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Slope stability of bioreactor landfills during leachate injection: Effects of heterogeneous and anisotropic municipal solid waste conditions Rajiv K Giri and Krishna R Reddy Waste Manag Res 2014 32: 186 originally published online 19 February 2014 DOI: 10.1177/0734242X14522492 The online version of this article can be found at: http://wmr.sagepub.com/content/32/3/186

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WMR0010.1177/0734242X14522492Waste Management and ResearchGiri and Reddy

Original article

Slope stability of bioreactor landfills during leachate injection: Effects of heterogeneous and anisotropic municipal solid waste conditions

Waste Management & Research 2014, Vol. 32(3) 186­–197 © The Author(s) 2014 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0734242X14522492 wmr.sagepub.com

Rajiv K Giri and Krishna R Reddy

Abstract In bioreactor landfills, leachate recirculation can significantly affect the stability of landfill slope due to generation and distribution of excessive pore fluid pressures near side slope. The current design and operation of leachate recirculation systems do not consider the effects of heterogeneous and anisotropic nature of municipal solid waste (MSW) and the increased pore gas pressures in landfilled waste caused due to leachate recirculation on the physical stability of landfill slope. In this study, a numerical two-phase flow model (landfill leachate and gas as immiscible phases) was used to investigate the effects of heterogeneous and anisotropic nature of MSW on moisture distribution and pore-water and capillary pressures and their resulting impacts on the stability of a simplified bioreactor landfill during leachate recirculation using horizontal trench system. The unsaturated hydraulic properties of MSW were considered based on the van Genuchten model. The strength reduction technique was used for slope stability analyses as it takes into account of the transient and spatially varying porewater and gas pressures. It was concluded that heterogeneous and anisotropic MSW with varied unit weight and saturated hydraulic conductivity significantly influenced the moisture distribution and generation and distribution of pore fluid pressures in landfill and considerably reduced the stability of bioreactor landfill slope. It is recommended that heterogeneous and anisotropic MSW must be considered as it provides a more reliable approach for the design and leachate operations in bioreactor landfills. Keywords Bioreactor landfill, capillary pressure, leachate recirculation, moisture distribution, municipal solid waste, pore-water pressure, slope stability

Introduction In recent years, bioreactor landfills have emerged as a successful means for the safe disposal of municipal solid waste (MSW). In bioreactor landfills, the collected leachate is recirculated into the MSW, in addition of supplementary liquids, to increase the moisture and result in fast biodegradation of MSW due to enhanced microbial activity (Barlaz et al., 1992; Chugh et al., 1998; Reinhart et al., 2002). Horizontal trenches (HTs), vertical wells, and/or drainage blankets are employed as leachate recirculation systems to recirculate leachate in bioreactor landfills. Constant injection pressure for a specified time period is needed to add moisture to the landfill (Xu et al., 2012). However, high injection pressures in leachate recirculation systems near the side slopes can generate excess pore fluid (i.e. water and gas) pressures and reduce the shear strength of the MSW due to decreased effective stress, which may endanger the stability of the landfill slope. Engineered landfill design and operation should consist of a careful assessment of landfill slope stability. Often,

these landfills are constructed near highly populated areas, which further increases the risk associated with landfill slope failure. Slope stability analyses of conventional landfills based on the geotechnical properties of MSW and underlying soils (i.e. unit weight, shear strength) have been reported in the literature (Eid et al., 2000; Mitchell et al., 1990; Gharabaghi et al., 2008; Zhan et al., 2008). In recent years, analyses of landfill slope stability during leachate operations have been getting more attention. For example, Koerner and Soong (2000) studied numerous landfill slope failures to analyse the developed pore-water pressures due to leachate University of Illinois at Chicago, Chicago, IL, USA Corresponding author: Krishna R Reddy, Department of Civil and Materials Engineering, University of Illinois at Chicago, 842 West Taylor Street, Chicago, IL 60607, USA. Email: [email protected]

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Giri and Reddy injection that resulted in slope failures. Kavazanjian and Merry (2005) reported the Payatas landfill failure due to elevated levels of leachate that led to excessive pore-water pressure in the landfilled MSW. Xu et al. (2012) carried out a single-phase flow modelling to determine the effects of pressurized leachate injection on the stability of a simplified bioreactor landfill slope. However, these studies only considered the effects of porewater pressure and neglected the pore gas (air) pressure. Also, the MSW was considered to be homogeneous, which does not represent the true nature of landfilled waste, as the MSW is found to be heterogeneous in nature with hydraulic conductivity, unit weight, and shear strength parameters varying with depth due to overburden stress and degree of decomposition (DOD) (Reddy et al., 2011). Using a numerical two-phase flow model, Reddy et al. (2013) showed that gas pressures can significantly exceed liquid pressure in a typical bioreactor landfill configuration during initial phases of leachate injection. Therefore, it is critical to evaluate the impact on the stability of bioreactor landfill slope particularly in response to moisture distribution and pore liquid and coupled gas pressures generated due to leachate injection using HTs for heterogeneous and anisotropic MSW (HTAW) conditions. In this study, a numerical two-phase flow modelling was used to determine the effects of heterogeneous and anisotropic nature of MSW under elevated injection pressure on the moisture distribution, generation and distribution of pore-water and capillary pressures (i.e. difference in pore gas pressure and water pressure) within the landfill, and, ultimately, the resulting impact on the stability of bioreactor landfill slope. The two-phase flow model validation has been presented elsewhere (Giri and Reddy, 2013) based on previously published studies using a single-phase flow model and stability analyses for a simplified bioreactor landfill configuration incorporating homogeneous and anisotropic MSW (HAW).

Methods Numerical two-phase flow and slope stability model The pores of unsaturated MSW were assumed to be filled with two immiscible fluids: namely the landfill leachate and landfill gas. The two-phase flow model incorporated modelling the flow of these two immiscible fluids (i.e. leachate considered as wetting fluid and landfill gas considered as nonwetting fluid). The capillary pressure is a function of leachate degree of saturation and can be represented using the model of van Genuchten (1980). The flow of leachate and landfill gas was described by Darcy’s law, whereas relative permeability of each fluid is based on leachate saturation by the empirical laws of van Genuchten function (ITASCA Consulting Group, 2011). In the numerical two-phase flow model, the governing equations of unsaturated MSW are given by the linear momentum balance and the fluid mass balance laws and are represented as:



ρ = ρ d + n ( S L ρ L + SG ρ G )

(equation 1)



 ∂q L   S ∂P ∂S  n L L + L  = − i  ∂t   K L ∂t  ∂xi 

(equation 2)



 ∂q G   S ∂P ∂S  n G G + G  = − i  ∂t   K G ∂t  ∂xi 

(equation 3)

where n is porosity, SL is leachate (liquid) saturation, SG is gas saturation, PL is pore liquid pressure, PG is pore gas pressure, ρL and ρG are fluid densities, ρd is matrix dry density, KL and KG are liquid and gas bulk modulus, respectively, and qiL and qiG are flow rate of liquid and gas given by Darcy’s law. The governing equations 1–3 were solved numerically with the Fast Lagrangian Analysis of Continua (FLAC) program using the finite difference method. The detailed mathematical formulations including governing equations related to the two-phase flow model are explained elsewhere (ITASCA Consulting Group, 2011, Reddy et al., 2013). Concurrently, slope stability analyses were performed using FLAC, wherein the strength reduction technique was adopted to compute factor of safety (FOS; Dawson et al., 1999). The Mohr–Coulomb failure criterion was combined with the strength reduction approach for stability analyses. In this approach, the FOS calculation was performed by successively reducing the shear strength parameters (cohesion and friction angle) of MSW until the slope reached on the verge of failure. Further information on the modelling approach and its successful application is presented elsewhere (Giri and Reddy, 2013).

Landfill configurations A two-dimensional bioreactor landfill, 175 m wide and 50 m deep with a side slope of 3:1 (horizontal/vertical), was created in FLAC to investigate the effects of HTAW under pressurized leachate addition. Figure 1a shows the landfill cell, known as base scenario, wherein the leachate was injected through a HT with a continual injection pressure of 49 kPa (i.e. equivalent to a 5-m water column head). The landfill model was considered to be completely filled with a homogeneous and anisotropic waste (HAW) throughout its entire depth for the numerical two-phase flow model validation (Figure 1a). The wet zone contour represented the extent of moisture surrounding the HT due to leachate injection. The top boundary was extended to a width of 25 m away from the side slope. A 0.3-m-thick leachate collection-andremoval system, consisting of free draining granular soil, was assumed to be located at the bottom of the landfill. A HT (1×1 m) was placed at an elevation of 30 m above the base of the leachate collection-and-removal system and at a setback of 30 m from the side slope. The landfill configuration was similar to that reported by Xu et al. (2012), who used the single-phase flow model

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Figure 1.  (a) Simplified bioreactor landfill configuration with homogeneous anisotropic waste (HAW), depicting the base scenario; (b) landfill configuration, depicting heterogeneous and anisotropic waste (HTAW).

SEEP/W and SLOPE/W, respectively, to evaluate pore-water pressures and their impact on slope stability analysis. The conceptual model did not consider the effects of a landfill cover system because this study was mainly focused on pressurized leachate injection and flow through landfilled waste when bioreactor landfill is in active state. Temperature effects, mechanical compression, and infiltrations were not included in the model. In bioreactor landfills, various biochemical processes and waste degradation generate significant amount of heat that result in long-term elevated waste temperatures (Yeşiller et al., 2005). The relatively high waste temperatures (~30–45°C) affect the process of landfilled waste decomposition that further varies the waste physical (e.g. void ratio, deformation) and geotechnical properties (e.g. unit weight, shear strength) as well as hydraulic properties (e.g. moisture content, saturation, pore pressure, leachate infiltration). The effects of elevated temperatures on the aforementioned waste properties are assumed to be even more prominent in bioreactor landfills than conventional dry landfills and should be considered for future research studies. However, the present study did not account for these coupled thermo-hydro-bio-mechanical interactions. To investigate the effects of HTAW conditions, the 50-m-deep landfill model was divided into 10 different layers, each layer

having a depth of 5 m. The unit weights, saturated hydraulic conductivities, and shear strength of MSW varied with depth and are explained in the next section. Figure 1b depicts a landfill configuration taking HTAW scenarios into account.

Material properties Limited data are available on shear strength of MSW and further research is needed to accurately predict the variation in shear strength properties of MSW with depth during leachate recirculation (Reddy et al., 2009a). Variation in unit weight of MSW with depth was given using the relationship proposed by Zekkos et al. (2006):

γ = γi +

z α +βz

(equation 4)

where γ is unit weight at depth z, α and β are modelling parameters for typical MSW, and γi is near surface in-place unit weight. The saturated hydraulic conductivity of MSW decreases with depth due to the increase in normal stress caused by overlying MSW and this can be expressed by the relationship proposed by Reddy et al. (2009b):

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Giri and Reddy Table 1.  Variations of heterogeneous and anisotropic MSW properties considered for model simulations. Layer

Depth (m)

HTAW-1 γ (kN m–3)

kv (cm s–1)

φ (°)

c (kPa)

γ (kN m–3)

kv (cm s–1)

φ (°)

c (kPa)

DOD (%)

γ (kN m–3)

kv (cm s–1)

φ (°)

c (kPa)

0–5 5–10 10–15 15–20 20–25 25–30 30–35 35–40 40–45 45–50

15 15 15 15 15 15 15 15 15 15

1.9×10–3 1.9×10–4 3.9×10–5 1.1×10–5 4.2×10–6 1.8×10–6 8.9×10–7 4.7×10–7 2.7×10–7 1.6×10–7

35

15

12.6 13.5 14.1 14.6 14.9 15.1 15.3 15.4 15.6 15.7

2.4×10–3 2.9×10–4 5.9×10–5 1.7×10–5 5.9×10–6 2.4×10–6 1.1×10–6 5.7×10–7 3.1×10–7 1.8×10–7

35

15

0 10 20 30 40 50 60 70 80 90

14.9 17.2 17.9 18.4 18.7 19.0 19.2 19.4 19.5 19.7

1.9×10–3 1.3×10–4 2.1×10–5 5.2×10–6 1.7×10–6 6.6×10–7 3.0×10–7 1.5×10–7 7.8×10–8 4.4×10–8

35.0 33.9 32.9 31.8 30.8 29.7 28.7 27.6 26.6 25.5

15.0 16.6 18.1 19.7 21.2 22.8 24.3 25.9 27.4 29.0

  10 (Top) 9 8 7 6 5 4 3 2 1 (Bottom)

HTAW-2

HTAW-3

γ, unit weight; kv, vertical hydraulic conductivity; φ, friction angle; c, cohesion; DOD, degree of decomposition.

Table 2.  Unsaturated hydraulic MSW parameters based on Breitmeyer and Benson (2011). Parameter

Value m–3)

Unit weight (kN Inverse of air-entry pressure α (kPa–1) Saturated moisture content φs Residual moisture content φr van Genuchten steepness parameter ‘n’ van Genuchten ‘m’



  σ '  kv = kv 1 +      pa   0

7.8 1.18 0.41 0.03 1.33 0.25

−5.3



(equation 5)

where kv0 is initial saturated hydraulic conductivity at zero normal stress (10–2 cm s–1), kv is saturated hydraulic conductivity under effective overburden of σ′, and pa is atmospheric pressure MSW is heterogeneous and anisotropic in nature. Therefore, to investigate this condition systematically, the bioreactor landfill cell (Figure 1b) was modelled for three different HTAW conditions with varying geotechnical properties with depth, and the results were compared with simplified HAW: •• HAW: For this condition, the MSW properties were assumed to be the same for entire depth of the landfill and were directly adopted from Xu et al. (2012). Unit weight (γ), cohesion (c), friction angle (φ), vertical saturated hydraulic conductivity (kv), and anisotropy (a; kh/kv) were set at 15 kN m–3, 15 kPa, 35°, 10–5 cm s–1, and 10 (Tchobanoglous et al., 1993). •• HTAW-1: The unit weight, anisotropy, and shear strength of MSW were taken to be exactly the same as that of HAW across the landfill cell. However, the saturated hydraulic conductivity for each layer was varied and calculated using equation 5, depending on the estimated overburden stress at the centre of each layer with the MSW unit weight of 15 kN m–3. Table 1 shows the MSW properties for HTAW-1.

•• HTAW-2: This represented a realistic heterogeneous nature of MSW found in bioreactor landfills immediately after placement, for which the unit weight and saturated hydraulic conductivity of MSW were varied with depth due to overburden stress. The unit weight of the MSW at the mid depth (25 m) of the landfill cell was exactly the same as that of HAW (i.e. γ=15 kN m–3) and unit weights for rest of the layers were varied with depth using equation 4. Saturated hydraulic conductivity of each waste layer decreased with depth and was calculated using equation 5. Shear strength and anisotropy (value 10) of MSW were constant throughout the landfill cell (Table 1). •• HTAW-3: Landfilled MSW, in the presence of moisture, undergoes microbial decomposition and this causes change in the geotechnical properties of MSW. To simulate this waste condition, the influence of DOD and overburden stress on the geotechnical properties of MSW were taken into account. To represent a typical state of degradation, a linear variation in the DOD and geotechnical properties of the MSW with depth was considered. It is assumed that the topmost layer had the geotechnical properties of fresh MSW (DOD=0 %), while the bottommost layer was nearly completely decomposed (DOD=95%). This approach has previously been adopted elsewhere (Reddy et al., 2011; Sivakumar Babu et al., 2010). Based on the DOD values, the geotechnical properties MSW at different depths were estimated and are summarized in Table 1. Unsaturated hydraulic parameters were kept constant for all MSW cases and followed the values given by Breitmeyer and Benson (2011) to evaluate the effect of two-phase flow (Table 2). Unsaturated hydraulic properties of the MSW were not varied with respect to the depth because: (1) very little published information is available on the evolution of unsaturated hydraulic properties of MSW as a function of overburden pressure; and (2) unsaturated hydraulic properties have a relatively small impact on the key design parameters at steady-state conditions (Hayder and Khire, 2005; Reddy et al., 2013).

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Initial and boundary conditions and model input parameters Mechanical boundary conditions were applied by fixing the base in both horizontal and vertical directions, so that the lateral and vertical deformations of the landfill at the base were zero. The lateral deformation was restrained on the right side boundary of the model, whereas the side slope was free to move in both directions and the top boundary was free to move only in the vertical direction. Hydraulic boundary conditions were taken into consideration by fixing the pore gas pressure and seepage at the top boundary and at the side slope. The pore gas pressure was atmospheric at the seepage boundary, which was impermeable to the liquid as long as the liquid pressure (water pressure) was negative: the gas pressure was taken as zero at boundary nodes where the condition was not met (ITASCA Consulting Group, 2011). The right-side boundary and the bottom of the landfill model were considered to be impermeable (i.e. free pore pressures and free saturation). All grid points were initially free to vary based on the net inflow and outflow from the neighbouring zones. Pore-water pressure was fixed to zero at the leachate collection-and-removal system to represent the drainage layer. The pore gas pressures were fixed to be zero initially at all grid points in order to establish initial mechanical equilibrium. The initial pore-water pressure was calculated based on the initial gas pressure by default. Thereafter, the gas pressures were set to vary for different flow conditions (ITASCA Consulting Group, 2011). The initial waste saturation of 40% at all grid points and an initial porosity of 40% at all zones were considered.

Model simulations Two-phase flow modelling, presented in this study, was validated based on the published studies using single-phase flow modelling and slope stability analysis under simplified conditions (e.g. HAW). The effects of continuous elevated injection pressures on the stability of bioreactor landfill slope were modelled and the results were compared with the single-phase flow study in terms of: (1) FOS vs. time, and (2) flow rate vs. time. For the validation purpose, continuous injection pressures of 49, 98, and 147 kPa were considered. In addition, the sensitivities of MSW geotechnical properties on the stability of landfill slope were analysed and validated with a two-phase flow model; these results are presented elsewhere (Giri and Reddy, 2013). All simulations were carried out using different continual injection pressures to examine transient leachate distribution until the steady state was reached or the injection time period for which the landfill slope design became unacceptable (i.e. FOSHAW>HTAW-2>HTAW-3. This signified the presence of higher wetted area (moisture distribution) under HTAW-1 and HAW for different injection pressures and

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Figure 3.  Saturation contours (leachate distribution) after 10 years with continuous injection pressure of 196 kPa for (a) HAW, (b) HTAW-1, (c) HTAW-2, and (d) HTAW-3.

indicated that, as the injection pressures were lowered, the wetted areas in HTAW-1 and HAW increased noticeably. Hence, HTAW, with varied unit weight and saturated hydraulic conductivity, must be considered for a reliable and optimal design of bioreactor landfills incorporating HTs, as the simplified HAW accounted for the larger distribution of leachate in landfills. In addition, HTAW-1 resulted in a higher wetted area with respect to HTAW-2, whereas HTAW-3 lowered it because of the DOD varying with depth.

Pore fluid pressures The generation and distribution of pore-water and capillary pressures and the degree of saturation for different waste conditions are shown in Figures 6 and 7. The results are shown for the first 30 days of injection. These observations were made at a location 5 m left of the HT and at an elevation of 30 m from the

base. The results show that, due to continuous leachate recirculation over time, the degree of saturation increased from the initial 40% until it became fully saturated. Capillary pressure, which was 17 kPa maximum initially, implying higher initial pore gas pressure than pore-water pressure, reduced gradually with time as the leachate was continuously injected and became zero once the MSW was fully saturated. However, the porewater pressure increased continually due to leachate injection. For the high injection pressure of 196 kPa over 30 days, the developed pore-water pressure at 5 m left of the trench was approximately 8% lower in HAW than in HTAW-2, as the relatively lower hydraulic conductivity of the successive deeper layers of the MSW in HTAW-2 resulted in generation of higher pore-water pressures than in simplified HAW. However, the developed pore-water pressure in the case of HTAW-1 was approximately 18% higher than HTAW-2, since the saturated hydraulic conductivities of HTAW-1 in the deeper layers of the MSW were lower than HAW, and HTAW-2 and, therefore, resulted in a maximum pore-water pressure. Furthermore, the developed pore-water pressure in the case of HTAW-3 was approximately 43% lower with respect to HTAW-2. A similar trend was observed for injection pressures of 147, 98, and 49 kPa. For example, using a low injection pressure of 49 kPa, during 4 weeks, the pore-water pressure was estimated to be 48% lower in HAW, while 37% higher in HTAW-1, when individually compared with respect to HTAW-2 (Figure 7). Furthermore, pore-water pressure was 133% lower in HTAW-3 than in HTAW-2. Therefore, the results imply that it is critical to consider the effects of real landfilled waste conditions (i.e. HTAW-2), otherwise the developed pore-water pressures would significantly be lower in simplified HAW. In addition, as the injection pressure reduced (from 196 to 49 kPa), the relative difference in pore-water pressure increased considerably across different MSW cases, so the effect of heterogeneity and anisotropy of MSW must be assessed. Similarly, the capillary pressures were also affected under different waste conditions. For example, with injection pressure of 49 kPa over 2 weeks, the developed capillary pressure in HAW was 28.5% lower than in HTAW-2. In addition, the capillary pressure in HTAW-3 was higher by approximately 21% than in HTAW-2. Similar relationships were observed for other injection pressures. However, as the injection pressure and duration increased, the capillary pressure subsequently reduced due to the continual increase in leachate saturation. Figure 8 summarizes the maximum developed pore-water pressure for different MSW conditions after 10 years of continual injection. For all injection pressures, the maximum developed pore-water pressure was in the following order: HTAW-1>HTAW2>HTAW-3>HAW. In the case of HAW, the pore-water pressures were significantly reduced and found to be the lowest amongst all MSW conditions due to the attainment of steady-state conditions at different injection pressures. Therefore, simplified HAW resulted in lower pore-water pressures than real field conditions, and this scenario may lead to unreliable and unsafe designs since,

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Figure 4.  Evolution of wetted area with time under elevated injection pressures for different MSW conditions: (a) HAW; (b) HTAW-1; (c) HTAW-2; and (d) HTAW-3.

difference in maximum developed pore-water pressure increased with decrease in injection pressures (from 196 to 49 kPa) for all MSW conditions.

Slope stability analyses

Figure 5.  Comparison of maximum wetted area for different MSW conditions under different injection pressures after 10 years.

in real field conditions, pore-water pressures would be much higher than in the case of simplified HAW. Furthermore, the developed pore-water pressure was highest in HTAW-1 due to relatively low permeability of deeper layers of the MSW. Also, the

The stability of the bioreactor landfill slope was evaluated in terms of FOS with injection time for different MSW conditions. Baseline (no leachate injection) FOS was computed to be 2.05, 2.06, 2.11, and 1.88 for HAW, HTAW-1, HTAW-2, and HTAW-3, respectively. The variation in the baseline FOS is mainly due to the varied geotechnical properties of MSW. As shown in Figures 9 and 10, increase in injection pressure during 10 years resulted in lowered FOS for all MSW conditions, primarily due to excessive pore fluid pressures. The influence of pore fluid pressures was minimal in HAW due to attainment of steady-state flow conditions and this led to relatively higher values of FOS during leachate injection. Hence, it can be interpreted that bioreactor landfill designs incorporating HAW would be unreliable and nonconservative, as the values of FOS computed for different injection pressures were 4–15% higher in HAW than in the case of real field conditions

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Figure 6.  Evolution of pore-water and capillary pressures and saturation over 30 days with continuous injection pressure of 196 kPa for (a) HAW; (b) HTAW-1; (c) HTAW-2; (d) HTAW-3.

Figure 7.  Evolution of pore-water and capillary pressures and saturation over 30 days with continuous injection pressure of 49 kPa for (a) HAW; (b) HTAW-1; (c) HTAW-2; (d) HTAW-3.

(i.e. HTAW-2). Maximum pore fluid pressures were observed in HTAW-1 due to relatively low hydraulic conductivities of deeper layers of the MSW. These higher pore pressures consequently yielded significantly lowered FOS for HTAW-1, especially at a high injection pressure of 196 kPa, for which the design of bioreactor landfill slope became unacceptable (i.e. FOS=1.48–1.5) after 10 years of continuous leachate injection (Figure 9a). Even though for different injection pressures (49– 196 kPa), the values of FOS were 2–10% lower in HAW than HTAW-2 due to higher developed pore pressures and resulted in unaccepted landfill designs (i.e. FOS

Slope stability of bioreactor landfills during leachate injection: effects of heterogeneous and anisotropic municipal solid waste conditions.

In bioreactor landfills, leachate recirculation can significantly affect the stability of landfill slope due to generation and distribution of excessi...
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