# Impact of Solidified Municipal Sludge as Temporary Covering Soil on the Stability of Landfill Slope
## 1 Introduction
Soil is the most typical material, and some industrial wastes may be used as alternative materials for temporary covering soil. Some scholars attempted to apply paper-making municipal sludge as temporary covering soil, but there were many problems in the practical application, such as inconvenient transportation and high kaolin content that resulted in low permeability [6; 7; 8; 9]. To better guide the practice, the U.K. and Ireland Environmental Protection Department and the U.S. Local Environmental Protection Bureau issued technical guidelines and recommended some alternative temporary covering materials, such as clay or sand, foam, geo-synthetics and paper-making municipal sludge [5; 11]. Soil for land-filling would occupy limited landfilling space. In the United States, the operating cost of a landfill is as high as USD2 million per hectare [12]. Since landfills are owned by private enterprises, the full use of space would impact the profits of enterprises. In China, landfills are usually taken charge of by domestic enterprises, but with the process of urbanization, there is less and less space for use. For instance, Qizishan Landfill in Suzhou could only be extended upward on the basis of the Phase I project. The constantly growing waste and finite storage capacity of landfills pose formidable challenges to sustainable urban development. It is not only an economic demand but also an environmental demand to utilize landfill space and save space resources as much as possible. The use of waste materials for the contemporary covering soil of landfill is an innovative approach to solve this problem. The research on temporary covering materials in Europe and North America mainly focuses on the resource recycling of waste materials, such as using municipal sludge, silt or sheared tire as the temporary covering material in landfills, to temporarily isolate solid wastes from the surroundings [13; 14; 15]. [PERSON] et al. [8] first proposed to use paper-making municipal sludge as the final covering soil in landfills. They conducted a five-year site leakage test and concluded that paper-making municipal sludge could be used as an effective hydraulic barrier. [PERSON] and [PERSON] [13] assessed the feasibility of using solidified municipal sludge as the temporary covering soil in landfill through laboratory experiments including unconfined compression strength, CBR (California Bearing Ratio), permeability and corrosion resistance tests. The results showed that the permeability of solidified municipal sludge was similar to other materials, and it was not obviously separate from the surface after 28 days. Additionally, its environmental indexes, including heavy metals and Escherichia coli, were acceptable, suggesting that solidified municipal sludge is safe to use in landfill. [PERSON] et al. (2016) compared clay with deeply dehydrated municipal sludge (municipal sludge containing 50% water was dried until the water content decreased to 45%) when considering the basic mechanical properties of the covering soil and the migration capacity of pollutants. They found that the unconfined compression strength of deeply dehydrated municipal sludge reached 77 kPa, and its permeability was 3.3\(\times\)10\({}^{-9}\) cm/s, implying that it could be an adequate impervious barrier. Currently, the research on the temporary covering materials of landfills mainly concentrates on laboratory tests of modified materials, and testing their mechanical and hydraulic properties. However, research on the permeability of daily covering soil, middle covering soil and final covering soil is still scarce, and the difference in permeability among the three materials remain unclear. Moreover, the permeability of all solidified municipal sludge is recommended to be less than 10\({}^{-7}\) cm/s, but whether such low-permeability material is suitable for landfills in China still needs further research. The landfill leachate level in China is generally high.