Failure of an unfilled landfill cell due to an adjacent steep slope and a high groundwater level: A case study

Journal Pre-proof Failure of an unfilled landfill cell due to an adjacent steep slope and a high groundwater level: a case study Shi-Jin Feng, Ji-Yun Chang, Hao Shi, Qi-Teng Zheng, Xing-Yu Guo, Xiao-Lei Zhang

PII:

S0013-7952(19)30704-5

DOI:

https://doi.org/10.1016/j.enggeo.2019.105320

Reference:

ENGEO 105320

To appear in: Received Date:

16 April 2019

Revised Date:

23 September 2019

Accepted Date:

29 September 2019

Please cite this article as: Feng S-Jin, Chang J-Yun, Shi H, Zheng Q-Teng, Guo X-Yu, Zhang X-Lei, Failure of an unfilled landfill cell due to an adjacent steep slope and a high groundwater level: a case study, Engineering Geology (2019), doi: https://doi.org/10.1016/j.enggeo.2019.105320

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Failure of an unfilled landfill cell due to an adjacent steep slope and a high groundwater level: a case study

Shi-Jin Fenga,*, Ji-Yun Changa, Hao Shia, Qi-Teng Zhenga, Xing-Yu Guob, Xiao-Lei Zhanga

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* Corresponding author

Key Laboratory of Geotechnical and Underground Engineering of the Ministry of

Education, Department of Geotechnical Engineering, Tongji University, Shanghai

Shanghai Geotechnical Investigations & Design Institute Company Ltd., Shanghai

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b

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200092, China

200032, China

Chang);

[email protected]

(Hao

Shi);

[email protected]

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(Ji-Yun

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E-mail addresses: [email protected] (Shi-Jin Feng); [email protected]

(Qi-Teng Zheng); [email protected] (Xing-Yu Guo); [email protected]

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(Xiao-Lei Zhang)

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Highlights: 

Systematic post-failure field investigations, finite element modeling and limit equilibrium analyses were performed.



Stability of landfills in regions with soft soils should be thoroughly assessed.



Interactions with surrounding structures deserve special attention for landfill stability.  Minimum safe distance to the surrounding structures should be guaranteed.

Abstract: Stability is a factor of vital importance for landfills, during both construction and operation. Slope failures are likely to occur in landfills located in regions with soft soils and a high groundwater table. In this study, the failure of an unfilled landfill cell, located in a typical coastal soft soil area, was explored. The failure was found to be caused by the instability of a continuous slope formed by the unfilled landfill cell and an adjacent construction waste landfill. A series of post-failure

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field investigations were performed, including cone penetration tests, monitoring of

horizontal displacement of the sliding body, and an investigation of the groundwater. Additionally, finite element modeling and limit equilibrium analysis were applied to

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study the failure mechanism and the factor of safety (FOS), respectively. Both the field

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investigations and the theoretical analyses indicated that the geometrical configuration and the soft substratum provided the basis for the formation of the sliding surface, while

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a rise of the groundwater level due to continuous rainfall and poor drainage condition

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triggered the failure. This case reveals that the stability of landfills in regions with soft soils should be thoroughly assessed, and that appropriate ground treatment is

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necessary to avoid potential sliding along soft substrata. Furthermore, special attention should be paid to determination of the safe distances to surrounding earth

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structures, considering the effect of any ground treatment applied.

Keywords: Municipal solid waste; Landfill; Soft soil area; Slope failure; Field investigation.

1

Introduction Landfilling is one of the most common methods of disposing of municipal solid

waste (MSW), and the slope stability of a landfill is of vital importance during both construction and operation. Some catastrophic slope failure cases in MSW landfills have been reported around the world during the recent decades. These failures can generally be classified into rotational failures and translational failures, depending on

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the sliding mode and mechanism (Qian and Koerner, 2009). For rotational failures, the sliding surface mainly develops within the waste body, as was the case in the

failures which occurred at the Istanbul landfill in Turkey (Kocasoy and Curi, 1995),

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the Hiriya landfill in Israel (Huvaj-Sarihan and Stark, 2008), and the Payatas landfill

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in Philippines (Merry et al., 2005). Translational failures develop completely or partially along the weak geosynthetic interface within the bottom liner system, as

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observed at the Kettleman Hills landfill (Mitchell et al., 1990) and the Mahoning

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landfill (Stark et al., 1998) in North America, and at the Shenzhen landfill in China (Peng et al., 2016). These two modes of failure are generally related to internal

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causative factors, such as high leachate level and low shear strength of the liner system.

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Apart from the two failure modes mentioned above, landfill failure can also be

related to external factors such as incompatible underlying geological conditions, especially in regions with soft soils. For example, the large landfill slope failure that occurred in Cincinnati, Ohio, on March 9th, 1996, with a volume of approximately 1.2 million m3, was mainly caused by the low shear strength of the underlying soft soils

(Eid et al., 2000; Stark et al., 2000; Chugh et al., 2007). Another severe landslide occurred at the Bandung landfill in Indonesia on February 21st, 2005, and was most likely triggered by high water pressure in the soft subsoil (Koelsch et al., 2005). These two cases provide valuable lessons for the design of landfills in regions with soft soils, such as the need to improve the bearing capacity of the foundation using dynamic compaction and vertical drains before constructing a landfill. However, in addition to

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poor geological conditions, other external threats also pose threats to the stability of landfills. One example is the surcharge caused by the newly constructed structures

near the landfill, a topic which, to the best of the authors’ knowledge, has been seldom

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studied.

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This paper describes the investigation of the severe failure of an unfilled landfill cell in Shanghai, China, located near the East China Sea. The area is characterized by

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the presence of soft soils and a high groundwater level. There was a large-scale

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construction waste landfill to the south of the landfill cell, separated by a distance of less than 20 m. Right before the landfill cell was filled with MSW, a severe sliding

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event occurred on August 16th, 2017, and the bottom of the unfilled landfill cell was uplifted by approximately 5 m. Some remedial measures were immediately taken to

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avoid further sliding movement, and these were followed by visual inspections, field investigations and extensive theoretical analyses exploring the failure mechanism and triggering factors. This study will concentrate on the mechanisms of interaction between landfills and the surrounding structures in regions with soft soils, and will provide valuable insights that will be useful in the rational design of landfill.

2

Background information This section will first introduce detailed information about the unfilled landfill

cell and the adjacent construction waste landfill including the plane layout, geological conditions and other design parameters that are needed in the subsequent numerical modeling. The failure event and the temporary remedial measures taken shortly

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thereafter are then presented.

2.1 General conditions

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The location and layout of the study area are shown in Fig. 1. A new landfill cell

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was constructed in the southernmost region of the MSW landfill and separated from the filled area by a waste dam. There was a large-scale construction waste landfill

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surrounded by an 8 m-high earth dam in the southern part of the plot (Fig. 1(b)), and

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the distance between the unfilled landfill cell and toe of the earth dam was about 18 m. Construction waste was continuously dumped into the construction waste landfill until

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August 2017, to a height of around 6.0 m. There were several leachate-regulating reservoirs in the east of the study area, and a piece of farmland to the west.

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The ground condition had been investigated through a systematic field survey

before constructing the unfilled cell, as summarized in Table 1. The ground consisted of, from the surface down, miscellaneous fill (2.2~4.0 m), dredger fill (2.8~4.8 m), sandy silt with silt clay (6.0~7.6 m), muddy clay (6.9~9.5 m), clay (8.3~11.4 m) and sandy silt (1.7~2.8 m). Specifically, there was a 15~21 m-thick clay layer containing

muddy clay and clay. The original field survey report regarded both the layer of sandy silt with silty clay and that of muddy clay as threats to slope stability due to their poor geological characteristics in terms of engineering considerations (high water content, high compressibility and plastic-flow state). As for the hydrological conditions, the groundwater level was at a depth of about 0.5 m and was mainly controlled by atmospheric precipitation and ground evaporation. Precipitation varies greatly by

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seasons in the study area, with most precipitation occurring from June to September.

Figure 2(a) and Fig. 2(b) present the conditions of the unfilled landfill cell before

construction on May 22nd, 2017 and after construction on August 1st, 2017. The earth

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dam of the construction waste landfill had been established for nearly one year before

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construction of the landfill cell. Plastic drainage plates were used to improve the bearing capacity of the subsoil layers beneath the base (Fig. 3(a)), but no measures

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were taken to strengthen the subsoil beneath the side slope of the cell. A typical

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composite liner system of geomembrane/geosynthetic clay liner (GMB/GCL) was applied in the landfill cell, with smooth GMB/GCL on the base (Fig. 3(b)) and

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textured GMB/GCL on the side slope (Fig. 3(c)). Textured GMB was used to minimize potential slippage along the geosynthetic interface of the side slope. On the

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southern side, the liner system was fixed in an anchor trench near the road, and on the northern side was anchored under the waste dam of the filled area.

2.2 Failure event On the afternoon of August 16th, 2017, a severe sliding event occurred in the

study area, uplifting nearly the entire base of the newly constructed landfill cell (Fig. 2(c)) to an elevation higher than that of the adjacent waste dam (Fig. 2(d)), within one minute. The sliding body also seriously bent a section of the road (blue dashed line in Fig. 4(a)), which had previously been straight (red dashed line in Fig. 4(a)), between the unfilled cell and the earth dam, and there were also some significant cracks formed on its surface (Figs. 4(b) and 4(c)). Furthermore, an approximately 1.5 m-wide

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continuous crack was observed on the northern side of the earth dam (Fig. 4(d)). Fig.

5(a) shows a three-dimensional model of the study area after failure, based on a topographical GPS survey, clearly reflecting the horizontal deformation of the road

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and the uplifting of the base. Section 3-3 in Fig. 5(a), which was perpendicular to the

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road and positioned at the middle of the failure area, was selected for the following quantitative analyses. In comparison with the topographical conditions of this section

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before failure, the maximum horizontal deformation of the road reached 8.5 m and the

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maximum uplift was about 5.0 m (red line in Fig. 5(b)). The unfilled landfill cell was

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completely destroyed by the unexpected slope failure.

2.3 Temporary remedial measures

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To prevent further sliding, some temporary remedial measures were promptly

carried out including crack-filling and slope-cutting. Over the first few days after the failure, cracks in roads and the earth dam were filled with cement mortar to avoid rainfall infiltration, and slope cutting was simultaneously conducted, owing to its effectiveness in reducing the sliding force. The top 2 m of the soil of the earth dam

adjacent to the unfilled landfill cell was removed layer by layer using excavators between August 19th and August 28th.

3

Field investigations

3.1 General arrangement Field investigations are of significant importance for monitoring further

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deformation and directly identifying the triggering factors, and by extension for

providing valuable knowledge that may be applicable to similar projects. Hence, the field conditions of the study area after failure were immediately and systematically

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investigated. Since the landfill owner still wanted to use both the unfilled landfill cell

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and the construction waste yard, the integrity of the uplifted liner system and the cracked body of the earth dam needed to be maintained. The geological conditions of

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the unfilled landfill cell had been investigated before construction, but without regard

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to the south side of the cell. Therefore, the post-failure investigations were conducted within the area between the unfilled landfill cell and the earth dam (Fig. 1(b)) to avoid

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puncturing the liner system at the bottom. As an efficient method to assess the physical and mechanical properties of soil,

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cone penetration tests (CPTs) were conducted on the southern side of the unfilled landfill cell (Fig. 1(b)). Both surface deformation and horizontal displacement at depth were monitored after the failure. Although there were a great number of field test points measured in the post-failure investigations, only five representative sections were selected for illustration and analysis in the following part of the

investigation. Each section included two CPT points, one surface displacement monitoring point and one deep horizontal displacement monitoring point (serial numbers in Fig. 1(b)).

3.2 Cone penetration tests (CPTs) In CPTs with a single bridge, the metal detector was pressed into the soil using a

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mechanical device under static pressure, during which time the specific penetration

resistance (ps) was recorded. ps has been proven to be an effective reference for

physical and mechanical properties of soils (Blight and Fourie, 2005; Feng et al., 2014;

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Tan and Lu, 2017). Figure 6 displays the variation of ps with depth for each section;

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according to its mean value, the soil was divided into six layers, which were generally consistent with those listed in Table 1. Although there were significant differences

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between the ps values of the top three layers, the thicknesses and mechanical

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properties of the soil layers below 15 m depth were consistent across all test points, which means that there had not been any disturbance from sliding within this depth

depth.

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range. Thus, it could be speculated that the sliding mainly developed above 15 m

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Table 2 summarizes the mean and standard deviation of ps for each soil layer in

the southern part of the failure area. The mean values of ps after failure (Table 2) were similar to those given in the original investigation report (Table 1), but for the top three layers, especially the layer of sandy silt with silt clay, the standard deviations were extremely high (>0.15), indicating obvious spatial differences. Given that the

soil could be disturbed by sliding, this failure sliding was most likely to occur within the top three soil layers, i.e., the layers of miscellaneous fill, dredger fill and sandy silt with silt clay. The high standard deviation of the sandy silt layer (0.49; layer 6) was negligible for two reasons. First, this layer exhibited a significant change in ps with depth, as shown in Fig. 6, which means that this property cannot be represented by the average value and standard deviation. Second, the overlying muddy clay layer and

3.3 Monitoring of horizontal displacement

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that the sliding surface was at least above layer 4.

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clay layer showed low standard deviations of ps over a uniform thickness, indicating

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Surface deformation was monitored with a total station at five measurement points (D1–D5, Fig. 1(b)) along the road bordering the unfilled area. As shown in Fig.

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7, the horizontal displacement rate of each point significantly declined over the first

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week (August 21st–28th) and then gradually approached zero. This proved the effect of the slope cutting on the top of the earth dam, which was finished around August 28th,

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as evidenced by the decrease in sliding force. Further, the total horizontal displacement at D3 on August 31st (approximately 94 mm) was larger than those of all

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the other four points (each less than 85 mm), revealing that section 3-3 was the most unfavorable section of the study area in terms of stability. Horizontal displacement with depth was measured by an inclinometer in a drilled hole. This method can facilitate the monitoring of sliding development and the identification of the sliding surface (Stark et al., 2000; Peng et al., 2016; Ma et al.,

2019). The incremental increases in horizontal displacement significantly decreased from August 24th to August 28th, reaching approximately zero after August 30th (Fig. 8), which is consistent with the results presented in Fig. 7. As the depth increased from 0 to 36 m, each measured horizontal displacement decreased slightly at first, increased to the maximum value at a depth of 7~9 m (within the layer of sandy silt with silt clay), and finally decreased significantly to almost zero at depths below 30 m

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(Fig. 8). Thus, it can be inferred that the sliding surface was located in the layer of

sandy silt with silt clay (7~9 m depth), and the sliding therein hauled both the muddy

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clay layer and the clay layer.

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3.4 Groundwater conditions

Groundwater distribution is essential for assessing the stability of landfills and

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similar structures in the earth (Koerner and Soong, 2000; Jiang et al., 2010; Feng et al.,

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2018b), and hence the groundwater level in the study area was measured using a water level gauge within an inclinometer pipe (Peng et al., 2016). In the unfilled

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landfill cell, in order to maintain the integrity of the liner system, the groundwater level was not directly measured, but rather inferred from the design documents and

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operation logs provided by the landfill owner. Data showed that the groundwater level under the unfilled landfill cell was maintained at 1 m below the liner system before failure, using inserted plastic drainage plates. A puddle along the northern part of earth dam (Fig. 9(a)) was observed at the southern part of the unfilled landfill cell, indicating a near-surface groundwater level

at the toe of the dam. This was attributed to the naturally high groundwater level (about 0.5 m below the surface, as mentioned in section 2.1) and continuous rainfall for nearly two weeks beginning on August 4th, 2017 (Fig. 9(b)). A similar but worse phenomenon was observed in the construction waste landfill, because waste with relatively high water content, such as construction garbage, dregs and slurry, were dumped in this non-standard disposal site lacking a drainage facility. Thus, there was

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a difference in the depth of the groundwater head of approximately 6.0 m between the northern and southern sides of the earth dam during the failure. As evidence of this,

during the post-failure investigation, the groundwater erupted from the inclinometer

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pipes (e.g., at CX3 in Fig. 9(a)) after installation, and continued to be expelled for

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several days. The stopping of the groundwater eruption might be attributed to the decrease in the groundwater level after failure, due to stoppage of the rainfall as well

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as seepage and evaporation.

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Overall, some key information was obtained from the above field investigations. First, the sliding mainly occurred within the top three soil layers, and the layer of

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sandy silt with silt clay played the role of the soft substratum, facilitating the failure. Second, the development of horizontal deformation was temporarily stopped through

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the slope cutting at the top of the earth dam. Third, the groundwater level within the construction waste landfill was very high before the failure event and there was a high groundwater head difference between two sides of the earth dam, both of which were factors extremely detrimental to slope stability. The field investigations clearly revealed the scale and characteristics of this failure event, and further theoretical

analyses were then conducted to explore the failure mechanism and identify the triggering factors.

4

Stability analyses

4.1 Numerical modeling Based on field investigations, the sliding in this case developed from the

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construction waste landfill to the unfilled MSW landfill cell, without significant deformation along the earth dam nor the southern edge of the unfilled cell, which means that this failure could be considered to be the result of a problem with plane

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strain perpendicular to the earth dam. Therefore, a two-dimensional finite element

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model (FEM) of the most typical section, section 3-3, was then established using the commercial software PLAXIS (Brinkgreve, 2002). The Mohr-Coulomb model was

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used to simulate several materials in this numerical study, including the stratified

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underlying soils (Table 1), MSW, the earth dam, construction waste, and 0.6 m-thick gravel above the liner system (Table 3). The geotechnical properties of the MSW in

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this landfill, the earth dam, and the construction waste were determined according to the literature (Sivakumar et al., 2004; Chen et al., 2014; Feng et al., 2018b; Zheng et

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al., 2019).

Given the significant deformation of the geosynthetics within the liner system

during the failure, detailed interactions between the geosynthetic elements were neglected and only the geomembrane (GMB) was modeled by flexible elastic elements in PLAXIS, as this material is much more sensitive to tension. For the GMB

used in the landfill cell, its elastic axial stiffness, EA, which is the ratio of the axial force per unit width to the axial strain, was provided by the manufacturer (Table 3). In the numerical model, the upper and lower interfaces of the simplified liner system were modeled as zero-thickness interface elements, considering the interactions with other geosynthetics, i.e., the geonet (GN) and the geosynthetic clay liner (GCL), following the Mohr-Coulomb criterion (Zania et al., 2010; Feng et al., 2018a; Chen et

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al., 2019). For the interface types GN/GMB and GMB/GCL shown in Fig. 3(b) (smooth GMB) and Fig. 3(c) (textured GMB), Table 3 gives the typical values of their

peak shear strength parameters (cpeak, φpeak) which were used in the following analysis

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according to their suggested ranges in the literature. The peak shear strength

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parameters of the geosynthetic interfaces were selected because the relative displacement of the interfaces was still limited in this newly constructed landfill cell.

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To model the anchoring of the liner system in the numerical model, the relative

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deformations between the GMB and the surrounding soil in the left- and right-hand anchorage regions were restricted by linking their nodes together.

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To explore the failure mechanism involved in this case, the entire construction process was simulated according to the engineering log. The balance of geostatic

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stress was first obtained in the presence of the earth dam, which had stood for a long time prior to the event, as shown in Fig. 2(a). To capture the development of the slope failure, the construction waste landfilling process within the earth dam was divided into three stages. In each stage, a 2 m-thick construction waste layer was added, with the groundwater level at the surface. Based on field investigations, the groundwater

level in the area between the landfill cell and the earth dam was 0.5 m below the ground surface, while it was 1 m below the surface within the landfill cell. Figure 10 displays the incremental horizontal displacement at the end of each stage, and the development of a sliding body underneath the earth dam and the side slope of the unfilled landfill cell can be observed. In stage 3, a dramatic increase in horizontal deformation occurred directly below the side slope of the unfilled landfill

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cell, and the sliding surface was mainly located in the layer of sandy silt with silt clay

(the region marked in red in Fig. 10(d)). The calculated distribution of horizontal displacement with depth at position CX3 is compared with the measured data

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obtained in the field tests in Fig. 11. The two displacement distributions were of

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different scales for the following three reasons: FEM modeled the entire sliding process including stages 1–3, while the field investigation was conducted after the

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failure event (after stage 3); the displacements which were measured after the failure

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event had been affected by remedial measures; FEM cannot accurately model a large deformation since high-degree distortion of the mesh will lead to numerical

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divergence. However, both the measured and calculated horizontal displacements showed dramatic increases within the layer of sandy silt with silt clay, revealing that

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this weak layer played an important role in this failure. Figure 12 displays the tensile strain of the GMB calculated at different stages,

with the left anchorage point represented by 0 m on the horizontal axis. There was a zone with a significant concentration of tensile strain in the GMB at a position 40 m from the left anchorage point (Fig. 12), which corresponds to the junction of the side

slope and the cell base and also to the exit of the sliding surface, as shown in Fig. 10. From this perspective, the tension concentration was caused by the large deformation of the underlying subsoil on the sliding surface. Another less-significant tension concentration was observed adjacent to the right anchorage point under the waste dam, which was caused by the uplifted base of the unfilled cell. The maximum tensile strain of the GMB, as shown in Fig. 12, was about 4.5%, which is far below its yield

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strain (generally 8~13%) (Qian et al., 2002; Giroud, 2005; Feng et al., 2019a). Hence, the liner system of the unfilled cell may not have been seriously damaged by the sliding event, and may still function well in the future, although this still requires a

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detailed in-situ check and reassessment.

4.2 FOS calculation

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FEM revealed the stress and strain states as well as failure development, as

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discussed in section 4.1, but is not able to indicate the factor of safety (FOS). To deepen the understanding of the failure mechanism involved in this case, the

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traditional limit equilibrium was obtained to calculate the FOS of the five representative sections shown in Fig. 1(b). The M-P method (Morgenstern and Price,

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1965) was used since it can satisfy both force and moment equilibria with consideration of both shear and normal interslice forces. Figure 13 displays the calculated FOS values of the different sliding surfaces in sections 2-2, 3-3, and 4-4, with start and exit ranges given, and the surface with the minimum FOS value, defined as the critical surface, highlighted in white. The critical

slip surface in each section was mainly located within the layer of sandy silt with silt clay, with exits near the toe of the side slope of the unfilled landfill cell, and the shapes were approximately consistent with those calculated by FEM shown in Fig. 10. The FOS values of sections 3-3 and 4-4 were both lower than 1.0, indicating a high possibility of slope failure, while section 2-2 had a FOS slightly greater than 1.0 but still far below the value required in practice. Thus, the above limit equilibrium

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analysis indicates that the continuous slope formed by the unfilled landfill cell and the

adjacent earth dam had already been unstable before the sliding event, since the height

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4.3 Determination of the sliding surface

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of the landfilled construction waste reached 6 m.

Both the numerical modeling and limit equilibrium analyses revealed that the

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sliding surface was primarily located in the layer of sandy silt with silt clay, which is

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consistent with the post-failure monitoring of horizontal displacement at depth described in section 3.3. Thus, the approximate sliding surface within section 3-3 was

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identified (Fig. 14), and was in decent agreement with the position of the crack on the earth dam and the measured maximum horizontal displacement at depth at CX3.

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However, the observed range of uplift of the base of the unfilled cell extended beyond the identified sliding body (red dashed line in Fig. 14) due to the existence of the liner system. A significant but local uplift should likely have occurred at the exit of the sliding surface if the liner system had not been present, but the geosynthetics in the liner are highly resistant to tension, thus resulting in the overall uplift of the cell.

4.4 Causes of failure It has been revealed that the continuous slope already had a high possibility of failure in this case, but the geometrical configuration and geotechnical parameters had remained unchanged for approximately one month before the failure. Thus, there must have been another triggering factor, which is theorized to have been the change in the

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groundwater level due to continuous rainfall as shown in Fig. 9(b). The groundwater levels under the unfilled landfill cell (1 m depth) and the road (0 m, at the surface)

adjacent to the earth dam had remained essentially unchanged before the failure,

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according to engineering logs and the field investigation as shown in Fig. 9(a).

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However, within the construction waste landfill, the groundwater level was already relatively high due to the dumping of construction waste with high water content into

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the landfill which lacked appropriate drainage facilities. The level then rose

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significantly due to the continuous rainfall, which in turn would have led to the generation of high pore pressure and a decrease in the effective stress of the

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substratum. A sensitive analysis on this factor is presented in Fig. 15, and the results indicate that the FOS values of the five sections in Fig. 1(b) decreased linearly by

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0.03–0.04 as the groundwater level rose from a depth of 6 m to 0 m (at the surface). Particularly, the FOS values of sections 3-3, 4-4, and 5-5 were near 1.0, or even below 1.0 in the worst case. Hence, it could be concluded that the failure was triggered by the rise of the groundwater level within the construction waste landfill resulting from the continuous rainfall and the poor drainage conditions.

5

Discussion

5.1 Characteristics of typical MSW landfill failures Some representative failure events of MSW landfills which have occurred in the past few decades are detailed in Table 4. There have been five previous failures involving landfills built upon soft substratum, in the USA, Philippines, and Indonesia,

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illustrating the detrimental effect of soft substrata on landfill stability and thereby

further supporting the conclusions made in this study. However, none of these previous failures were the result of interactions between the landfill and surrounding

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structures, indicating that this case may be of value as a reference since this problem

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will become increasingly common with growing land shortages.

The failure studied in this paper was caused by a sliding motion from the

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construction waste landfill toward the unfilled MSW landfill cell, primarily through

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the layer of sandy silt with silt clay. Two detrimental factors, specifically the soft substratum and the adjacent engineering structure, exerted a coupled influence in this

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case, and thus a parametric study was conducted to explore the influence of the interactions between the two factors. As illustrated in Fig. 14, the coupled influence

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was characterized by the distance to the unfilled MSW landfill cell combined with the shear strength parameters of the layer of sandy silt with silt clay. Following the Mohr-Coulomb strength criterion, the shear strength of the soft substratum can be normalized using the effective normal stress (σn−u) as follows:

 n  u

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c'  tan  ' n  u

(1)

where τ is the shear strength, σn is the normal stress, and u is the pore water pressure. Only the variation of the effective friction angle (φ') was considered in the parametric study since the normalized cohesion (the underlined part in Eq. (1)) is relatively negligible. The FOS value of the continuous slope is closely related to both the distance between the unfilled MSW landfill cell and the construction waste landfill (d), and the

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shear strength of the soft substratum (tanφ') (Fig. 16). For the failure case studied (d = 18 m, tanφ' = 0.24), the resultant FOS value of less than 1.0 is marked by the solid

grey square in Fig. 16. In order to reach a FOS value of 1.2, a distance of 29 m and

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tanφ' of 0.24 would be required. This required distance could be shortened to 18 m if

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the substratum between the two landfills had a tanφ' value of 0.36. Therefore, two approaches could be adopted to avoid possible instability caused by the soft

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substratum and surrounding structures. The first option is to ensure a safe distance

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between the landfill and existing structures, with softer substrata requiring a larger safe distance. If the safe distance cannot be satisfied, then the other method is to treat

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the ground between the two structures.

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5.2 Implications

Many landfills are located in or near developed and populated coastal areas

around the world, which means that a soft substratum is a common threat to landfill stability. Based on the analyses described herein, this specific failure was mainly caused by human errors; specifically, not enough attention was paid to the soft

substratum and the slope was not stable enough, which led to the deficient ground treatment. This case thus provides three implications for the design and management of landfills in regions with soft soils, from a geotechnical point of view. First, the stability of MSW landfills built on soft substrata should be thoroughly assessed, and appropriate ground treatments should be applied when necessary, to avoid potential sliding along a soft substratum. Second, other structures in the earth near a landfill

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deserve special attention, ensuring a safe distance between the landfill and other

structures, or applying a treatment to the substratum between them when the recommended safe distance is not feasible. The safe distance is closely related to the

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shear strength of the soft substratum, as illustrated in Fig. 16. Third, man-made

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construction waste landfills face significant risks due to non-standard designs, such as the absence of an appropriate drainage system, especially in China. Apart from the

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case described in this paper, the catastrophic landslide of the construction waste

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landfill at Guangming, Shenzhen, China (Feng et al., 2019b) was another typical

6

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example of such an issue.

Conclusions

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Stability is a major problem for MSW landfills in areas with soft soils, and thus

deserves special attention during its entire life cycle. In this work, detailed field investigations were conducted in combination with numerical simulations and limit equilibrium analyses to systematically investigate the severe failure of an unfilled landfill cell in Shanghai, China. Based on the geotechnical and hydrological

conditions revealed by the post-failure field investigations, the failure mechanism and triggering factors were explored theoretically. Some major conclusions that were drawn are as follows. First, the failure of the unfilled landfill cell was the result of the instability of a continuous slope formed by the landfill cell itself and an adjacent construction waste landfill. The base of the landfill cell was uplifted by a maximum of around 5.0 m, and

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horizontal deformation of the road induced by the sliding reached 8.5 m.

Next, the sliding surface was found to have developed mainly within the layer of

sandy silt with silt clay, and the failure was believed to have been triggered by a rise

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in the groundwater level within the construction waste landfill due to continuous

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rainfall and poor drainage conditions.

Finally, the stability of MSW landfills built on areas with soft soils should be

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thoroughly assessed to avoid potential sliding along soft substrata, and special

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attention should be given to determination of the safe distance to surrounding earth

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structures, further considering the effect of ground treatment when necessary.

Conflict of interest

Jo

The authors declared that they have no conflicts of interest to this work.

Acknowledgments Much of the work described in this paper was supported by the National Natural Science Foundation of China under Grant Nos. 41725012, 41572265 and 41661130153,

the Newton Advanced Fellowship of the Royal Society under Grant No. NA150466, and the Shanghai Science and Technology Innovation Action Plan under Grant No. 18DZ1204402. The writers would like to greatly acknowledge all these financial supports and express their most sincere gratitude.

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Figure and Table Captions Table Captions Table 1 Properties of the soil layers according to the geotechnical investigation report. Table 2 Values of ps for each soil layer in the southern part of the failure area. Table 3 Parameters of materials and interfaces used in the numerical analysis. Table 4 Slope failures of MSW landfills in recent decades.

Figure Captions

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Figure 1. Unfilled landfill cell: (a) general conditions; (b) detailed layout. Figure 2. A brief history of the unfilled landfill cell: (a) before construction; (b) after construction; (c) and (d) after failure.

Figure 3. Design details of the landfill cell: (a) ground treatment with plastic drainage plates; (b) liner system on the base; (c) liner system on the side slope.

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Figure 4. Failure characteristics on the southern side of the unfilled landfill cell: (a) deformation of the road; (b) cracks in the road; (c) close-up details of the

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cracks in the road; (d) a crack on the earth dam.

Figure 5. General characteristics of the failure area: (a) 3D model based on GPS

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measurements; (b) cross-section of section 3-3.

Figure 6. Variation in specific penetration resistance (ps) with depth: (a) C1-1 and C1-2; (b) C2-1 and C2-2; (c) C3-1 and C3-2; (d) C4-1 and C4-2; (e) C5-1 and C5-2

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(see Fig. 1(b) for locations of each section). Figure 7. Results of monitoring of horizontal displacement at D1–D5 (see Fig. 1(b) for location).

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Figure 8. Variations in horizontal displacement with depth: (a) CX1; (b) CX2; (c) CX3; (d) CX4; (e) CX5.

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Figure 9. Indicators of the high groundwater level: (a) pictures of the puddle and groundwater erupting from an inclinometer pipe; (b) distributions of daily and cumulative rainfall.

Figure 10. Incremental horizontal displacements at the end of (a) the balance of geostatic stress, (b) stage 1, (c) stage 2, and (d) stage 3 of the construction waste landfilling process, as obtained from FEM modeling. Figure 11. Comparison of horizontal displacements at position CX3 (see Fig. 1(b) for location) obtained from numerical analyses and field measurements.

Figure 12. Tensile strain of the geomembrane within the liner system as obtained through numerical analyses. Figure 13. Results of slope stability analyses using the M-P method: (a) section 2-2, (b) section 3-3, (c) section 4-4 (see Fig. 1(b) for location). Figure 14. Determination of the sliding surface in section 3-3 (see Fig. 1(b) for the location of this section). Figure 15. Effect of groundwater depth within the construction waste landfill on the factor of safety (FOS) of the five representative sections (see Fig. 1(b) for locations).

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Figure 16. Effects of normalized shear strength of the soft substratum (tan φ') and the distance between the MSW landfill and the construction waste landfill on the

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factor of safety (FOS) of the continuous slope.

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na

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(a)

(b)

Figure 1. Unfilled landfill cell: (a) general conditions; (b) detailed layout.

(b)

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(a)

(c)

(d)

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Figure 2. A brief history of the unfilled landfill cell: (a) before construction; (b) after

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construction; (c) and (d) after failure.

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(b)

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(a)

(c)

Figure 3. Design details of the landfill cell: (a) ground treatment with plastic drainage

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plates; (b) liner system on the base; (c) liner system on the side slope.

(b)

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(a)

(c)

(d)

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Figure 4. Failure characteristics on the southern side of the unfilled landfill cell: (a) deformation of the road; (b) cracks in the road; (c) close-up details of the cracks in the

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road; (d) a crack on the earth dam.

f oo pr Pr

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na l

(a)

(b)

Figure 5. General characteristics of the failure area: (a) 3D model based on GPS measurements; (b) cross-section of section 3-3.

f oo pr ePr na l

(a)

(b)

(c)

(d)

(e)

Figure 6. Variation in specific penetration resistance (ps) with depth: (a) C1-1 and C1-2; (b) C2-1 and C2-2; (c) C3-1 and C3-2; (d) C4-1 and C4-2;

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(e) C5-1 and C5-2 (see Fig. 1(b) for locations of each section).

30

Total displacement: D1 D2 D3 D4 D5

120

20

80

10

40

0

2017/08/22

2017/08/25

2017/08/28

2017/08/31

Date

Total displacement (mm)

160 Displacement rate: D1 D2 D3 D4 D5

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Displacement rate (mm/d)

40

0

Figure 7. Results of monitoring of horizontal displacement at D1–D5 (see Fig. 1(b) for

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location).

f oo pr ePr na l

(a)

(b)

(c)

(d)

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Figure 8. Variations in horizontal displacement with depth: (a) CX1; (b) CX2; (c) CX3; (d) CX4; (e) CX5.

(e)

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40

Daily rainfall Cumulative rainfall

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35

Slope failure occurred

25

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Daily rainfall (mm)

30

20 15

na

10 5

210 180 150 120 90 60 30 0

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08 /0 3 08 /0 4 08 /0 5 08 /0 6 08 /0 7 08 /0 8 08 /0 9 08 /1 0 08 /1 08 1 /-1 2 08 /1 3 08 /1 4 08 /1 5 08 /1 6 08 /1 7 08 /1 8

ur

0

240

Cumulative rainfall (mm)

(a)

Date

(b)

Figure 9. Indicators of the high groundwater level: (a) pictures of the puddle and groundwater erupting from an inclinometer pipe, (b) distributions of daily and cumulative rainfall.

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Figure 10. Incremental horizontal displacements at the end of (a) the balance of

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geostatic stress, (b) stage 1, (c) stage 2, and (d) stage 3 of the construction waste

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landfilling process, as obtained from FEM modeling.

0

Measured horizontal displacement (mm) 4 8 12 16 20

0

24

Dredger fill 6

Sandy silt with silt clay Calculated results: Stage 1 Stage 2 Stage 3 Measured results after failure: 2017/08/25 2017/08/26 2017/09/13

18 24 30 36

0

30

Muddy clay

Clay

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Depth (m)

12

Sandy silt

60 90 120 150 180 Calculated horizontal displacement (mm)

210

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Figure 11. Comparison of horizontal displacements at position CX3 (see Fig. 1(b) for

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na

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location) obtained from numerical analyses and field measurements.

Tensile strain (%)

4.0

Side slope

Base Stage 1 Stage 2 Stage 3

Anchorage

Anchorage

5.0

3.0 2.0

0.0

0

20

40 60 Position (m)

80

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1.0

100

Figure 12. Tensile strain of the geomembrane within the liner system as obtained

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na

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through numerical analyses.

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(a)

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na

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(b)

(c)

Figure 13. Results of slope stability analyses using the M-P method: (a) section 2-2, (b) section 3-3, (c) section 4-4 (see Fig. 1(b) for location).

f oo pr ePr na l

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Figure 14. Determination of the sliding surface in section 3-3 (see Fig. 1(b) for the location of this section).

Section 1-1 Section 2-2 Section 3-3 Section 4-4 Section 5-5

1.20 Factor of safety (FOS)

Ground surface

1.25

1.15 1.10 1.05

0.95

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1.00

-6 -5 -4 -3 -2 -1 0 Groundwater depth of construction waste landfill (m)

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Figure 15. Effect of groundwater depth within the construction waste landfill on the

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na

lP

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factor of safety (FOS) of the five representative sections (see Fig. 1(b) for locations).

FOS=1.2

f

e-

35 FOS=1.0 30

20

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.05

10

na l

Before failure

0.50

15

0.55

Pr

25

0.45

Distance (m)

40

1.7 1.6 1.5 1.4 1.3 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3

pr

45

oo

FOS

50

tan '

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Figure 16. Effects of normalized shear strength of the soft substratum (tan φ') and the distance between the MSW landfill and the construction waste landfill on the factor of safety (FOS) of the continuous slope.

Table

Table 1 Properties of the soil layers according to the geotechnical investigation report.

1

Miscellaneous fill

2.2~4.0

18.1

2

Dredger fill

2.8~4.8

16.6

3

Sandy silt with silt clay

6.0~7.6

18.2

4

Muddy clay

6.9~9.5

5

Clay

8.3~11. 4

6

Sandy silt

1.7~2.8

Frictio n angle φ', (°) 30.0

1.92

9.0

14.5

0.92

4.0

16.0

2.04

14

11.5

0.62

17.7

17

13.5

0.97

18.5

4.0

29.5

4.42

re

16.6

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na

lP

Note: CPT ps = value of specific penetration resistance as obtained through cone penetration tests.

CPT ps (MPa)

5.0

-p

Soil name

L ayer

Cohesi on c', (kPa)

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Unit weight (kN/m3 )

Thickn ess (m)

Table 2 Values of ps for each soil layer in the southern part of the failure area.

Av

Mean value of ps (MPa)

ps Stan

erage

L Soil layer

C

ayer 1

Miscellan

C 2

2

C 3

C 4

1

2

C

value

geotechnical deviation

5

2

according to dard

(MPa) 1

2.0

1

0.19 .02

Dredger

.81 1

.28 0

.19 1

.80 1

0

2 .03 Sandy silt

.95 2

.12 1

.13 1

.73 1

3 .65

Muddy

.52 0

.37 0

4 .75

.70 1

Clay .00

.80 0

.94

2 .09

0

.85

1

.02

3

na

2

6

0

lP

clay

.87

2

ur Jo

.20

.34

.88

.17

0.62

0.08

0.97

0.49

4.42

1.0 5

3 .10

0.05 8

3.0

Sandy silt

.46

2.04

0.7

1

3

0.45

0

.82

.13

0.92

1.9

0

1

0.15

9

re

with silt clay

0.9

-p

fill

1.92

1

ro of

eous fill

5

report (MPa)

0

Table 3 Parameters of materials and interfaces used in the numerical analysis.

Unit

weight

Cohesion

c

Friction

angle

Material (kN/m3)

φ(°)

(kPa)

20

5.0

40

Construction waste

17

10

25

MSW

9.8

23

20

Gravel

20

5.0

40

GMB

Axial stiffness, EA = 176 kN/m

Interface type

Source

cpeak (kPa)

φpeak (°)

Literature a

1.0-8.2

8.5-29

This paper

2.0

15

-p

GN/GMBS Literature b

8

0.0-8.0

18.9-29.4

This paper

5.0

25

Literature d

0.0-10.9

14.5-24.4

3.0

18

na

lP

Literature c

GMBT/GCL

6.49-11

0.0

This paper

GN/GMBT

0.0

re

GMBS/GCL

ro of

Earth dam

This paper

Jo

ur

Note: GN = Geonet GMBS = smooth geomembrane GMBT = textured geomembrane Literature a: Stark and Poeppel (1994), Fowmes et al. (2008), Shen et al. (2018) Literature b: Koerner and Narejo (2005), Bergado et al. (2006) Literature c: Jones and Dixon (2005), Fowmes et al. (2008) Literature d: Chen et al. (2010), Ross and Fox (2015)

Table 4 Slope failures of MSW landfills in recent decades.

1 988 1 989 1 993 1 996 1 996 1 997 2 000 2 005 2

California,

Sliding along interfaces within the composite liner system Multirotational slope failure along underlying soft clay layer

USA

Yes No Yes

Istanbul, Turkey

Heavy rains, excessive leachate level

No

Cincinnati, Ohio, USA

Softening of underlying native soils

Yes

Mahoning, Ohio, USA

Failure along wet bentonite layer of the unreinforced GCL

No

Hiriya, Israel Payatas, Philippines

Manila,

Steep slopes, lack of drainage controls, high moisture content

No

Failure along MSW and clay subsoil induced by heavy rains

Yes

Bandung, Indonesia Shenzhen, China

High water pressure in the soft subsoil High water level within landfill

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na

008

Kettleman, USA

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984

Failure along tidal marsh induced by rapid rise of phreatic level

USA

-p

1

Soft substratum

Reason description

re

ear

Location

lP

Y

50

Yes No