# Impact of Solidified Municipal Sludge as Temporary Covering Soil on the Stability of Landfill Slope
## 1 Introduction
In most cases, the leachate level of large-scale municipal solid waste landfills in cities such as Chengdu, Guangzhou, Xi'an, Suzhou, Hangzhou, Shanghai and Ningbo is only 3-5 cm below the landfill face, and there are many spill points on the slope (17; 18). [PERSON] (18) studied some landfills in South China and found that the permeability decreased with depth, and the clogging of the discharge guide layer not only increased the landfill water level, but also led to stagnant water levels in some areas. [PERSON] and [PERSON] (19) summarized the reasons for stagnant water in landfills in European and North American countries. The layered landfill of waste and the low-permeability daily covering material resulted in the multi-layer structure of landfills, while the horizontal extension of plastics, paper and other waste under pressure led to the anisotropy of permeability, which was high in the horizontal direction and low in the vertical direction. The leachate forms stagnant water on the temporary covering soil or aquitard, and seeps from the landfill slope in the horizontal direction. Currently, scholars have come up with some suggestions on the permeability of temporary covering soil, for instance, ranging between 10\({}^{-4}\) cm/s and 10\({}^{-5}\) cm/s (20), but the formation of stagnant water is influenced by rainfall infiltration and variations in climate from place to place. Under different climatic conditions, research on the appropriate permeability of solidified municipal sludge as a temporary covering soil is still needed for land-filling. A high stagnant water level is one of the primary reasons for the declining safety factor of landfills. Many scholars have conducted statistical analyses of landfill instability cases. [PERSON] and [PERSON] (2011) analyzed the instability of fifteen large landfills, eight of which failed due to the rapid increase in leachate level. In order to investigate the influence of various adverse conditions on the stability of landfills, many scholars have studied the safety and stability of existing landfill slopes (22; 23; 24; 25; 26; 27; 28; 29; 30). [PERSON] et al. (2010) tested the unit weight of waste and permeability of Jiangcungou Landfill, and calculated the stability of landfill slope in the presence of middle covering soil based on the assumption that the permeability decreased with depth. They found that the leachate was distributed in layers in the landfill owing to the existence of middle covering soil and there was stagnant water in each layer of temporary covering soil, and the safety factor of the landfill slope in the rainy season was less than 1.0. [PERSON] et al. (2017) conducted back analysis of slope instability of a landfill in Shenzhen in June 2008, and found that the increase in water level led to an increase in MSW (municipal solid waste) weight. Accordingly, the stress of underlying backing straps exceeded the residual strength of liner materials, which was the fundamental reason for landfill instability. In terms of the damage of the failure surface inside the waste body, [PERSON] et al. (2011) divided variation of waste degradation over time into four (4) stages, and evaluated the change in slope stability caused by waste degradation in the different stages. It was found that the cohesion of the waste was 0, and the safety factor decreased gradually when the internal friction angle decreased from 33\({}^{\circ}\) to 24\({}^{\circ}\). Particularly for landfills with a slope configuration of 2H:1V, the safety factor dropped below 1.0. In recent years, research has shown that rainwater infiltration has become the primary cause of declining safety and stability of landfills. [PERSON] et al. (2018) simulated the water transport and safety of Qizishan Landfill under different rainfall patterns, and found that the stagnant water level inside the landfill rose rapidly after 7 days of heavy rain, and the safety factor of the landfill slope dropped from 1.314 to 1.016.