# Impact of Solidified Municipal Sludge as Temporary Covering Soil on the Stability of Landfill Slope
## 1 Introduction  
Municipal sludge is a solid waste with a high content of organic matter, which is a by-product generated in the process of sewage disposal. Its composition is complex, and proteins and lignins are its primary contents [1]. With more and more attention paid to sewage disposal, the sewage collection rate grows year by year, and so does the yield of municipal sludge. By the end of 2019, the urban and county sewage disposal capacity in China reached 52,585 million m\({}^{3}\) and 9501 million m\({}^{3}\), respectively, while the sewage disposal rate reached 95.81% and 93.55%, respectively. If 8 tons of municipal sludge could be produced by 10,000 tons (assuming gravimetric water content of 80%) of sewage, it is estimated that 60 million tons of municipal sludge would be produced in 2021. The municipal sludge disposal level is relatively low in China, since both the technology and equipment are at the primary level. As reported [2], the percentage of sewage disposal equipment that can effectively dispose of the sludge is less than 50%, and mature technology and complete supporting facilities only account for 10%. At present, municipal sludge disposal measures feature reduction, harmlessness and resource recycling. Sludge solidification technology is characterized by a large disposal volume and short disposal period, and it can recycle municipal sludge of a high water content and low strength. The treated sludge has potential to be reused as the subgrade material for pavement, and as temporary covering soil for landfills. Instability of landfill slopes occurs frequently around the world, and some accidents have attracted extensive attention due to their large scale and severe casualties. Rumpke Landfill in Cincinnati, Ohio, USA [3], and Payatas Landfill in the Philippines [4], among others, suffered major accidents, with more than a hundred people dead or missing. In such cases, the landfill leachate level is a primary factor that influences the slope stability. For instance, Payatas Landfill, near Manila, the capital of the Philippines, was closed later than expected. Slope failure then occurred, which was mainly caused by the declining effective stress of the failure surface due to the increase in pore water pressure during rainfall. Improperly disposed municipal sludge is also a factor causing slope instability in landfills. The municipal sludge pits of Shanghai Laogang Landfill, Chengdu Chang'an Landfill and Shenzhen Xiaping Landfill experienced piping. Landfill leachate is a kind of high-concentration organic fluid produced after undergoing compaction by the weight of refuse dump, rainwater infiltration, microbial decomposition and biochemical reactions in the long-term land-filling process. The water quality of landfill leachate is hard to estimate, because it may contain various harmful inorganic and organic substances, and it is one of the main factors driving numerous problems in landfills. In order to minimize the impact of landfill waste on its surroundings, the waste is covered by materials to isolate it from the surroundings. There are three common covering materials, namely final covering soil, middle covering soil and daily covering soil. Daily covering soil is usually applied to cover the working face or side of the landfill, to reduce the impact of the temporary working face on the surroundings, local residents' health and the quality of life of surrounding communities. Daily covering soil can be used after the workday or workweek of land-filling. Middle covering soil is used temporarily when land-filling will not be carried out for a long period of time. A final covering system must be strictly designed, and it includes a soil erosion control layer, protection layer, drainage layer, impervious layer, exhaust layer, foundation layer, etc. Requirements for the temporary covering soil of landfill were first proposed in the Summer Report [5], which included the use of some non-perishable and non-degradable materials.