# Field Treatment of Storage Sludge and Stability Analysis of Overlying Municipal Waste Landfilling
## 5 Stability Analysis  
The sludge pond treatment in this study was designed to achieve the subsequent landfilling of waste, so the stability of the slope body for vertical waste landfilling above the sludge pond needed to be calculated and analyzed. In this analysis, the treated sludge pond was used directly as the foundation of the landfilling area. There were two possibilities for the sludge pond overburden sliding along the slope body, which were the internal sliding of the sludge pond along the treated sludge, and the sliding along the impermeable liner at the bottom of the sludge pond. The shear strength of the raw sludge was close to zero, hence, the waste directly piled above the fluid sludge pond was detrimental to the stability of the body of the downstream waste [20]. The shear strength of the solidified sludge improved significantly compared to that of the raw sludge, which strengthened the stability of the slope pile. To analyze the stability of the waste landfilling above the sludge pond, a two-dimensional stability analysis was carried out using the Morgenstern-Price method and the limit equilibrium slice method. By the end of November 2019, the maximum height of the landfill waste above the sludge pond had reached 34 m, and the average landfill height was about 25 m. The elevation of the landfill waste was 110 m (Figure 9b). The analytical model was based on a survey of the terrain profile that was conducted in 2015 (Figure 14). The composite liner, the interface of which had a low shear strength, was the weak link in the stability analysis. In this analysis, the composite liner was simulated as a 1-m thick soft soil layer. The site investigation demonstrated the layered sedimentation of the waste  
Figure 13: The mean shear strength parameters within each range of water content among the solidified sludge in the shallow layer and deep layer: (**a**) the mean cohesion of the solidified sludge varying with the water content and (**b**) the mean internal friction angle of the solidified sludge varying with the water content. at the Qizishan landfill, and that the physical properties vary with the depth. Thus, the landfill model was set by layers, representing the strength parameters of the fresh waste, the shallow waste, and the old waste, determined according to the geological prospecting of the Qizishan landfill. The stability calculation analyzed the landfill slope, consisting of a 40-m height of waste over the sludge pond in two cases. Case I analyzed the stability of the slope above the raw sludge pond based on the layered shear strength properties of the raw sludge. From the bottom to the top in the raw sludge pond, there was fluid-plastic sludge, fluid sludge, and mud-water mixing sludge, respectively. Additionally, the leachate level of the waste slope body was defined as the half-height, and the leachate level of the raw sludge pond, measured in the field, reached the surface of the sludge pond. Due to solidification, the leachate level of the solidified sludge pond was not considered. Case II used the non-uniform solidified sludge as a part of the foundation for the waste slope, hence, the different shear strength properties of the solidified sludge were applied in the stability analysis. The shear strength parameters of the solidified sludge used for the analysis are described in Section 4.2.2. The shear strength parameters used for the stability analysis are shown in Table 2. Additionally, the sliding surface of the waste slope was assumed to be cambered, and after setting the radius and center ranges, it was automatically searched by the software of GeoStudio [32]. Figure 14 shows the critical failure surface of the waste slope for the two cases. The landfill body slides along the inside of the raw sludge pond, and the safety factor (Fs) of case I is 0.934 (Figure 14a). According to CJJ 176-2012 [33], Fs should not be less than 1.3, which indicates that the raw sludge pond does not satisfy the requirement as the foundation for the waste.