# Field Treatment of Storage Sludge and Stability Analysis of Overlying Municipal Waste Landfilling
## 4 Analysis of the Solidified Sludge
### Physical Properties
#### 4.2.2 Strength Properties  
Direct shear tests were conducted according to AASHTO procedure T236-08 [30]. The sheared speed was set as 0.80 mm\(\cdot\)min\({}^{-1}\). Figure 12 shows the direct shear test results for solidified sludge samples. In the solidified samples, the measured cohesion and internal friction angle were different. The parameters of the direct shear test for the  
Figure 11: Water content of the solidified sludge, varying with the depth. samples decreased with the increased buried depths. The average cohesion and internal friction angle of all solidified samples was 23.52 kPa and 16.60\({}^{\circ}\), respectively. There was a sudden fall in the average cohesion of the solidified sludge at the buried depth of 10 m. That is consistent with the variation of the water content and is because of the layered method of solidification. The cohesion of the solidified sludge within 10 m ranged from 5.50 kPa to 55 kPa, and the internal friction angle ranged from 4.50\({}^{\circ}\) to 31.70\({}^{\circ}\). The cohesion of the solidified sludge below 10 m ranged from 3.2 kPa to 58.4 kPa, and the internal friction angles ranged from 2.10\({}^{\circ}\) to 32.60\({}^{\circ}\). Compared to the shear strength of the raw storage sludge (Figure 6), the shear strength of the solidified sludge increased significantly. From the analysis of the distribution in the cohesion and internal friction angle of the solidified sludge with different buried depths, compared to the solidified sludge within 10 m, the solidified sludge below 10 m had a lower average value. The reason for this is the solidification mechanism difference between the two solidification methods. The solidified sludge in the deep layer was not only strengthened via the solidification but also enhanced by the squeezing generated by the piles. Thus, the stress state of the sampled solidified sludge from the deep layer showed differently and led to a loss of strength in this layer. Additionally, the relationships between the shear strength parameters and the water content of the solidified sludge were explored. The relationships herein reflect the influence of the water content on the strength of the solidified sludge from the shallow layer and the deep layer. they also serve to explore the mechanisms by which different treatment methods and the stratified state of the raw sludge affect the treated sludge. Figure 13 shows the shear strength parameters varying with the water content, per 10%. The cohesion generally decreased exponentially with the increasing water content. The reason for this is the solidified sludge builds a greater strength by consuming water, growing hydration products, and increasing density. The internal friction angle of the solidified sludge also decreased exponentially with the increasing water content. [PERSON] et al. [3] pointed out that different internal friction angles depend on the composition of the sludge. The trend lines in Figure 13 show that the shear strength parameters of the deep sludge decreased more dramatically with the water content than that of the shallow sludge. This means that the shear strength parameters of the deep sludge vary more sensitively with the water content than those of the shallow sludge. This is due to the difference in the solidification methods. In the solidified sludge, the strength improvement came via the evolution of the structure and composition. The different structures in the solidified sludge caused by the water content were reflected in the water film, which surrounded the sludge flocs  
Figure 12: The shear strength parameters of the samples taken from the boreholes: (**a**) the cohesion of the solidified sludge varying with the depth and (**b**) the internal friction angle of the solidified sludge varying with the depth. and particles, weakening the suction between soil particles [31]. The composition of the solidified sludge differed in the number of clay particles and solidification products.