# Strength Characteristics of Biochar-Amended Clay Covered Soil Mixed with Methane-Oxidizing Bacteria
## 3 Experimental Results and Analysis
### Influence Mechanism of Strength Properties of Biochar-Clay Mixed Soils with Methanotrophs  
Figure 7 shows the biochar-clay mixed soil and methane oxidizing bacteria with a carbon content of 15%. The soil was magnified 1000 times using SEM, and microscopic scanning images at 0 d, 5 d, and 10 d of aeration time were obtained.  
It can be seen from the figure that with an increase in aeration time, the microstructure characteristics, such as soil pore size and skeleton morphology, of the soil samples changed with the methane filling time. When the methane oxidizing bacteria were incorporated into the biochar-clay mixed soil on the 0 th day, the soil samples with 15% showed two main distribution forms of biochar in the soil. One is that the biochar particles were used to fill the gaps between the clay particles and clay aggregates. The other was that the carbon particle skeleton was formed by mutual contact between the biochar particles. [PERSON] et al. [30] showed that the most probable pore diameter of the standard sand is larger than biochar particle size. Therefore, the pores among sand particles could be filled by biochar. [PERSON] et al. [31] and [PERSON] et al. [14] studied the optimal pore size distribution of clay, which is between 0.01 and 1 \(\upmu\)m from the MIP results of clay, that is, the pore size of  
Figure 6: Cohesion force and friction angle of biochar–clay mixed soil with methanotrophs and various biochar content. (**a**) \(\varphi\)–\(\alpha\). (**b**) \(c\)–\(\alpha\).  
those pores with maximum probability. Therefore, it can be judged that the biochar acts as the soil skeleton in the mixtures.  
Figure 7 shows that the strength characteristics of the biochar-clay mixture with methane-oxidizing bacteria are affected by the biochar content.  
It can also be seen from Figure 7b,c that with an increase in methane filling time (5 d and 10 d), the extracellular polymers (such as polysaccharides and proteins), which have strong surface adsorption, good biological flocculation, and stable soil structure, increase the shear strength of the biochar-clay mixture. [PERSON] et al. [32] investigated multi-year-old desert algal crusts, field artificial algal crusts, and indoor artificial algal crusts with different strengths and concluded that the algae in the biological crusts on the desert surface can secrete exopolysaccharides. These bind the loose sand grains to achieve sand fixation and thereby enhance the sand's strength. To enhance the strength of cohesive-free soils and remediate soil contaminants, [PERSON] et al. [33] used microbially produced metabolites to show that combining _Bacillus sphareticus_ and Candida for biological treatment can form CaCO\({}_{3}\) and PbCO\({}_{3}\) in sandy soil, which improves its strength, reduces permeability, and increases the fixation of pollutants. For granular soil, the flocculation of the extracellular polymer tends to make the particles stick together to form larger agglomerates, and the pores' contact form changes from surface-surface to point-point. Therefore, the pores between the soils increase with an increase in aeration time.