# Strength Characteristics of Biochar-Amended Clay Covered Soil Mixed with Methane-Oxidizing Bacteria
## 1 Introduction  
During the service period of a typical landfill, gas is released due to the degradation of organic matter in the waste. Landfill emissions contain two greenhouse gasses: methane (55% to 60%) and carbon dioxide (40% to 45%). Potentially, methane's global greenhouse effect is about 25 times that of carbon dioxide [1], and between 5% to 50% of landfill methane escapes into the atmosphere even if a complete gas collection device is installed. In small, medium-sized, or old landfills, where such devices are rarely installed, methane suppression by other means is necessary [2]. Therefore, to suppress and reduce the escape of methane gas, this study proposes a biochar-clay mixture with methanotrophic bacteria as an alternative material for the landfill's final cover. The research results have practical implications for global warming. Methane oxidizing bacteria are microorganisms that use methane as their only carbon and energy source and are easily enriched in the landfill coating [3]. [PERSON] et al. [4] showed that adding methane oxidizing bacteria to the landfill's overburdened soil can promote the oxidation of escaping CH\({}_{4}\). However, the growth and reproduction of the soil's aerobic methane oxidizing bacteria require a suitable environment (such as optimal temperature, pH, and oxygen content), and the traditional clay overburden has high clay content and poor aeration. It is also acidic and, therefore, generally not suitable for the growth of methane oxidizing bacteria [5]. With its low density, high specific surface area, high pH and stability, and well-developed pore structure [6; 7], biochar has received considerable attention as a sustainable and eco-friendly material for overburdened soil improvement in landfills [8]. Added to soil, it can reduce soil density, increase soil porosity, and affect the distribution of soil aggregates and pore size [9; 10; 11]. Biochar's porosity also provides a good habitat for methane-oxidizing bacteria [12]. It can improve the soil's microclimate and physical-chemical properties, both of which promote bacterial growth and enzyme activity [13], and increase the methane oxidation efficiency of a landfill's biochar-clay cover mixed with methanotrophs. Although suggested biochar as a modification material for the final landfill, most traditional landfill cover layers are composed of clay and are affected by the atmosphere's dry-wet cycle or the uneven settlement of waste degradation [14]. For this reason, the cover layer often cracks, a phenomenon closely related to the microscopic pore structure of the overlying soil, which in turn affects the soil's strength properties [15; 16]. [PERSON] [15] investigated the effect of biochar content and particle size on the soil's shear strength and showed that the latter gradually increased with the amount of biochar. When the biochar's particle size increases, the soil's shear strength also increases. [PERSON] et al. [17] similarly explored whether biochar improved the shear strength of cohesive sandy soil and found that, when the amount of biochar increases from 5% to 15%, the soil's shear strength parameters--cohesion and internal friction angle--also gradually increased. These studies show that the strength properties of biochar-amended soil are an important consideration in soil modification research. Although many studies have investigated the strength characteristics of biochar-amended soil, most have focused on the influence of biochar content and particle size on the soil's strength characteristics. For example, [PERSON] et al. [18] and [PERSON] et al. [17] studied the effect of biochar content on the soil's shear strength and showed that it increased as the amount of biochar increased. [PERSON] et al. [19] investigated the strength properties of various biochar-modified soils (5%, 10%, and 20% of biochar) and particle size (size \(-10\), size \(-20\), and size \(-40\)) and found that the samples' shear strength not only increased as the biochar content increased but also as the biochar's particle size decreased.