Orthogonal Experimental Study on Crack Resistance of Biochar and Sisal Fiber Composite Improved Expansive Soil

. Desiccation shrinkage is a key factor contributing to the engineering hazards in expansive soil. To inhibit the initiation and expansion of cracks, a composite improvement approach using biochar and sisal fibers was applied to expansive soil from Nanning, Guangxi. An orthogonal test method was employed to conduct desiccation cracking tests on the improved soil, investigating the effects of biochar and sisal fibers on crack length, crack ratio and crack width. The results indicate that different proportions of sisal fibers and biochar can effectively inhibit soil crack development. The biochar content is the primary factor affecting crack length, while sisal fibers mainly inhibit the development of crack ratio and average crack width. The optimal proportion for composite improvement approach was found to be 6‰ of 3 cm sisal fibers and 4% biochar. The study findings can provide a new approach for the expansive soil improvement and for controlling engineering problems caused by soil cracking.


Introduction
Expansive soil is widely distributed in more than 20 provinces in China. It is a type of special soil composed of highly hydrophilic clay minerals such as montmorillonite, illite, and kaolinite. Its significant shrinkage by losing water leads to severe soil cracking, resulting in various engineering problems such as foundation subsidence, slope instability and building structure craze. Therefore, the development of economically effective management measures to inhibit cracking in expansive soil is necessary for controlling and mitigating engineering hazards in expansive soil areas.
Currently, numerous researchers from both domestic and international institutions have devoted efforts to the study of expansive soil improvement and have achieved certain research outcomes. Common improvement materials include gravel [1] , fibers [2] , weathered sand [3] , lime [4] , fly ash [5] , cement [6] , et al. However, the effectiveness of single-method improvement for expansive soil has certain limitations, leading to an increased focus on the composite improvement [7,8] using multiple materials. Hao et al [9] conducted a composite improvement using fly ash and sisal fibers, and found that the surface crack ratio of the specimens was lower than that of untreated soil and soil improved with single materials, effectively inhibiting crack development and expansion. Wang et al [10] used soil stabilizers with polypropylene fibers to effectively control of the crack expansion ratio in expansive soil.
Although the aforementioned improvement methods are capable of enhancing the engineering properties of expansive soil, they present challenges such as high economic costs, limited improvement effects, and environmental pollution. Therefore, with the goals of economic feasibility and environmental friendliness in mind, this study selects sisal fibers and biochar to improve expansive soil. The orthogonal experimental method is employed to investigate the effects of biochar content, sisal fiber content, and fiber length on the soil cracking behavior through desiccation cracking tests. The study aims to propose an innovative improvement technique to inhibit the initiation of cracks in expansive soil and provide an economically effective and environmentally friendly solution for mitigating the engineering problems caused by expansive soil cracking.

Experimental Materials
The investigated soil was obtained from Nanning, Guangxi, China. The soil exhibited a yellow-brown color, and its basic physical properties were determined and presented in Table 1. The biochar was produced by the pyrolysis of timber under high-temperature and oxygenlimited conditions. The biochar had a specific surface area of 51.7m 2 /g and a density of 0.47g/cm 3 . The sisal fibers were produced from Guangxi, China. It had a density of 1.47g/cm 3 , fiber diameter of 0.12 mm, maximum tensile strength of 564.39 MPa, and elongation at break of 2.10%.

Experimental parameters
The orthogonal experimental method was employed in this study. The experimental factors included the biochar content, sisal fiber content and length. The measured parameters were the crack length, crack ratio, and crack width of the expansive soil. Among them, the biochar content is 4%, 6%, 8% and 10%; the sisal fiber content is 1.5‰, 3‰, 4.5‰ and 6‰; and the sisal fiber length is 1cm, 2cm and 3cm.

Experimental Schemes
According to the orthogonal design method, 16 experimental schemes considering the factors of biochar content, sisal fiber content, and fiber length were determined. Additionally, a control group consisting of untreated soil without any additives was included as a reference. In total, 17 experimental schemes were devised, as presented in Table 2.

Experimental Methodology
The air-dried expansive soil was crushed and sieved through a 2mm sieve. Then, predetermined amounts of distilled water, biochar and sisal fiber were added to the soil thoroughly mixed. The soil mixture was then compacted to a specimen size of 250mm × 250mm × 15mm. A thin layer of lime powder was evenly spread on the surface of the compacted soil specimens. Subsequently, the specimens were placed on an electronic balance. The heating lamp, LED light, camera, and electronic balance were sequentially turned on for the desiccation cracking test. The experimental temperature was 26°C and the humidity was 60%. The electronic balance recorded the change in sample mass, while the camera captured the desiccation cracking process and surface crack evolution. Finally, MATLAB programming was used to process the captured images and obtain the crack length, crack ratio, and crack width.

Experimental results
The results of the 17 experimental schemes are shown in Table 3. The experimental results indicate that the crack parameters of the improved expansive soil are smaller than those of the untreated soil. Therefore, it can be concluded that biochar and sisal fiber can effectively inhibit the development of cracks in the expansive soil. Among them, the improved soil mixed with 3cm sisal fiber with a content of 6‰ and biochar with a content of 4% has fine fissures, and the improvement effect is most obvious on the basis of economy and environmental protection.

Effect of factors on crack length
The mean values of crack length (k 1 , k 2 , k 3 , and k 4 ) in the improved soil under different factors and levels can be obtained based on the experimental data from Table 3. Consequently, the extreme differences R k can be calculated as shown in Table 4. Table 4 shows that the effect order of each factor on crack length in the improved soil is as follows: biochar content → sisal fiber content → sisal fiber length. The influence of each factor on the crack length in the improved soil is shown in Figure 1. The crack length has a non-positive correlation with the increase in biochar content. The crack length does not decrease monotonically with increasing material admixture and length, so it is necessary to determine the appropriate value to achieve the optimal ratio and thus optimize the crack length. The crack lengths obtained are smaller than the other values for the same factors when the biochar content is 10%, the sisal fiber content is 4.5‰ and the sisal fiber length is 3cm.

Effect of factors on the crack ratio
The mean values of crack ratio (f 1 , f 2 , f 3 and f 4 ) in the improved soil under different factors and levels can be obtained based on the experimental data from Table 3. The extreme differences R f can be obtained as shown in Table  5. Table 5 shows that the effect order of each factor on crack ratio in the improved soil is as follows: sisal fiber content → biochar content → sisal fiber length. The effect of each factor on the crack ratio of the improved soil is shown in Figure 2, which shows that the crack ratio exhibits a negative correlation with the increase in biochar content. The effects of sisal fiber length and content on the crack ratio are non-monotonic. When the biochar content is 10%, sisal fiber content is 4.5‰, and sisal fiber length is 1cm, the obtained crack ratio is smaller than the corresponding values for other levels of the same factor.

Effect of factors on crack width
The mean values of crack width (g 1 , g 2 , g 3 and g 4 ) in the improved soil under different factors and levels can be determined using the experimental data from Table 3. Consequently, the extreme differences R f can be calculated as shown in Table 6. Table 6 shows that the effect order of each factor on crack width in the improved soil is as follows: sisal fiber content → biochar content → sisal fiber length. The impact trends of various factor on the crack width of the improved soil are depicted in Figure 3. The effect of biochar content on crack width exhibits a decreasing linear relationship, while the effect of sisal fiber length and content on crack width shows a non-monotonic correlation. When the biochar content is 10%, the sisal fiber content is 4.5‰, and the sisal fiber length is 2cm, the obtained crack width is smaller than the values obtained under the same factors.

Conclusion
(1) The improved soil obtained by using different proportions of sisal fiber and biochar exhibits lower crack indicators compared to the untreated soil, indicating the effective suppression of crack development in expansive soil when these two materials are combined.
(2) The optimal experimental scheme in this study involves adding 6‰ of 3cm sisal fiber and 4% biochar, resulting in fine cracks in the improved soil and the most effective improvement on the basis of economy and environmental protection.
(3) The biochar content is the most significant factor affecting crack length, while sisal fiber primarily inhibits the development of crack ratio and average width.
(4) To achieve the best improvement in crack length in expansive soil, it is recommended to use sisal fibers with a length of 3cm, a sisal fiber content of 4.5‰, and a biochar content of 10%. For improving crack ratio, the optimal scheme involves sisal fibers with a length of 1cm, a sisal fiber content of 4.5‰, and a biochar content of 10%. Lastly, for enhancing crack width, using sisal fibers with a length of 2cm, a sisal fiber content of 4.5‰, and a biochar content of 10% is the recommended approach.