Impact of Solidified Municipal Sludge as Temporary Covering Soil on the Stability of Landfill Slope

Abstract

Solidified municipal sludge is widely used as temporary covering soil in landfill. Due to the complex leachate of landfill, solidified municipal sludge has the problems of stagnant water and strength degradation. In order to investigate the influence of solidified municipal sludge on the stability of landfills, a landfill slope model with SEEP/W and SLPOE/W was established, using the actual infiltration as the boundary condition. Based on the changes in permeability and strength of the intermediate overburden layer, the migration law of leachate in a landfill under the condition of an intermediate overburden layer was analyzed. The relationship between landfill slope safety, climatic conditions and deterioration of the intermediate overburden layer was further explored. The results show that the permeability of the intermediate overburden layer affects the distribution of leachate and the height of stagnant water. During the rainstorm period, the safety factor of the landfill slope decreased rapidly from 1.4 to 1.0 or even lower. With the decrease in the shear strength of the intermediate overburden, the safety factor of the landfill slope was reduced to less than 1.0. Therefore, maintaining the permeability and strength of solidified municipal sludge at a certain level is required to ensure the safety and stability of landfill slopes.

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 m³ and 9501 million m³, 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. 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 [6,7,8,9,10]. 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]. Moo-Young 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. Chung and Lee [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. Chen et al. [16] 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 3.3×10⁻⁹ 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. 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]. Lan [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. Dixon and Jones [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. Qian and Koerner [21] 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]. Yang et al. [30] 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. Peng et al. [27] 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, Hossain et al. [31] 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° to 24°. 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. Qiu et al. [28] 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.

When solidified municipal sludge becomes the intermediate cover of a landfill, the refuse body is divided into refuse units and refuse layers. The influence of water retention on the safety of the landfill slope is not clear. At the same time, it is not clear whether the intermediate layer will become a weak sandwich due to the erosion of the solidified municipal sludge. The effect of solidified municipal sludge as temporary soil cover on the safety of landfill slope is a problem to be solved when solidified municipal sludge is applied. In this study, two problems that may cause the slope instability of landfills were taken into account when solidified municipal sludge is used as temporary covering soil for land-filling. One is the formation of stagnant water, and the other is the declining strength of solidified municipal sludge in the leachate environment.

2. Modelling Simulation

2.1. SEEP/W and SLOPE/W Coupling Landfill Slope Stability Analysis

The impact of solidified municipal sludge as middle covering soil on landfill slope stability was evaluated with the seepage of porous media (SEEP/W) and slope stability (SLOPE/W) analysis of GeoSlope. SEEP/W is widely used to investigate saturated–unsaturated seepage of groundwater in geotechnical engineering, and can be used for unsaturated finite element calculation. Many scholars have used the SEEP/W calculation program to investigate the migration of landfill leachate. SLOPE/W not only has non-arc sliding, but also can search through soft interlayers. In this study, the landfill slope safety factor was calculated using the Morgenstern–Price method. The material parameters in the model were determined according to the effective stress method. The distribution of stagnant water and pore water pressure under different climatic conditions were first calculated by SEEP/W, and then used in SLOPE/W as the background parameter values for subsequent coupling calculations. The deterioration of temporary covering soil was taken into account in the simulation, and the landfill slope safety factor was also obtained.

2.2. Establishment of Model

Figure 1 shows the distribution of the leachate level obtained from the field monitoring results of Jiangcungou Landfill in China [2]. Middle covering soil about 10 to 30 cm thick was laid on each layer of waste. Due to the low permeability of the middle covering soil, there were many layers of stagnant water inside the landfill, and the top layer was only 3 m below the working face.

The model in Figure 2 was built by referring to this multi-layer middle covering soil structure. The model was 157 m long and 40 m high, and the slope was 3H:1V. There were two layers of middle covering soil, and each layer was 30 cm. The two layers divided the waste into three layers. Owing to the different land-filling periods and overlying surcharge loading, the three layers of waste were each of a different nature.

2.3. Parameters and Boundaries

2.3.1. SEEP/W Model Parameters

Due to the difference in waste degradation and consolidation stress, the permeability of waste decreases with the depth. The hydraulic properties of the waste reported by Zhang [32] were adopted in this study. Figure 3 and Figure 4 show the soil water retention curve and unsaturated permeability of waste in different depths in Suzhou Qizishan Landfill measured by Zhang Wenjie from Zhejiang University. The upper layer of newly filled garbage has not been effectively rolled, and the permeability coefficient is high. Due to the subsequent overlying load, the permeability decreases rapidly. At the same time, the unsaturated permeability coefficient changes with the generation. The saturated permeability levels for the three layers from top to bottom were estimated as 4.81 × 10⁻² cm/s, 3.5 × 10⁻³ cm/s and 2.75 × 10⁻⁴ cm/s, respectively.

The water content of waste is relatively high, and the permeability of middle covering soil or daily covering soil is relatively low. It could be judged from the formation of stagnant water that the saturation degree of temporary covering soil was close to 1. To reduce the complexity of model calculation, the saturated permeability model was selected for the temporary covering materials, and the permeability levels were 10⁻⁴ cm/s, 10⁻⁵ cm/s, 10⁻⁶ cm/s and 10⁻⁷ cm/s.

2.3.2. SEEP/W Boundary Conditions and Initial Conditions

The 2012 version of GeoSlope used VADOSE/W to calculate the interaction between the unsaturated soil layer and the climate, and the new version after 2018 version deleted the VADOSE/W module and adopted TEMP/W and SEEP/W coupling to calculate the evaporation, runoff and infiltration. However, it was prone to non-convergence and the number of calculation cycles was too large for complicated models. To simplify the calculation process, UNSAT-H in different climatic conditions and Loam as the actual infiltration condition of temporary covering soil were adopted as the upper infiltration boundary of SEEP/W [33]. The lower boundary of the landfill was a clay liner with thickness of 2 m and permeability lower than 10⁻⁷ cm/s or equivalent to geosynthetics. Sufficient drainage measures were set on the liner to ensure that the main head of the landfill was controlled within 30 cm. Although it is difficult for most landfills to guarantee the 30 m head control height, a 30 m head boundary was directly set for the lower boundary as required, and the right was the symmetrical boundary [28,34].

In the design of a landfill, a waste dam could be used as a permeable edge for collecting and discharging the leachate. It is assumed that the leachate can be drained on the permeable edge freely; therefore, the area near the waste dam in the model was set as zero head boundary. The drainage system in the landfill could simulate the location 10 m away from the slope on each layer of middle covering soil through the drainage boundary. The water content of waste in China is usually above 50%, which is much higher than that in European and American countries. By referring to the soil–water characteristic curve shown in Figure 3, the suction was between −3 and −4 kPa. The initial suction of the waste in the model was −4 kPa.

2.3.3. SLOPE/W Model Setting

An important factor of slope stability is the strength of the landfill body. Much research has been conducted on the strength of waste [34,35,36,37,38]. However, it is not easy to obtain a representative and reliable value of the strength of waste, since it is influenced by many factors, such as the type of waste, composition, degradation degree, land-filling depth, landfill characteristics, density, internal structure, stress history, etc. [19]. Moreover, it is related to the strength test method, sample size and strength criteria adopted. In terms of land fill age, there are abundant fibrous materials in the components of waste dumped in the first three years, so it exhibits a strong cohesive behavior, but as age increases, muck gradually becomes predominant component, and such cohesion drops gradually, even to zero [36]. Some field and laboratory test results in other studies are shown in

Table 1. Strength parameters of MSW.

Reference	Test Method	Result	Remark
Zhan et al. [36]	Triaxial Test	c = 10–33 kPa, φ = 8–27°	Normal stress: 50~400 kPa
Feng [39]	Large Triaxial Shear Apparatus	0 < σ < 60 kPa, c = 20 kPa, φ = 10° σ ≥ 60 kPa, c = 20 kPa, φ =22°	Normal stress: 100~400 kPa
Liu et al. [40]	Laboratory Triaxial	c = 16.5~40 kPa, φ = 21~31°	Normal stress: 50~200 kPa
Kavazanjian [41]	-	σ < 30 kPa, c = 24 kPa, φ = 0° σ ≥ 30 kPa, c = 0 kPa, φ = 33°	-

. As reported [28,36], the strength of waste is greatly influenced by the landfill age (depth). Therefore, referring to the on-site layered sampling data of Suzhou Qizishan Landfill, the entire model is divided into three layers, and the strength parameters of each layer are shown in

Table 2. Strength parameters of MSW with different ages.

	Depth (m)	Cohesion c (kPa)	Internal Friction Angle φ (°)
Surface waste	0–9	23.3	9.9
Middle waste	9–24	20.2	21.9
Deep waste	24–39	0	26

.

Daily covering soil or middle covering soil would also undergo the deformation and failure process. The strength of solidified municipal sludge is mainly contributed by the solidified product of cement clinker. In the immersion test in acid solution, the cohesion of solidified municipal sludge dropped rapidly in the solution with pH = 2. Based on the consideration of the worst conditions, the cohesion of solidified municipal sludge c =0 kPa, and the internal friction angle φ =7°, 14° and 21°. The simulation was conducted with the above-stated shear strength parameters, and it was found that when the thickness of middle covering soil was 0.5 m, SLOPE/W could retrieve the potential sliding surface of middle covering soil automatically.

3. Results

Due to the existence of aquitards, the leachate in a landfill not only migrates downward vertically, but also horizontally, in 2D and even 3D processes. Owing to the existence of daily covering soil and middle covering soil, the migration of leachate is even more complicated. In this study, the migration of leachate in the landfill (in the non-existence of middle covering soil, in the existence of middle covering soil, in the existence of daily covering soil) was analyzed in different conditions by taking the climate in Nanjing as an example.

3.1. Migration of Leachate in the Non-existence of Middle Covering Soil

Figure 5 shows the distribution of leachate in the condition without any aquitard. When there are no aquitards in middle covering soil, the infiltrating rainwater and leachate of waste can move downward to the bottom of landfill and be discharged smoothly by the internal drainage system, which is an optimal situation of drainage. It is clear that the leachate can be discharged from the landfill through the drainage system by gravity, and there is no stagnant water in the landfill.

3.2. Impact of Middle Covering Soil k_s on the Formation of Stagnant Water in the Landfill

Impacts of permeability of the middle covering soil on stagnant water were analyzed. Figure 6 shows the changes in leachate distribution inside the landfill after heavy rainfall and five days after the rainfall with the permeability of the middle covering soil of 10⁻⁴ cm/s. It is indicated that when the permeability is 10⁻⁴ cm/s, the downward-moving stagnant water quickly accumulates on the middle covering soil and rapidly moves downward. According to Figure 6b, there is barely any stagnant water in the middle covering soil. When the saturated permeability of the middle covering soil is 10⁻⁵ cm/s, the distribution of stagnant water is similar, but the infiltration rate declines.

Figure 7 shows the distribution of stagnant water in the landfill during the heavy rainfall period with the saturated permeability of the middle covering soil of (a) 10⁻⁶ cm/s and (b) 10⁻⁷ cm/s. From Figure 7a, when the permeability decreases from 10⁻⁴ cm/s to 10⁻⁵ cm/s, it is difficult for the stagnant water to move downward by gravity. The permeability in the landfill exhibits anisotropic characteristics, and the leachate is discharged from the landfill through the drainage system along the middle covering soil layer in the horizontal direction. If the drainage system is blocked, the leachate may seep from the slope surface of the landfill, and this has been observed in many landfills in South China.

When the saturated permeability of the middle covering soil is 10⁻⁷ cm/s, stagnant water is more obvious. Stagnant-water-saturated areas take up half of the landfill space, and the head of the stagnant water level may reach 7–8 m. The pressure head distribution on the right boundary of the model is shown in Figure 8, and it is clear that the stagnant water is only 5 m below the working face, consistent with the situation of most landfills [28]. The first layer of stagnant water below the working face is as thick as 10 m, and the waste 10 m below the middle covering soil is in a saturated state. Moreover, it is indicated that the pressure head increases gradually with the increase in depth (i.e., decrease in altitude) in the stagnant water area. It is clear that the landfill has a layered structure due to the existence of middle covering soil, and there are remarkable differences in water pressure head between the upper and lower layers of the middle covering soil. If the drainage approach is inappropriate, the stagnant water in the landfill might be higher, thus imposing a greater impact on the slope stability of the landfill.

3.3. Impact of Daily Covering Soil on the Formation of Stagnant Water

When there is daily covering soil, the landfill becomes several relatively independent units, which are isolated by the daily covering soil layer, thus making leachate infiltration more difficult. The migration of leachate in the landfill in the presence of daily covering soil is shown in Figure 9.

Due to the daily covering soil, the distribution of leachate is more complicated, and there are layers of stagnant water. For instance, the relationships between the leachate pressure head and land-filling depth in section A-A and section B-B are shown in Figure 10. It is clear that the surface of the landfill is in an unsaturated state, but with greater depth, the saturated state is reached at an approximate depth of 5 m. Since it is separated into units by the daily covering soil, the pressure head also exhibits a similar trend. Compared with Figure 8, where there is only middle covering soil, the existence of daily covering soil causes a more complicated stagnant water level in the landfill, and the saturated region becomes larger.

4. Discussion

When solidified municipal sludge is adopted as the temporary covering material of a landfill, the landfill has a multi-layer structure. On one hand, there may be aquitards, which would result in the accumulation of leachate in the landfill and slope instability. On the other hand, the strength of solidified municipal sludge dropped due to the erosion caused by the leachate, and there is doubt whether the continuous middle covering soil would become a weakness plane of the landfill. Therefore, it is necessary to further study its influence on the slope stability.

4.1. Slope Safety Factor and Sliding Failure Surface

The relationship between rising stagnant water levels and the slope safety factor is calculated based on the permeability of middle covering soil, which is 10⁻⁶ cm/s in the climatic conditions in Nanjing.

Figure 11 shows the highest stagnant water head of middle covering soil and slope safety factor with the climatic change. It is clear that the safety coefficient of the landfill is around 1.4, which is safe, but it fluctuates with the stagnant water head. As it rains more, the infiltrating leachate increases, so the entire saturated area of the landfill enlarges, and the safety coefficient of landfill slope decreases rapidly. When the head of stagnant water increases to 7 m, the slope safety coefficient of the landfill drops to 0.78, far lower than the requirement for minimum slope stability. It is relatively dry in the winter and there is less rain, so the slope safety coefficient increases rapidly. In Figure 11, it is demonstrated that the safety factor is impacted by rainfall infiltration. For instance, the safety coefficient is low in the summer since it is rainy, but it increases in the winter, displaying periodical changes over the year.

Figure 12 shows the slope failure surface on the 100th and 204th days. In Figure 12a, little stagnant water is observed, and the waste body slips through the deep middle covering soil. The leading edge of the most dangerous sliding body slips out of the waste dam, while the trailing edge of the waste body passes through the multi-layer middle covering soil and then the waste body. In Figure 12b, there is obvious stagnant water on the two layers of middle covering soil in the rainy season. Due to the impact of overlaying stagnant water, the leading edge of the most dangerous sliding body directly slips out along the first layer of the middle covering soil, while the trailing edge passes through the waste dam. The rainfall infiltration changes the distribution of leachate in the landfill, as well as the safety factor of the landfill. On the other hand, in the two above-stated cases, middle covering soil becomes a weak inter-layer of the landfill body, and the slope slides along this weak inter-layer.

4.2. Impact of Climatic Condition on Safety Factor

According to the rules of the formation of stagnant water in landfills, the water equilibrium mechanism in temporary covering soil varies under different climatic conditions. The actual infiltration levels are different in Guangzhou, Nanjing, Xi’an and Yinchuan, being 1340 mm, 765 mm, 303 mm and 75 mm, respectively. Taking the saturated permeability of middle covering soil, 10⁻⁶ cm/s, as an example, the safety factor of a typical landfill is calculated, as shown in Figure 13. It is clear that with the increase in rainfall and infiltration, the slope safety coefficient of the landfill declines gradually. The rainfall and infiltration levels are relatively low in Yinchuan, so its safety coefficient is the highest, and there is barely a slope instability issue. In Guangzhou, both the rainfall and infiltration rates are high, and there are usually short-range rainstorms, so the safety risks are more prominent. When the internal friction angle is 14°, the slope safety coefficient under the climatic conditions in Nanjing and Guangzhou is close to 1.0, and slope instability issues are likely to occur.

4.3. Impact of Deteriorating SMS as Temporary Covering Soil on the Stability of Landfill Slope

The existence of temporary covering materials such as middle covering soil and daily covering soil changes the migration of leachate in a landfill, and the internal stagnant water level increases, influencing the overall slope stability and reducing the safety factor of the landfill. According to the requirements in Technical Code for Municipal Solid Waste Sanitary Landfill (

Table 3. Safety factor of landfill slope.

	Security Level	First-Level Slope	Second-Level Slope	Third-Level Slope
Calculation Methods
Plane sliding method		1.35	1.30	1.25
Broken line sliding method
Arc sliding method		1.30	1.25	1.20

), the safety factor of the landfill slope should meet several requirements.

The cohesion c and internal friction angle φ of OPC30 and SAC30 can reach 12.7 kPa, 24.6° and 68.0 kPa, 18.9°, respectively, after being solidified for 7 days. However, with acidic corrosion, especially in strong acid environments, i.e., pH = 2, the cohesive force declines rapidly, and it could be close to zero under certain conditions. Considering cohesion of zero, the influence of shear strength parameters (i.e., internal friction angle) on the landfill slope safety factor was analyzed, as shown in Figure 14. On one hand, with the decrease in the permeability of the middle covering soil, the leachate may form a high stagnant water level. Meanwhile, the strength of solidified municipal sludge decreases due to corrosion, which is likely to form a weak layer in the landfill, and further leads to the decrease in the safety coefficient. Considering F_s = 1.3 as the bottom line, as shown in Table 3, the permeability of the middle covering soil and internal friction angle should be at least 10⁻⁵ cm/s and 21°, respectively, in order to ensure the safety of the landfill slope.

Based on the coupling calculation of SEEP/W and SLOPE/W, the influence on the landfill slope safety factor was calculated from the angle of the permeability coefficient and strength parameters of the intermediate soil cover. In wet climates, the safety factor of a landfill site is obviously affected by the rainy season. The decrease in strength of the intermediate layer may reduce the safety factor of a landfill to 1.0 and result in potential landfill slope failure. The stability of the permeability coefficient and the strength of the intermediate soil cover are the requirements to ensure the safety of the landfill slope.

5. Conclusions

Analyzing the problem that solidified sludge as temporary covering soil may impact the safety of landfill slope, a SEEP/W and SLOPE/W coupling landfill slope model was established with the actual infiltration as the boundary condition. Based on the changes in permeability and strength of the middle covering soil, the migration rules of the leachate in a landfill with middle covering soil were analyzed, and the relationship between the slope safety coefficient of the landfill and climatic conditions as well as deteriorating middle covering material was investigated. The conclusions are as follows:

(1) When there is no middle covering soil or blockage, the leachate can be smoothly discharged by the drainage system. The permeability of the middle covering soil should not be too low, since this may lead to a relatively high level of stagnant water on the middle covering soil. If the daily covering soil has low permeability, the distribution of the leachate will become more complicated.

(2) The impact of climatic conditions on the safety of landfills was analyzed with the SEEP/W and SLOPE/W coupling model. We concluded that under the same conditions, the slope safety coefficients of landfills in Guangzhou and Nanjing are much lower than those in Yinchuan and Xi’an. Landfills in wet areas have more prominent safety problems in the rainy season since the infiltration forms the stagnant leachate in a relatively short period of time. When the stagnant water increases, the slope safety coefficient of the landfill decreases to an unacceptable level. When the intensity of rainfall decreases and the level of stagnant water drops, the landfill slope safety coefficient begins to increase. In addition, the waste body may pass through the middle covering soil in the rainy season.

(3) As the shear strength of the middle covering soil decreases, the safety coefficient of the landfill may drop to 1.0 or lower. In order to guarantee the safety of the landfill slope, the permeability of the middle covering soil and the internal friction angle should be at least 10⁻⁵ cm/s and 21°, respectively.