on Mechanical Properties of Soil at Different Degrees of Compaction

As shown in Figure 4, the effective cohesion of the 70% DOC-5% BC and 70% DOC-10% BC groups increased by 27% and 54% compared to bare soil, respectively. For the 90% DOC-5% BC and 90% DOC-10% BC groups, cohesion increased by 0.2 to 0.7 times compared to bare soil, respectively. Test results indicate that increasing the biochar content enhances the effective cohesion of the soil. Moreover, the increment of effective cohesion of the biochar–soil mixture at 90% DOC was more extensive than that of 70% DOC. The main reason is that adding biochar changes the particle size distribution of the soil mixture [42,43]. Approximately 79% of biochar is less than 0.002 mm in diameter. It should be noted that the more biochar is added, the higher the content of fine particles in the soil mixture, which could lead to an increase in the effective cohesion of soil. Figure 4 further indicates that when DOC increases from 70% to 90%, the absolute increase in cohesion under the same biochar content shows an amplifying trend. This means that in a highly compacted skeleton, fine-grained biochar is more likely to fill pores and exert a binding effect, thereby explaining why the increase in cohesion is more significant under 90% DOC conditions.
Biochar also slightly increased the internal friction angle of soil, regardless of DOC (as shown in Figure 4b), likely due to the fact that woody-based biochar has more angular and sharp edges [44], which can enhance particle interlock and increase the soil’s internal friction angle.
After five wetting–drying cycles, both the cohesion and internal friction angle of the soil and biochar–soil mixture decreased. This phenomenon is attributed to the expansion and contraction of soil volume during the cycles, which alters the contact forms and arrangement patterns of soil particles [45]. After five wetting–drying cycles, cracks between the surface and interior of the soil progressively expanded, with these internal cracks unable to fully close during subsequent rainfall. Consequently, the effective cohesion of the soil decreases after the cycles. At 70% DOC, the effective cohesion of soil without biochar was decreased by 54%, while the effective cohesion of soils amended with 5% and 10% BC was decreased by 27% and 35%, respectively. At 90% DOC, the decrease in effective cohesion after the wetting–drying cycles was less pronounced across all groups than in the 70% DOC groups. These findings suggest that biochar mitigates the adverse effects of wetting–drying cycles on effective soil cohesion.
Under 70% DOC conditions, the effective internal friction angle of soil without biochar after wetting–drying cycles decreased to 93% of its initial value, while soils amended with 5% and 10% biochar retained 97% and 99% of their original values, respectively. At 90% DOC, the decrease in effective internal friction angle after wetting–drying cycles was more pronounced in biochar-amended and non-amended soils compared to the 70% DOC conditions. These results suggest that biochar reduces the weakening effects of wetting–drying cycles on the effective internal friction angle of soils. Wetting–drying cycles compromise soil structure by forming microcracks and inducing particle rearrangement, causing the soil to compact more tightly [46]. During the wetting–drying cycles, the soil undergoes irreversible compaction, where smaller biochar particles would get stuck in the pores of larger soil particles, leading to increased compaction. This enhanced compaction is associated with an increased internal friction angle [47].

3.3. Effects of Biochar on Intact Rooted-Soil Properties at Different Degrees of Compaction

At 90% DOC, the effective cohesion of the soil initially increased and then decreased with increasing addition of Biochar (Figure 5a). Specifically, the effective cohesion of the 90% DOC-5% BC group increased by 23% compared to soil without biochar. This increase is primarily attributed to the higher root content in the 5% BC group, which facilitated the formation of aggregates through root binding with soil particles, enhancing interparticle adhesion and cementation, thereby improving soil cohesion. Generally, plant root systems exhibit greater tensile strength than soil [48]. When the soil is subjected to shear forces, the shear stress is transmitted to the roots through interfacial friction between the roots and the soil. In addition, plant root systems enhance soil cohesion by secreting exudates [49]. The frictional interaction between roots and soil effectively combines the roots.
Tensile strength with the compressive strength of soil forms a root–soil composite that collectively resists stress-induced failure. Furthermore, biochar improves soil porosity and uniformity of pore distribution, adsorbing and stabilizing nutrients, which in turn promotes microbial proliferation and growth. Microbial activity enhances soil cohesion by secreting polysaccharides and extracellular polymeric substances [50]. However, the effective cohesion of the 10% BC group increased by only 7.1% compared to the soil without biochar, a smaller increase than observed in the 5% BC group. This is likely due to the greater root content in the 5% BC group compared to the 10% BC group (Figure 3c,d). The addition of biochar increased soil pH [51], leading to a higher concentration of hydroxide ions, which inhibited root growth and reduced root biomass, ultimately resulting in a decrease in the soil’s effective cohesion.
At 70% DOC, the increase in biochar content results in changes in the effective cohesion of intact rooted soil, following a similar trend to that observed at 90% DOC (Figure 5b). Under the optimal biochar content of 5%, the enhancement in shear strength of intact rooted soil is more pronounced at a lower degree of compaction. Specifically, in the case of 70% DOC-5%BC, the effective cohesion and effective internal friction angle of intact rooted soil were 1.4 and 1.1 times greater, respectively, compared to the intact rooted soil without biochar. Additionally, compared to the 0% BC group, the effective cohesion growth rate in the 5% BC group under 70% DOC was 20 percentage points higher than that under 90% DOC. However, at 70% DOC, the effective internal friction angle of intact rooted soil initially increased and then decreased as biochar content increased, differing from the pattern observed at 90% DOC. This indicates that the mechanisms influencing the effective internal friction angle in the biochar–root–soil system are complex at different DOC levels. Biochar amendment significantly enhanced the growth of the roots, which increased the effective internal friction angle and effective cohesion of the soil. Therefore, it can be concluded that biochar significantly improves the mechanical properties of planted soil under lower compaction conditions. Future studies could focus on the particle size of biochar, root distribution patterns, and soil organic matter content, which may play a critical role in the interaction between biochar and plant roots.

3.4. Effects of Biochar on the Mechanical Properties of Remolded Rooted Soils at Different Degrees of Compaction

Figure 6 illustrates the variation in the effective cohesion and effective friction angle of remolded soil with different root content under various DOCs. At 90% DOC, the effective cohesion of remolded soil increased with the increase in root content, which is consistent with the behavior in intact rooted soil. The increase in effective cohesion was particularly pronounced when root content increased from 0.15% to 0.2%. This can be attributed to the fact that at lower root content, the spacing between roots is greater, and their contribution to effective cohesion primarily stems from the one-dimensional tensile reinforcement provided by individual roots [52]. However, as root content increased, the roots extended in multiple directions, creating a more complicated three-dimensional network that connected with soil particles in various orientations, leading to a more substantial increase in effective cohesion [53].
At 70% DOC, the effective cohesion of remolded soil increased with rising root content until it reached 0.1% (Figure 6b). However, with a 5% biochar (BC) addition, once root content exceeded 0.1%, the rate of increase in effective cohesion slowed down. When the BC content was raised to 10%, effective cohesion began to decrease as root content surpassed 0.1%. Specifically, at a root content of 0.2%, the effective cohesion of soil with 10% BC was only 76% of that observed at 0.1% root content. In theory, as more roots are incorporated into the soil, root density along the shear surface increases, allowing roots to distribute shear stress more effectively. However, excessive root content may lead to root clumping [54], causing uneven root distribution in the soil and reducing the efficiency of root bonding within soil particles. Related study (Pradhan et al., 2012) [53] indicated that excessive root fibers can lead to uneven root distribution in the soil, diminishing the beneficial effects of roots on soil shear strength.
Figure 6a,b reveal that at 90% DOC, the effective internal friction angle is nearly constant under different root content. At 70% DOC, the effective internal friction angle decreased slightly as BC content increased. This may be due to roots displacing soil particles from their original positions, reducing the effective contact area between soil particles and, consequently, lowering the internal friction angle [55]. Overall, no clear pattern emerges regarding the impact of root addition on the effective internal friction angle, which is consistent with findings from other researchers [56].
Comparing intact rooted soil samples to remolded rooted soil samples, it was evident that under 90% DOC, the intact soil exhibits significantly lower shear strength than the remolded soil (Figure 7a). Specifically, at 0%, 5%, and 10% BC content, the shear strength of intact soil was 91%, 87%, and 83% of that of remolded soil, respectively. This finding indicates that remolded soil overestimates the strength of intact rooted soil, which has some implications for engineering design and soil stability assessments. The overestimation is because plant roots penetrate the soil and exert mechanical force on soil particles, leading to particle dispersion. At a high degree of compaction (90% DOC), this mechanical action causes soil particles to recombine, thereby increasing soil porosity. As the contact area between particles gradually decreases, the adhesive forces between particles also weaken [57], ultimately causing a decrease in shear strength of soil.
Figure 7b shows that the intact 70% DOC-0% BC group exhibits higher shear strength than remolded soil. However, as the biochar content increased, the shear strength of intact soil was lower than that of remolded soil. Without BC, the effective internal friction angle of intact soil is influenced by plant roots. Unlike remolded soil, plant roots in intact soil grow into the pores of soil, connecting soil particles. Additionally, small biochar particles adhere to the root surface [58], making the root system more tightly embedded in the soil. In addition, the biochar surface and its vicinity form a fine organic layer through a series of binding mechanisms, including the adsorption of cations, anions, nanoscale mineral particles, and organic compounds [59]. These surfaces contain numerous nanoscale pores that can tightly bind molecules [60] and adsorb organic compounds and adhesive substances released by roots, thus increasing the effective internal friction angle and enhancing the effective shear strength. However, after adding biochar, various factors can influence intact soil’s effective internal friction angle.
It is worth noting that at both 90% DOC and 70% DOC, the optimal BC content is 5%, at which the shear strength is maximized. An appropriate amount of biochar can supply plants with the necessary organic carbon, improve soil fertility, promote plant growth, and enhance the reinforcing effect of plant roots. In addition to root effects, microorganisms in the soil also play a crucial role in processes such as organic matter decomposition, nutrient transformation, and soil aggregation [61]. The remolded biochar–root–soil composites tend to overestimate the shear strength of intact soils, especially as BC content increases. Excessive biochar addition can raise soil pH, affect the diversity and activity of the microbial community, and alter the release pattern of microbial metabolites in intact rooted soil [62]. Some microbial metabolites may have adhesive and lubricating effects, reducing the contact and friction between soil particles and lowering soil shear strength. It should be noted that the specific mechanisms of these effects can be influenced by various factors, such as biochar characteristics, soil type, microbial community composition, and environmental conditions. Therefore, understanding the impact of biochar content on the effective shear strength of root–soil composites requires further tests to figure out the underlying mechanisms.

4. Conclusions

This study investigated biochar–root–soil composites by cultivating Bermuda grass in 300 mm high columns at 70% and 90% degrees of compaction and subjecting them to five wetting–drying cycles. Biochar made from coconut shells was added at 0%, 5%, or 10%, and mechanical properties were determined by direct shear tests on both intact and remolded soil samples. The principal findings are as follows:
(1)
At different DOCs, the root content of Bermuda grass initially increased and then decreased with an increase in biochar content. The optimal biochar content was found to be 5%. The effects of biochar on root content were more remarkable under lower compaction conditions.
(2)
Biochar enhances the effective cohesion of soil, with a more pronounced effect as the degree of compaction of soil increases. After five wetting–drying cycles, the effective cohesion of the soil decreases, while the effective internal friction angle increases. Biochar mitigates the detrimental effects of wetting–drying cycles on soil strength. At 70% DOC, the effective cohesion of soil without biochar was reduced by 54%, while the effective cohesion of soils with 5% and 10% BC was reduced by 27% and 35%, respectively.
(3)
As biochar content increases, the effective cohesion of intact rooted soil initially rises and then falls. At 70% DOC, the effective cohesion and internal friction angle of intact rooted soil were 40% and 10% higher than those of rooted soil without biochar, respectively. However, at 90% DOC, the effective internal friction angle of intact rooted soil remained relatively insensitive to changes in biochar content.
(4)
Some studies have found that remolding soil destroyed the in situ structure of the root and inflated shear strength [63]. However, in this study, the shear strength of intact rooted soil is lower than that of remolded rooted soil, indicating an overestimation of soil strength in the remolded state. In remolded rooted soil, roots primarily enhance soil strength by increasing effective cohesion. In intact rooted soil, shear strength is influenced by the interaction of biochar, roots, and microorganisms.

Author Contributions

B.W.: Investigation, Data curation, Writing—original draft, Writing—review and editing, Validation. F.W.: Investigation, Data curation, Formal analysis, Visualization, Writing—original draft. H.L.: Investigation, Methodology, Data curation, Writing—review and editing. H.X.: Conceptualization, Methodology, Writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (Grant Nos. 42175320, 42172309, and 52268053), the Key Laboratory of Geohazard Prevention of Hilly Mountains, Ministry of Natural Resources (Grant Nos. FJKLGH2024K004).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data will be made available on request.

Acknowledgments

The constructive comments from the anonymous reviewers are also greatly appreciated.

Conflicts of Interest

Author Bo Wang was employed by the company South Zhejiang Comprehensive Engineering Institute Co., Ltd. of Investigation and Mapping. Author Feng Wang was employed by the company Nuclear Industry Southwest Geotechnical Investigation & Design Institute Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Abbreviations

BCBiochar content (%)
c′Effective cohesion (kPa)
DOCDegree of compaction (%)
GsSpecific gravity of soil (dimensionless)
GWCGravimetric water content (%)
RcRoot content (%)
RvRoot volume ratio (dimensionless)
SMSilty sand with gravel
USCSUnified soil classification system
τIShear strength of intact soil (kPa)
τRShear strength of remolded soil (kPa)
φ′Effective internal friction angle (°)

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Figure 1. Biochar FT-IR spectrum.
Figure 1. Biochar FT-IR spectrum.
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Figure 2. Schematic diagram of Bermuda grass cultivation devices.
Figure 2. Schematic diagram of Bermuda grass cultivation devices.
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Figure 3. Bermuda grass characteristics under different biochar content and degrees of compaction: (a) green coverage; (b) shoot length; (c) root biomass; (d) root volume ratio.
Figure 3. Bermuda grass characteristics under different biochar content and degrees of compaction: (a) green coverage; (b) shoot length; (c) root biomass; (d) root volume ratio.
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Figure 4. The shear strength index of soil with different biochar content under different degrees of compaction before and after five wetting–drying cycles: (a) effective cohesion (c′); (b) effective internal friction angle (φ′).
Figure 4. The shear strength index of soil with different biochar content under different degrees of compaction before and after five wetting–drying cycles: (a) effective cohesion (c′); (b) effective internal friction angle (φ′).
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Figure 5. Influence of biochar content and different compaction states on shear strength of intact rooted soil after wetting–drying cycles: (a) 90% degree of compaction (DOC); (b) 70% degree of compaction (DOC) (Rc represents root content).
Figure 5. Influence of biochar content and different compaction states on shear strength of intact rooted soil after wetting–drying cycles: (a) 90% degree of compaction (DOC); (b) 70% degree of compaction (DOC) (Rc represents root content).
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Figure 6. Influence of biochar content on the shear strength indicators of remolded rooted soils under different compaction levels: (a) 90% degree of compaction (DOC); (b) 70% degree of compaction (DOC).
Figure 6. Influence of biochar content on the shear strength indicators of remolded rooted soils under different compaction levels: (a) 90% degree of compaction (DOC); (b) 70% degree of compaction (DOC).
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Figure 7. Influence of biochar content on shear strength of intact and remolded rooted soil under 100 kPa vertical pressure at different compaction levels: (a) 90% degree of compaction (DOC); (b) 70% degree of compaction (DOC).
Figure 7. Influence of biochar content on shear strength of intact and remolded rooted soil under 100 kPa vertical pressure at different compaction levels: (a) 90% degree of compaction (DOC); (b) 70% degree of compaction (DOC).
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Table 1. Index properties of test soil.
Table 1. Index properties of test soil.
Properties0% BC5% BC10% BCReferences
USCS soil classification *SM * ASTM D2487 [22]
Specific gravity2.632.572.50ASTM D854 [23]
pH7.878.148.57ASTM D4972 [24]
Optimum moisture content (%)18.419.820.6ASTM D698 [25]
Maximum dry density (g/cm3)1.751.671.61ASTM D698 [25]
Liquid limit (%)37.838.540.0ASTM D4318 [26]
Plastic limit (%)28.225.224.6ASTM D4318 [26]
Plasticity index (%)9.613.315.4ASTM D4318 [26]
Particle size distribution (%) ASTM D6913/D613M [27] ASTM D7928 [28]
Sand (>0.075 mm)53.7
Silt (0.075–0.002 mm)44.2
Clay (<0.002 mm)2.1
Note *: BC refers to biochar content; USCS soil classification refers to the Unified Soil Classification System that determines the type of soil; SM refers to silty sand with gravel.
Table 2. Basic parameters of biochar.
Table 2. Basic parameters of biochar.
Specific GravityAcidityParticle Size Distribution (%)
GS * pH >0.075 mm 0.075–0.002 mm <0.002 mm
1.719.368.0113.2778.72
Note *: Gs refers to the specific gravity of soil, which is the ratio of the mass of soil particles to the mass of distilled water at 4 °C of equal volume.
Table 3. Test conditions of drying–wetting cycles.
Table 3. Test conditions of drying–wetting cycles.
First SecondThirdFourthFifth
Rainfall duration (h)21121
Rainfall intensity (mm/h)5261815970
Rainfall return period *205205010
Note: * Reference to extreme rainfall in the Fuzhou area [33].
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Wang, B.; Wang, F.; Liu, H.; Xu, H. Effects of Biochar on the Mechanical Properties of Bermuda-Grass-Vegetated Soil in China. Sustainability 2025, 17, 7596. https://doi.org/10.3390/su17177596

AMA Style

Wang B, Wang F, Liu H, Xu H. Effects of Biochar on the Mechanical Properties of Bermuda-Grass-Vegetated Soil in China. Sustainability. 2025; 17(17):7596. https://doi.org/10.3390/su17177596

Chicago/Turabian Style

Wang, Bo, Feng Wang, Hongwei Liu, and Hui Xu. 2025. "Effects of Biochar on the Mechanical Properties of Bermuda-Grass-Vegetated Soil in China" Sustainability 17, no. 17: 7596. https://doi.org/10.3390/su17177596

APA Style

Wang, B., Wang, F., Liu, H., & Xu, H. (2025). Effects of Biochar on the Mechanical Properties of Bermuda-Grass-Vegetated Soil in China. Sustainability, 17(17), 7596. https://doi.org/10.3390/su17177596

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