# Strength Characteristics of Biochar-Amended Clay Covered Soil Mixed with Methane-Oxidizing Bacteria
## 3 Experimental Results and Analysis  
### Effects of Normal Stress on Strength Properties of Biochar-Clay with Methane-Oxidizing Bacteria  
Figure 3 shows the shear stress of biochar-clay with methane oxidizing bacteria with 0% and 10% biochar under different vertical loads (\(\sigma_{\mathrm{v}}\) = 100 kPa, \(\sigma_{\mathrm{v}}\) = 200 kPa, and \(\sigma_{\mathrm{v}}\) = 400 kPa). Figure 3: \(Cont.\)  
It can be seen from Figure 3a that when the biochar content was 0%, the shear stress and strain curves of the soil samples of the biochar-clay with methane oxidizing bacteria under vertical stress of 400 kPa have no peak strength. When the strain increases, the sample's stress-strain curve shows a strain hardening state. Under 100 kPa and 200 kPa of vertical stress, the peak strength of the shear stress and strain curve of the soil sample is not obvious; that is, the softening phenomenon is not apparent. When the carbon content is 10%, as shown in Figure 3b, the shear stress and strain curves of the biochar-clay soil with methane oxidizing bacteria under different vertical loads (\(\sigma_{\rm v}\) = 100 kPa, \(\sigma_{\rm v}\) = 200 kPa, and \(\sigma_{\rm v}\) = 400 kPa) have obvious peaks, and the shear strength increases as the vertical normal stress increases. The principle of effective stress can be used to explain this phenomenon; that is, when normal stress increases, the soil particles can be arranged more closely, and the soil's particle skeleton can resist soil deformation. With the same amount of biochar and a higher vertical normal stress, which indirectly increases the frictional resistance between the soil particles, the compaction effect of the soil sample is more significant. Thus, the greater the sample's normal stress, the greater its shear strength. Figure 3 shows the shear stress-strain curve of the mixed soil, from which we can obtain the elastic modulus of the soil sample at the initial approximate elastic stage of the curve. It can be seen from the figure that with an increase in vertical stress, the elastic modulus of the samples with biochar content of 0% and 10% showed a gradually increasing trend. Effects of Methane Filling Time on the Strength Properties of Biochar-Clay with Methane-Oxidizing Bacteria  
Figure 4 shows the relationship between the shear stress and strain of the biochar-clay soil with methane oxidizing bacteria when the biochar content is 15% and the methane filling time is 0 d, 5 d, and 10 d. The figure shows that the stress-strain curves of the soil samples are all strain softening under the biochar content. Under 400 kPa normal stress, the peak strength of the soil samples increased gradually as the methane filling time increased, but the peak strengths of the samples were similar at 5 d and 10 d. The ductility and residual strength of the biochar-clay mixture with methane oxidizing bacteria improved with the methane filling time of 10 days. This is because the longer the methanogenesis time, the more metabolites (extracellular polymers) the methanotrophs produced. Figure 3: Stress-strain curves of biochar–clay mixtures with methane oxidizing bacteria under various normal stresses: (**a**) \(\alpha\) = 0%, \(t\) = 0 d. (**b**) \(\alpha\) = 10%, \(t\) = 0 d.  
Effects of Carbon Content in Biochar on the Strength Properties of a Biochar-Clay Mixed Soil with Methane-Oxidizing Bacteria  
Figure 5 shows the relationship between the shear stress and strain of biochar-clay mixed soil with methane oxidizing bacteria with various amounts of biochar. It can be seen from the figure that with an increase in biochar, the soil's stress-strain curve transitioned from strain hardening to strain softening. However, with an increase in the biochar, the small particles fill the voids in the soil and the large particles form carbon-soil and carbon-carbon frameworks. Due to the weak strength of the biochar particles, as the confining pressure increases, the carbon particles are compressed, and the soil's stress-strain curve peaks, showing strain softening.