As waste landfills continue to expand, accidents can occur if these landfills are not properly stabilized. In this study, samples of municipal solid waste (MSW) from a waste landfill in Xi'an, China were collected through on-site drilling. Considering the effects of eight landfill ages (1, 2, 3, 11, 12, 13, 21, 22, and 23 y) and six moisture contents (natural, 20, 40, 60, 80, and 100%), 324 groups of MSW were tested in the laboratory with a direct shear test apparatus. The results indicate the following: (1) with an increase in horizontal shear displacement, the shear stress of MSW gradually increases without a peak stress phenomenon, which is a displacement hardening curve; (2) with an increase in landfill age, the shear strength of MSW increases; (3) with an increase in moisture content, the shear strength of low-age MSW first increases and then decreases, but the relationship between shear strength and moisture content of medium- and high-age MSW is not significant; (4) with an increase in landfill age, the cohesion (c) decreases, and the internal friction angle (φ) increases; (5) with an increase in moisture content, the c and φ of low-age MSW first increases and then decreases. Based on these results, a shear strength, landfill age, and moisture content correlation model is established, and the model has a high degree of fit with the overall experimental data. The results of this study can provide a reference for the stability calculations of MSW landfills.
Research Article
Effects of moisture content and landfill age on the shear strength properties of municipal solid waste in Xi'an, China
https://doi.org/10.21203/rs.3.rs-1971719/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 19 Apr, 2023
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Municipal solid waste
direct shear test
landfill age
moisture content
shear strength
landfill stability
At present, there are three main methods for environmentally sound disposal of municipal solid waste (MSW): sanitary landfill, incineration, and composting. Sanitary landfills are the most frequently used to treat MSW (Wang et al. 2015; Eskandari et al. 2016; Falamaki et al. 2018; Madon et al. 2019), especially in developing and underdeveloped countries, owing to their economic and environmental benefits (Reddy et al. 2009; Falamaki et al. 2019). As waste continues to be added to landfills, the height of the pile becomes increasingly steeper; the lack of sufficient knowledge regarding the factors affecting the strength of MSW landfills (Zhu et al. 2009; Guerrero et al. 2013) has caused destabilization damage to occur in some areas. For example, in 2016, the Wuma landfill in Shenzhen, China, suffered destabilization due to a high water pressure in the landfill after heavy rainfall; in 2017, a destabilization accident at the Meethotamulla landfill in Sri Lanka killed more than 30 people, while a localized slip destabilization incident occurred at Greentree Landfill, USA in the same year (Wang et al. 2021).
Several researchers have performed shear strength tests on MSW (Landva and Clark 1990; Zhang et al. 2000; Machado et al. 2002; Zhang et al. 2003; Fucale et al. 2005; Feng et al. 2005; Gao et al. 2007; Bray et al. 2009; Shariatmadari et al. 2009; Zekkos et al. 2010; Shariatmadari et al. 2011; Karimpour-Fard et al. 2011; Karimpour-Fard et al. 2013; Zhang et al. 2014; Zhan et al. 2015; Babu et al. 2015; Zhang et al. 2015; Li et al. 2016; Feng et al. 2017; Shariatmadari et al. 2017; Fei et al. 2018; Falamaki et al. 2019; Alidoust et al. 2021), and the relevant references on the shear strength properties of MSW are presented in Table 1. Some researchers found that the shear strength of MSW decreases with increasing landfill age (Caicedo et al. 2002; Hossain et al. 2009; Fei et al. 2015; Karimpour-Fard et al. 2019). However, other researchers believe that the shear strength of MSW increases with landfill age (Ma et al. 2010; Zekko et al. 2017). Some researchers believe that as landfill age increases, c (cohesion) increases and φ (internal friction angle) decreases (Gabr et al. 2007; Reddy et al. 2011); however, others believe that as landfill age increases, c decreases and φ increases (Chen et al. 2009; Ramaiah et al. 2017; Abreu et al. 2017).
The moisture content of MSW in landfills in different regions varies, and the moisture content of MSW in the same landfill can change over time. For the many landfills that have adopted bioreactors, the moisture content of MSW also increases during leachate recharge. A change in the moisture content can greatly affect the shear strength parameters of MSW. However, there are relatively few studies related to the relationship between MSW shear strength parameters and moisture content, and there is no unified conclusion. For example, Zhu et al. (2012) and He et al. (2018) found that MSW c increases and φ decreases with increasing moisture content, whereas Zhao et al. (2014) found that as the moisture content increases, c first increases and then decreases, and φ increases. Moreover, the existing research examines only a small range of moisture content, which is insufficient for determining the effect of low-to-high moisture content on the strength properties of MSW. Therefore, it is necessary to study the moisture content as a variable. In this study, we not only consider the effect of variation in moisture content on the shear strength of MSW but also analyze the relationship between the shear strength, moisture content, and landfill age.
2.1 Materials
The landfill in Xi'an, which started in 1994, covers an area of approximately 687,337 m² with a storage capacity of 49 × 106 m3. It is responsible for 90% of landfill waste treatment in Xi'an (covering all urban areas), with a daily waste output of over 10,000 t.
The test materials obtained by drilling samples from the landfill in Xi'an are shown in Fig. 1. MSW with landfill ages of 1–3 y, 11–13 y, and 21–23 y was defined as low-age, medium-age, and high-age MSW, respectively.
The MSW samples (5 kg) were placed into an oven at 65 °C, and the samples were dried for 24±1 h, to a constant weight weighed on a balance. The components were then separated using manual sorting, and the percentage of each component was calculated. The paper content in MSW in China is minimal, and in manual sorting, paper is difficult to identify; thus, it is classified among indistinguishable materials.
The influence of composition on shear strength is mainly dependent on the content of reinforced and basic materials. In this study, textiles, plastics, and wood are classified as reinforced materials, and metals, stones, rubber, glass, indistinguishable materials, and soil as basic materials. An increase in moisture content increases the compressibility of MSW (Xu et al. 2019); therefore, textiles and plastics are classified as high-compressibility components, and the remaining materials are low-compressibility components. The MSW composition at each landfill age is shown in Table 2.
|
Age(y) |
Composition (%) |
||||||||
|---|---|---|---|---|---|---|---|---|---|
|
Plastic |
Rubber |
Textile |
Wood |
Stone & ceramics |
Glass |
Metal |
Indistinguishable Materials |
Soil (< 0.5 mm) |
|
|
1 |
45.49 |
0.81 |
3.6 |
1.67 |
28.2 |
0.85 |
0.5 |
2.88 |
16 |
|
2 |
37.67 |
0.03 |
11.01 |
17.14 |
13.21 |
2.2 |
1.43 |
5.48 |
11.83 |
|
3 |
30.11 |
0 |
12.25 |
5.84 |
14.88 |
1.66 |
3.93 |
5.77 |
25.56 |
|
11 |
18.1 |
5.77 |
1.39 |
5.29 |
24.66 |
1.8 |
0.68 |
9.74 |
32.57 |
|
12 |
22.17 |
2.8 |
4.71 |
9.68 |
9.99 |
5.6 |
3.2 |
8.07 |
33.78 |
|
13 |
17.89 |
2.79 |
2.67 |
6.29 |
15.78 |
3.6 |
1.68 |
8.67 |
40.63 |
|
21 |
8.81 |
0.19 |
0.53 |
1.4 |
29.66 |
0.85 |
1 |
6.64 |
50.92 |
|
22 |
5.26 |
0.14 |
0.76 |
2.32 |
22.39 |
0.73 |
0.25 |
8.42 |
59.73 |
|
23 |
10.98 |
3.41 |
4.21 |
3.39 |
9.09 |
2.32 |
3.47 |
7.72 |
55.41 |
2.2 Direct shear test apparatus
In this study, a direct shear (DS) apparatus was used (Fig. 2), with a stepper motor, a continuous variable, and an input speed within the range of the test protocol for shearing. Furthermore, the apparatus had a relatively large box with a diameter of 185 mm (upper and lower shear box heights of 40 and 20 mm, respectively).
2.3 Test process
Direct shear tests were conducted on MSW with landfill ages of 1, 2, 3, 11, 12, 13, 21, 22, and 23 y for this study. The tests were carried out in accordance with the technical specifications for geotechnical waste tests (CJJ/T 2013), and the maximum particle size was not larger than 1/8 of the diameter of the test apparatus, i.e., 185/8 = 23.125 mm. To fully utilize the reinforced materials, the waste was screened and manually sheared to control its length at approximately 20 mm.
The sample preparation and test procedure were performed by using a 1 y old landfill, which comprised a 20% moisture content, as the control. The specific steps were as follows: a 1000-g MSW sample was dried in an oven at 65 ℃ for 24 ± 1 h, to a constant weight. Subsequently, 200 ml of purified water was sprayed evenly on the sample with a watering can, while the sample was turned continuously. Finally, the sample was stored for 24 ± 1 h. The two shear boxes on the top and bottom of the direct shear test apparatus were aligned, two fixed pins were inserted, and the prepared MSW was filled into the shear box in layers, with each layer filled in equal amounts. A simple tamping hammer was then used to evenly compact the sample and ensure the same plane. The vertical pressure corresponding to each moisture content (natural, 20, 40, 60, 80, and 100%) was 25, 50, 100, 150, 200, and 300 kPa, respectively. After compressing the sample for 24 ± 1 h under vertical pressure, the fixed pins were removed, and the shearing test was performed. In this study, these tests were strain controlled, and the test rate was 1 mm/min (Abreu et al. 2017). The horizontal load was recorded each time the sample produced 0.5-mm shear horizontal displacement. The test ended when the ratio of the shear horizontal displacement to the sample diameter reached 20%.
3.1 Physical properties of MSW
The physical properties of MSW at each landfill age were tested. The density of the sample was obtained using the sample bucket weighing method. Then, MSW (5 kg) was taken from each bucket, divided into two equal portions, and baked in an oven at 65°C for more than 24 h to reach a constant weight. The two portions were then weighed to obtain the moisture contents (dry weight basis), and the average moisture content was obtained. A 50-g sample was prepared according to the composition, and the specific gravity was determined using the volumetric flask method. The density, moisture content, and specific gravity of MSW at each landfill age are shown in Fig. 3.
As can be seen in the figure, the density of MSW increases with an increase in landfill age; this is because with an increase in landfill age, the depth and vertical pressure increase, thereby causing an increase in density. The natural moisture content of MSW does not appear to have a significant relationship with landfill age. The specific gravity of MSW increases with an increase in landfill age, which is due to the high organic content of MSW in the Xi'an landfill; as the landfill ages, the organic materials decompose, whereas the inert material content increases; thus, the specific gravity increases.
3.2 Organic content
Organic matter tests were conducted on MSW at various landfill ages using a muffle furnace, and the relationship between organic content and landfill age is shown in Fig. 4. The figure indicates that the MSW contained a large amount of organic materials, especially the low-age MSW; however, with an increase in landfill age, the organic materials gradually decomposed, and the lowest organic matter content was 8.91%. The organic content of some high-age MSW was higher than that of low-age MSW, which indicates that the organic content is not only influenced by the landfill age but also related to the composition and degradation conditions (moisture content, temperature, microbial activity, etc.). For example, Gurijala et al. (1993) found that MSW degradation was faster when the moisture content was close to the field moisture capacity. Kumar et al. (2019) found that the rate of MSW degradation increased when the temperature was increased from 20 to 50 ℃. Therefore, the change in organic content is influenced by several factors, one of which is landfill age.
3.3 MSW field moisture capacity
A permeability and compression correlation tester was designed and built to conduct field moisture capacity tests. Field moisture capacity can be expressed in three ways: dry weight, wet weight, and volumetric. In this study, dry weight field moisture capacity was adopted. The relationship between field moisture capacity and landfill age is shown in Fig. 5. The moisture in waste is different from that in soil because it exists not only in the form of bound water and free water but also as intracellular water stored inside the organic cells, which cannot be transferred under the action of gravity or air pressure and can greatly affect the field moisture capacity of MSW. As the landfill age increases, the organic content decreases, the intracellular water content decreases, and the field moisture capacity decreases.
3.4 Shear stress–displacement behavior
As an example, the shear stress–displacement behavior of MSW at landfill ages of 1 and 21 y and moisture contents of 20% and 80% is shown in Fig. 6. The shear stress–displacement relationship exhibits the displacement hardening phenomenon. During the test, some samples showed a sudden rise or fall in shear stress, which may have been caused by overstretching or pulling the reinforced materials in shear.
3.5 Relationship between shear strength of MSW and landfill age
The shear stress of MSW exhibits a displacement hardening state, making it difficult to obtain a peak value. Gao et al. (2010) concluded that a landfill destabilizes after the MSW strain reaches a certain level. At the maximum strain that an internal pipe network can withstand, the damage strain of MSW is often taken as 10–15% when considering the combined deformation of the domestic waste and the bottom liner of the landfill (Feng et al. 2005). In this study, according to the deformation that the landfill can withstand, the shear strength corresponding to a sample strain of 10% was selected as the breaking strength for this test.
The relationship between the shear strength of MSW and landfill age is shown in Fig. 7. When the vertical pressure is low, the relationship between the shear strength of MSW and landfill age is not obvious, but with an increase in the vertical pressure, the overall shear strength of MSW increases with the landfill age; the higher the vertical pressure, the greater the impact of landfill age on the shear strength of MSW. This is because as the MSW landfill ages, c decreases, and φ increases. According to Coulomb theory, φ dominates in the process of increasing vertical pressure; thus, the shear strength of MSW tends to increase with an increase in MSW landfill age.
3.6 Relationship between shear strength of MSW and moisture content
The relationship between shear strength of MSW and moisture content at each landfill age is shown in Fig. 8. With an increase in moisture content, the shear strength of low-age MSW first increases and then decreases.
However, when the moisture content of low-age MSW is lower than the optimum moisture content, an increase in moisture content allows the reinforced materials to become better interspersed in the composition; thus, the reinforced materials can provide better tensile strength, and the shear strength increases. Additionally, an increase in moisture content increases the compressibility of the sample, and the sample becomes more compact under the same vertical stress; thus, the shear strength increases.
When the low-age MSW moisture content is higher than the optimum moisture content, under the action of vertical pressure, a portion of the gravity water flows out from the shear surface, which produces a lubricating effect on the materials on the shear surface, allowing them to easily slide. Meanwhile, the remaining water in the sample lubricates the reinforced materials in shear, allowing them to easily slide with upper and lower shear box displacement; thus, the shear strength reduced.
The optimum moisture content of low-age MSW is 60–80%. When the moisture content is close to the field moisture capacity, water exists mainly in the forms of bound and intracellular water. The reinforced materials can provide the maximum tensile strength, and the materials on the shear surface can provide the maximum friction and occlusion force.
The reduced reinforced materials and lower percentage of highly compressible materials in the middle- and high-age MSW compared with those in the low-age MSW make it harder for the moisture content to increase the shear strength through these two aspects. The materials on the shear surface may slide more easily to reduce the shear strength due to the lubrication effect of water. This makes the effect of increasing moisture content on shear strength full of uncertainty.
3.7 Relationship between shear strength parameters of MSW and landfill age
The relationship between shear strength parameters of MSW and landfill age is shown in Fig. 9. As landfill age increases, c decreases, and φ increases. This is a result of the change in composition; as the MSW landfill age increases, the amount of reinforced materials, such as plastic, that provide c decreases, whereas the components, such as indistinguishable materials, that provide φ increase. Low-age MSW has a higher plastic content, and plastic has a smaller φ; thus, the overall φ is lower.
3.8 Relationship between MSW shear strength parameters and moisture content
The relationship between shear strength parameters of MSW at each landfill age and moisture content is shown in Fig. 10. With an increase in the moisture content, the c and φ of low-age MSW first increases and then decreases.
When the moisture content of low-age MSW is less than the optimum moisture content, an increase in moisture content allows the reinforced materials to become better interspersed between the remaining composition and, thus, increases c. Additionally, an increase in moisture content increases the compression of the sample and, thus, increases φ.
When the low-age MSW moisture content is greater than the optimum moisture content, under the action of vertical pressure, a portion of the gravity water flows out from the shear surface; the remaining water acts as a lubricant on the materials, allowing the reinforced materials to easily slide with the upper and lower shear box displacement during shear, thus reducing c, and allowing the materials on the shear surface to easily slide, thus reducing φ.
Middle- and high-age MSW have fewer reinforced materials, fewer high-compressibility materials, and lower field moisture capacity than low-age MSW, which makes the effect of increasing moisture content on c and φ full of uncertainty.
3.9 Relationship between shear strength parameters–moisture content–landfill age for low-age MSW
The relationship between shear strength parameters–moisture content–landfill age for low-age MSW is shown in Fig. 11. With an increase in the moisture content, c and φ of low-age MSW first increase and then decrease. The relationship between shear strength parameters, moisture content, and landfill age can be fitted as a nonlinear surface. The fitting equation is shown in Eq. (1), and the fitting correlation coefficients R2 of the c and φ equations are 0.75 and 0.72, respectively. The degree of fit is relatively high.
where ω is the moisture content (%) and y is the landfill age (y).
4.1 Relationship between MSW shear strength parameters and landfill age
The shear strength parameters of MSW at natural moisture content were selected for comparison with the experimental results of previous researchers, and the relationship between shear strength parameters and landfill age is shown in Fig. 12, where the arrow direction indicates an increase in landfill age. Reddy et al. (2011) indicated that c of MSW increases and φ decreases as the landfill age increases, and Hossain et al. (2009) also suggested that φ of MSW decreases with increasing landfill age. In contrast, Chen et al. (2009), Ramaiah et al. (2017), and Abreu et al. (2017) suggested that c of MSW decreases, and φ increases with increasing landfill age.
The changes in MSW composition were obvious with an increase in landfill age. Gabr et al. (2007) and Hossain et al. (2009) concluded that as landfill age increases, the plastic content increases, and plastic as reinforced materials will increase c; however, plastic has a small φ, which will decrease φ. Bareither et al. (2012) concluded that the increased soil, gravelly, and inert materials in MSW increase φ as landfill age increases. Chen et al. (2009) and Ramaiah et al. (2017) concluded the opposite, suggesting that as landfill age increases, the reinforced materials (plastics and fibers) in the waste decrease, and thus, c decreases, while an increase in the soil-like component increases φ of the MSW. The effect of landfill age on the shear strength parameters of MSW lies more in the change of composition than in the change of the properties of the materials themselves. The results of the present experiment are similar to those of Chen et al. (2009) and Ramaiah et al. (2017) regarding composition and indicate that as landfill age of MSW increases, the proportion of reinforced materials, such as plastic and fiber, decreases, and the proportion of soil increases substantially; thus, c decreases, and φ increases with an increase in landfill age.
The samples in this study were collected from a landfill in Xi'an, which is located in northwest China. The landfill contains a certain amount of stones and ceramics, which are materials with smoother surfaces that will reduce φ. Using 10% strain as the breaking strength of the sample will also make φ smaller.
4.2 Relationship between shear strength parameters of MSW and moisture content
Various study results on moisture content were collected, integrated, and analyzed, and the relationship between MSW shear strength parameters and moisture content is shown in Fig. 13.
Figure 13 shows the inconsistency of the conclusions on the relationship between shear strength parameters and moisture content from studies conducted by various researchers on MSW from different landfills. First, the different conclusions are related to the content of reinforced materials; when there are more reinforced materials, an increase in moisture content will allow the reinforced materials to become better interspersed between the components, thus increasing c. Reddy et al. (2009) and Zhao et al. (2014) had fewer reinforced materials, such as plastic, in their composition; thus, changing the moisture content could not easily increase c by increasing the spatiality. In contrast, the sample examined by Zhu et al. (2012) contained more reinforced materials, and c increased with increasing moisture content, which matches the results of this study in the same moisture content interval.
Second, the different conclusions are related to the content of high-compressibility materials. φ increases with the friction and occlusion force on the upper and lower sides of the shear surface. The sample examined by Zhao et al. (2014) had a large proportion of high-compressibility materials, such as cooking and garden waste, in its composition, and φ increased with increasing moisture content, which matches the results of this study in the same moisture content interval.
Finally, the variability in the conclusions is related to the control of variables, size of the apparatus, and moisture content interval. Bareither et al. (2012) and Karimpour-Far et al. (2018) changed both the test material and moisture content; because the shear strength of MSW is not only related to the moisture content but also to the composition, organic content, landfill age, and other factors, their conclusions were different from this study. When the direct shear apparatus size is changed, the conclusions will be affected; the larger the size of the direct shear apparatus, the more consistent the results will be with the field test. The difference in the moisture content interval adopted by researchers is also a factor that impacts the results.
In this study, the effects of moisture content, landfill age, and vertical stress on MSW stability were considered, and a direct shear test apparatus was used on 324 samples to study the shear strength properties of MSW. The conclusions are as follows: (1) with an increase in horizontal shear displacement, the shear stress of MSW gradually increases without a peak stress phenomenon, which indicates a displacement hardening curve. Changes in moisture content and landfill age do not appear to affect the displacement hardening of MSW; (2) the effect of landfill age on the strength properties of MSW was investigated, and the results indicated that MSW at each moisture content showed an overall trend of increasing shear strength, decreasing c, and increasing φ with an increase in landfill age. With an increase in vertical stress, the increase in landfill age has an increasingly significant effect on the improvement of shear strength; (3) the effect of moisture content on the strength properties of MSW at low ages was investigated, and the test results indicated that the shear strength of MSW first increased and then decreased as the moisture content increased, and the optimum moisture content range was 60–80%. As the moisture content increased, c and φ first increased and then decreased; (4) the effect of moisture content on the strength properties of middle- and high-age MSW was investigated, and the experimental results indicated that the shear strength parameters of MSW at middle and high age were not directly related to the moisture content and fluctuated only due to the change in moisture content; (5) a theoretical model was developed for the low-age shear strength parameter–moisture content–landfill age correlation. This model considers the effects of moisture content and landfill age simultaneously and can simulate and predict the variation in shear strength of MSW with moisture content and landfill age; (6) the c of MSW range is 6.04–18.69 kPa, and the φ range is 10.78–18.26°.
Ethics approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Availability of data and materials
The datasets used and analysed in the current study are available from the corresponding author on reasonable request.
Competing interests
The authors declare that they have no competing interests.
Funding
This research was funded by the National Natural Science Foundation of China (Contract Nos. 51978625 and 51678532), supported by the Zhejiang Provincial Natural Science Foundation of China under Grant No. LZ21E080003, and key research and development plan of Zhejiang Province under Grant No. 2021C02039.
Authors' contributions
MH conducted the direct shear test on municipal solid waste in landfill of Xi'an, and analyze the relationship between the shear strength of MSW and moisture content and landfill age. JL, JZ and HX conducted the direct shear test on municipal solid waste in landfill of Xi'an; ZZ analysed the test data and reviewed and revised the manuscript. All authors read and approved the final manuscript.
Acknowledgements
We thank the Xian Jiangcungou Landfill for their assistance.
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Table 1 is available in the Supplementary Files section.