# Strength Characteristics of Biochar-Amended Clay Covered Soil Mixed with Methane-Oxidizing Bacteria
## 3 Experimental Results and Analysis
### Analyze the Relationship between Macro Strength Characteristic Change and Microstructure  
The results of the macroscopic shear strength test show that the soil's shear strength can be improved with an increase in the amount of biochar and aeration time. The soil sample's macro-strength characteristics are related to its microstructure, pore size structure, particle gradation, biochar particle size, relative particle size, and the interaction effect between them. For materials whose clay particle size is larger than that of biochar, the latter is used to fill the pores of the particles and render the particle distribution of the soil sample denser, as shown in Figure 8A. However, when the biochar content is larger than a certain optimal biochar content value, some biochar particles fill the gaps between the soil's particles, and some form a carbon particle skeleton. It can be seen from the left side of Figure 8B that when the biochar content is greater than 10%, a biochar particle skeleton forms in the soil sample. Converting the biochar's particle size curve in Figure 1 into a particle size distribution curve, the optimal particle size is about 70 \(\upmu\)m. From the MIP results of the clay, it can be seen that the optimal pore size distribution is between 0.01 and  
Figure 7: SEM images of biochar–clay mixed soil with bacteria (0 d, 5 d, and 10 d). (**a**) \(\alpha=15\%\), 1000 times magnification, 0 d. (**b**) \(\alpha=15\%\), 1000 times magnification, 5 d. (**c**) \(\alpha=15\%\), 1000 times magnification, 10 d.  
1 \(\upmu\)m [34] and that the biochar's optimal particle is much larger than the clay's optimal pore size. This shows that the distribution of biochar in clay is chiefly in the form of filling and forming the carbon skeleton. Due to the biochar's low strength, there is a trend of decreasing peak strength of the mixed soil sample at 0 d (see Figure 4). The reason for this may be that when the aeration time is 0 d, the soil's bacterial liquid has not played its role. At this time, the strength of the soil sample chiefly includes the force between the soil particles and the biochar particles and the bonding force between them. Figure 8: _Cont._  
Soil is a heterogeneous multiphase system. Its microorganisms are living organic colloids that directly participate in mineral formation and evolution. Therefore, the activities of the microorganisms influence the soil's macroscopic properties. [PERSON] et al. [35] suggested that microbial activity can improve the strength of soils with various properties. For clay, the soil strength is chiefly improved by microorganisms and their metabolites. Bacteria secrete an extracellular polymer during the growth and reproduction process, which changes the soil microstructure. An extracellular polymer, a mushroom-like biofilm structure secreted by bacteria in the process of growth and reproduction, attaches to the surface or around the bacteria. Scholars who study microorganisms generally believe that EPS (extracellular polymeric substance) is relatively complex. [PERSON] et al. [36] investigated the role of the extracellular polymer matrix (EPM) components in biofilms. Figure 8: Microstructure and schematic diagram of biochar–clay mixed with bacteria. (**A**) SEM images of biochar–clay mixed soils with various biochar content: (**a**) \(\alpha=0\%\); (**b**) \(\alpha=5\%\); (**c**) \(\alpha=10\%\); (**d**) \(\alpha=15\%\). (**B**) Schematic diagram of the microstructure evolution of the biochar–clay mixed with bacteria. (**C**) Fourier transform infrared spectroscopy analysis of biochar–clay mixed soil. (**D**) Fourier transform infrared spectroscopy analysis of biochar–clay mixed soil. Extracellular polymers widely exist on cell surfaces, have strong surface adsorption and good biof flocculation, can stabilize soil structure [37], and have an important role in the process of microbial mineralization.