Two-Phase Flow Modeling of Leachate Injection Effects on Stability of Bioreactor Landfill Slopes

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1 Two-Phase Flow Modeling of Leachate Injection Effects on Stability of Bioreactor Landfill Slopes Paper #12826 Rajiv K. Giri Graduate Research Assistant, University of Illinois at Chicago, Department of Civil & Materials Engineering, 842 West Taylor Street, Chicago, IL 60607, Krishna R. Reddy Professor, University of Illinois at Chicago, Department of Civil & Materials Engineering, 842 West Taylor Street, Chicago, IL 60607, ABSTRACT In bioreactor landfills, the leachate is recirculated using leachate recirculation systems (LRS) under pressure to enhance municipal solid waste (MSW) degradation and reduce overall settlement period. Pressurized leachate injection leads to higher moisture distribution, which results in faster degradation of MSW. However, the high injection pressure in recirculation system near side slopes can generate excess pore fluid pressures and reduce effective shear strength of MSW, leading to instability of bioreactor landfill slopes. In this study, a numerical two-phase flow modeling is used to predict the moisture distribution, generation and distribution of pore-water and pore-gas pressures, and their impacts on stability of bioreactor landfill slopes in heterogeneous and anisotropic waste conditions at a high injection pressure. Two-phase flow model is preferred because landfill leachate and gas exist simultaneously in unsaturated MSW pores, and the model enables the realistic generation and distribution of moisture and pore pressures through porous MSW. Strength reduction technique was employed to perform slope stability analyses as it takes into account of the transient and spatially varying pore water and gas pressures. The model results were validated based on the published studies using single-phase flow modeling and slope stability analysis under simplified conditions (e.g., homogeneous MSW). This study then simulated the effects of heterogeneous and anisotropic waste conditions by incorporating different waste layers of varying unit weight and saturated hydraulic conductivity. The unsaturated hydraulic properties of MSW with waste dry unit weight of 7.8 kn/m 3 were taken based on laboratory studies. Overall, it was concluded that high injection pressure in recirculation systems near side slope and heterogeneous-anisotropic waste properties can greatly affect the stability of bioreactor landfill slope, and therefore, the effects of pore water and gas pressures need to be assessed for stability analyses of bioreactor landfill. INTRODUCTION Bioreactor landfills have emerged as a latest trend for the safe disposal of municipal solid waste (MSW) in solid waste management practices. In bioreactor landfills, leachate is injected into the waste using leachate recirculation systems (LRS) at constant injection pressure. This practice results in higher moisture distribution, increased microbiological and nutrient activities, faster biodegradation of organic matters, reduced settlement period, recovery of the used landfill space, 1

2 and increase in landfill gas (primarily methane) production. However, if high injection pressure is applied into the LRS near landfill side slope, it may generate excess pore fluid pressures and significantly reduce the effective shear strength of MSW, which can lead to landfill slope instability. In the past, bioreactor landfill slope failures have been reported due to poorly controlled leachate recirculation. 1 Some of these landfill failures are: (a) The Bulbul landfill failure, Durban, South Africa, 1997: in this case, the buildup of excessive pore fluid pressures in the newly placed uncompacted zone led to the landfill slope failure; (b) The Dona Juana bioreactor landfill, Bogota, Columbia, 1997: in this particular case, progressive toe failure was reported due to excessive pore fluid pressure caused by poorly controlled leachate injection, and (c) Payatas landfill failure, Quezon City, Manila, Philippines, 2000: in this case, failure of landfill occurred due to heavy rainfall for a very short duration, which resulted in generation of excessive pore water pressure. Therefore, controlled leachate injection is needed for stable landfill slopes. The current design of recirculation systems is empirical, and a rational design methodology is needed for effective, controlled leachate recirculation that ensures the physical stability of the landfill. In this study, first a numerical two-phase flow modeling is validated based on the results from previous published studies using single-phase flow model and stability analysis for a simplified bioreactor landfill configuration. Then, the two-phase model is used to determine the effects of continuous leachate injection under elevated pressures of 49 kpa, 98 kpa, 147 kpa, 245 kpa, 294 kpa, and 390 kpa in homogeneous anisotropic waste conditions. Finally, the numerical two-phase flow model is used to predict the moisture distribution, generation and distribution of pore-water and pore-gas pressures, and their impacts on stability of bioreactor landfill slopes in heterogeneous anisotropic waste conditions under a high injection pressure of 390 kpa. METHODOLOGY Numerical Two-Phase Flow and Slope Stability Model The void spaces (pores) of unsaturated MSW are assumed to be filled with two immiscible fluids, namely: the leachate and landfill gas. Two-phase flow model incorporates modeling the flow of these two immiscible fluids (i.e., leachate considered as wetting fluid and landfill gas considered as non-wetting fluid). The pressure difference between pore gas and pore water, also known as capillary pressure, is a function of leachate degree of saturation, and can be represented by the van Genutchen (1980) model. 2 The flow of wetting-leachate and non-wetting landfill gas is described by Darcy's law, whereas relative permeability of each fluid is based on wetting leachate saturation by empirical laws of the van Genuchten function. 3, 4, and 5 The governing equations for the two-phase flow model incorporating flow-only mode are given as: L S L PL S L qi n + = (Eq. 1) K L t t xi 2

3 G S G PG SG qi n + = (Eq. 2) K G t t xi Where: n = porosity S L = wetting leachate (liquid) saturation S G = non-wetting gas saturation P L = wetting pore liquid pressure P G = non-wetting pore gas pressure K L and K G = liquid and gas bulk modulus, respectively q and L i q = flow rate of wetting liquid and non-wetting gas given by Darcy s law G i The mathematical formulation including governing equations and numerical formulations related to the two-phase flow model are explained in detail elsewhere. 5 A finite difference code known as Fast Lagrangian Analysis of Continua (FLAC) is used for the modeling purpose. The application of the FLAC model in studying the generation and distribution of leachate, pore water, and pore gas pressures in bioreactor landfills has been successfully carried out in a previous study. 6 and 7 The stability analyses of bioreactor landfill slopes are also performed using FLAC program, wherein strength reduction technique is adopted to determine slope stability in terms of factor-ofsafety. 8 In general, Mohr-Coulomb failure criterion is combined together with strength reduction approach for stability analyses. In this approach, factor-of-safety calculation is performed by successively reducing the shear strength parameters of landfilled wastes (cohesion and friction angle) until the slope reaches on the verge of failure. FLAC uses a bracketing approach to determine initial stable and unstable bracketing state solutions for a given factor-ofsafety trial (F trial ) value. 8 and 9 Wherein, stable bracketing state refers to F trial value for which solution converges and unstable bracketing state corresponds to F trial value for which the solution does not converge. Subsequent trials are carried out to obtain an intermediate F trial value between the two bracketing states, and further analyses are performed. Once the solutions converge, the new F trial value supersedes the stable bracketing state value; otherwise, the unstable bracket value is replaced. The difference between the stable and unstable solutions is continuously reduced until the difference reaches below a specified tolerance limit. 5 This modeling approach was used successfully in a previous study. 10 Landfill Configuration A simplified two-dimensional bioreactor landfill configuration, 175 m wide and 50 m high with a side slope of 3H: 1V, was created in FLAC using graphical interface to investigate the effects of heterogeneous anisotropic waste (HTAW) conditions, leachate injection under high pressure, and their resulting impacts on pore fluid distribution and the stability of landfill slope. Figure 1 shows this simplified bioreactor landfill configuration. The landfill model configuration and overall modeling approach is similar to that reported by Xu et al. (2012) 11 who used the singlephase flow model SEEP/W and SLOPE/W (Geo-Slope International, Canada), respectively, to evaluate pore water pressures and its resulting impacts on slope stability analysis. 11 3

4 Figure 1. Simplified Bioreactor Landfill, Depicting the Base Scenario Using Xu et al. (2012) 25 m Horizontal Trench (1m x Municipal Solid 30 m 3 1 Wet 30 m 50 m Leachate Collection and Removal System 175 The conceptual landfill model does not consider the effects of a landfill cover system because this study is mainly focused on pressurized leachate injection and flow through landfilled waste, when the bioreactor landfill is in active condition. The bioreactor landfill is considered to be completely filled with a homogeneous anisotropic waste (HAW) throughout its entire depth. A 0.3 m thick leachate collection and removal system (LCRS), consists of free draining granular soil, is assumed to be located at the bottom of the 50 m deep landfill. A horizontal trench (1m x 1m) is placed 30 m above the base of the LCRS and at a setback of 30 m from the side slope. Figure 1 shows the landfill model depicting these scenarios, wherein the leachate is injected into the horizontal trench at a constant pressure. The wet zone contour represents the extent of elevated moisture distribution surrounding the trench due to leachate injection. The top boundary is free to extend laterally away from the side slope up to any location, and in this study, it is assumed to be extended to a width of 25 m away from the side slope. The model is discretized into cells, and all the external boundaries are simulated as zero flow boundaries. For all the modeling cases, a grid cell size of 1m x 1m is selected to obtain accurate results. To simulate leachate collection and removal system (LCRS) at the bottom, zero pressures are defined for all cells in the bottom-most model layer and the sum of outflow from these cells is calculated as outflow through LCRS. 6 In order to investigate the effects of heterogeneous-anisotropic waste (HTAW) conditions, the 50 m deep landfill model is divided into 10 different layers; with each layer having a depth of 5 m. The unit weights and saturated hydraulic permeabilities of MSW vary with depth and are explained in the next section. Figure 2 depicts a simplified bioreactor landfill configuration taking into consideration of heterogeneous-anisotropic waste conditions. 4

5 Figure 2. Landfill Configuration for Heterogeneous-Anisotropic Waste (HTAW) Condition 25 m Horizontal Trench (1m x 1m) 1 Waste lifts 3 LCRS 175 m k v = 2.4 x10-3 ; γ = 12.6 k v = 2.5 x10-4 ; γ = 13.5 k v = 4.7x10-5 ; γ = 14.1 k v = 1.3 x10-5 ; γ = 14.6 k v = 4.4 x10-6 ; γ = 14.9 k v = 1.8 x10-6 ; γ = 15.1 k v = 8.2 x10-7 ; γ = 15.3 k v = 4.1 x10-7 ; γ = 15.4 k v = 2.3 x10-7 ; γ = 15.6 k v = 1.3 x10-7 ; γ = 15.7 Note: Vertical hydraulic conductivity (k v ) is in cm/s, and unit weight (γ) is in kn/m m MSW Properties To evaluate the effects of heterogeneous anisotropic waste in bioreactor landfill, the MSW is modeled for two waste conditions: (a) homogeneous anisotropic waste (HAW), and (b) heterogeneous anisotropic waste (HTAW). In both the conditions, it is assumed that the MSW is a freshly placed waste with zero degree of decomposition (e.g., DOD = 0%). For HAW conditions, the MSW properties (i.e., unit weight, cohesion, friction angle, saturated hydraulic conductivity, and anisotropy) are assumed to be the same for the entire depth of the landfill. These MSW properties are directly adopted from Xu et al. (2012) and shown in Table 1. Table 1: MSW Properties for Heterogeneous Anisotropic Waste (HTAW) Conditions Unit Weight (kn/m 3 ) Cohesion (kpa) Friction Angle (degree) Saturated Vertical Hydraulic Conductivity, kv ( cm/s) x Anisotropy (k H /k V ) In order to simulate HTAW condition in this study, the bioreactor landfill is divided into ten different layers, and the unit weight of the landfilled waste for each layer is calculated using unit weight and depth relationship given by Zekkos et al. (2006): 12 Where: γ = γ z i + (Eq. 3) αz + β 5

6 γ = unit weight of MSW at depth z α = modeling parameter = 3.0 m 4 /kn for typical MSW β = modeling parameter = 0.2 m 3 /kn for typical MSW γ i = near surface in-place unit weight (kn/m 3 ) For HTAW conditions, it is considered that the unit weight of the MSW at the mid depth (25 m) of the landfill is exactly the same as that of HAW condition (i.e., γ = 15 kn/m 3 ), and unit weights for rest of the layers vary with depth according to the above mentioned equation. 12 In addition, the saturated hydraulic conductivity of each waste layer decreases with depth due to the increase in normal stress caused by overlying MSW. Reddy et al. (2009) reported a laboratory study on saturated hydraulic conductivity of MSW as a function of effective vertical stress, and this data can be expressed by the following relationship: 13 k v = k v 0 σ ' 1 + pa 5.3 (Eq. 4) Where: k v0 = initial saturated hydraulic conductivity at zero normal stress (10-2 cm/s) k v = saturated hydraulic conductivity under effective normal stress of σ' p a = atmospheric pressure (101.3kPa) In this study, the shear strength parameters (i.e., cohesion and friction angle) of heterogeneousanisotropic waste are assumed to remain constant throughout the landfill and their values are taken from Xu at al. (2012). 11 In addition, the anisotropy is considered by taking k h = 10 k V for the MSW. Table 2 shows the MSW properties used in the present study for HTAW conditions. Table 2: MSW Properties for Heterogeneous Anisotropic Waste (HTAW) Conditions Layer Depth (m) Depth to mid layer (m) Unit Weight (Zekkos et al., 2006) (kn/m 3 ) Vertical Hydraulic Permeability (Reddy et al., 2009) (cm/s) 10 (Topmost) x x x x x x x x x (Bottom) x Friction Angle (degree) Cohesion (kpa) 6

7 Model Input and Initial & Boundary Conditions The unsaturated hydraulic properties of MSW based on experimental studies have been reported in the literature. 14, 15, and 16 All of these studies assumed the van Genutchen model (1980) to report the corresponding MSW unsaturated hydraulic properties. Of all these reported studies, the MSW used in Breitmeyer and Benson (2011) study had the maximum MSW dry unit weight (γ = 7.8 kn/m 3 ), and resembles closest to the landfilled waste being considered in this study. Therefore, the unsaturated parameters given by Breitmeyer and Benson (2011) are used to define the unsaturated hydraulic properties of MSW. Table 3 summarizes the unsaturated hydraulic properties of the landfilled waste. Table 3: Unsaturated Hydraulic MSW Properties Based on Breitmeyer and Benson (2011) Matric suction α (1/kPa) Saturated moisture content θs Residual moisture content θr van Genuchten n van Genuchten a van Genuchten b van Genuchten c In general, boundary conditions are categorized into (a) mechanical boundary conditions, and (b) hydraulic boundary conditions. Mechanical boundary conditions are applied by fixing the base in both horizontal and vertical directions, so that the lateral and vertical deformations of the landfill at base are zero. Furthermore, the lateral deformation is restrained on the right side boundary of the model, whereas the side slope is free to move in both directions, and the top boundary is free to move only in the vertical direction. Hydraulic boundary conditions are taken into consideration by fixing the pore gas pressure at the top boundary and at the side slope. The rightside boundary and the bottom of the landfill model are considered to be impermeable (i.e., free pore pressures and free saturation). The grid points were initially free to vary based on the net inflow and outflow from the neighboring zones. Pore water pressure was fixed to zero for the grid points at the LCRS (0.3 m above the base) to represent the drainage layer. The initial volumetric moisture content of 15% (v/v, by volume) at all grid points and an initial porosity of 40% at all zones were considered. For an atmospheric air pressure and almost zero residual saturation, the negative initial pore water pressure was calculated based on the van Genutchen empirical laws 9 and was used at all grid points. In addition, the initial pore gas pressures were assumed to be zero at all grid points. Model Simulations Validation For validation purpose, the base scenario set-up of the single-phase flow study with a constant liquid injection pressure of 49kPa is simulated with the two-phase flow model. 11 Firstly, a sensitivity analysis is carried out using the two-phase flow model to determine the effects of material properties (i.e., unit weight, cohesion, friction angle, hydraulic conductivity, anisotropy, and side slope) on the baseline factor of safety calculations. The results obtained from this study are compared with the results reported by Xu et al. (2012) 11. 7

8 Thereafter, the effects of continuous elevated injection pressures on stability of bioreactor landfill slope are modeled, and the results are compared with the single-phase flow study in terms of (1) factor of safety v/s time, and (2) flow rate v/s time. For the validation purpose, continuous injection pressures of 49kPa, 98kPa, and 147kPa are considered. Effects of Elevated Injection Pressures Once the two-phase flow model is validated, it is employed to determine the effects of continuous leachate injection under different elevated pressures for HAW conditions. For this purpose, the elevated injection pressures of 49 kpa, 98 kpa, 147 kpa, 245 kpa, 294 kpa, and 390 kpa were used to evaluate their resulting impacts on the stability of landfill slope in terms of evolution of FOS calculations with time. Effects of Heterogeneous Anisotropic MSW and High Injection Pressure In order to investigate the effects on landfill slope stability due to HTAW properties under high injection pressure, the leachate is injected into both HAW and HTAW conditions under a continuous high injection pressure of 390 kpa through the horizontal trench system. For HTAW condition, in this study, the 50 m deep landfill model is divided into 10 different waste layers with an equal thickness of 5 m for each waste layer, and the unit weight and saturated hydraulic conductivity of each waste layer are determined based on Zekkos et al. (2006) 12 and Reddy et al. (2009) 13, respectively. RESULTS AND DISCUSSION Validation The numerical two-phase flow model used in the present study is validated based on the published studies using single-phase flow modeling and slope stability analysis under simplified conditions (e.g., homogeneous MSW). Figure 3 compares the sensitivity of waste properties on stability of landfill slope for the base scenario in terms of FOS calculations derived from the present study as well as the single-phase flow study carried out by Xu et al. (2012). 11 8

9 Figure 3. Sensitivity Analysis: Effects of Cohesion, Friction Angle, Hydraulic Conductivity, Anisotropy, Side Slope, and Unit Weight on Slope Stability Present Study Xu et al. (2012) 2.2 Present Study Xu et al. (2012) Cohesion (kpa) Friction Angle (Degree) Present Study Xu et al. (2012) Present Study Xu et al. (2012) e-5 4e-5 6e-5 8e-5 1e Vertical Hydraulic Permeability (cm/s) Anisotropy Present Study Xu et al. (2012) 2.5 Present Study Xu et al. (2012) H:1V 2H:1V 3H:1V 4H:1V 5H:1V Side Slope Unit Weight (kn/m 3 ) 9

10 The baseline factor of safety obtained using both models is computed to be 5. It is evident that the two-phase model adopted in this study provides similar results as reported by Xu et al. (2012). 11 For baseline conditions, the shear strength parameters of MSW (cohesion and friction angle), and geometry of side slope have greater impacts on stability of landfill slopes rather than MSW unit weight and saturated hydraulic conductivity. Figure 4 shows the effects of continuous elevated injection pressures on slope stability. For the validation purpose, the results obtained from the present study are compared with the published results. Xu et al. (2012) 11 reported a reduction in FOS from 5 to 4 under a continuous injection pressure of 49 kpa for a period of 10 years, whereas the present study results in FOS reduction from initial 5 to 3 after the same duration of 10 years for the injection pressure of 49 kpa. As the injection pressure increases, leachate spreads more in the lateral direction than vertically due to the waste anisotropy. This results in larger waste saturation near the trench location, and leads to increase in pore fluid pressures. Consequently, factor of safety of the slope model reduces with time. Application of 98 kpa injection pressure for 10 continuous years leads to reduced FOS from 5 to 1.98 (present study), and 5 to 0 (Xu et al., 2012) 11, respectively. Similarly, a continuous injection pressure of 147 kpa for a time period of 10 years causes decrease in FOS from 5 to 1.95 (Xu et al., 2012) 11, and 5 to 1.94 (Present study). Figure 4. Effects of Elevated Injection Pressures of 49 kpa, 98 kpa, and 147 kpa kpa - Xu et al. (2012) 98 kpa - Xu et al. (2012) 147 kpa - Xu et al. (2012) 49 kpa - Present Study 98 kpa - Present Study 147 kpa - Present Study Time (day) 10

11 The determination of unit outflow rates (m 3 /s per meter) with time due to different elevated injection pressures has been carried out in the present study (Fig. 5) and compared with the results reported by Xu et al. (2012). 11 It is clear that the effects on unit flow rates due to continuous leachate injection at the pressure range used are very limited and thus can be ignored. Figure 5. Comparison of Flow Rate v/s Time for Injection Pressure of 49 kpa, 98 kpa, and 147 kpa Ouflow Rate (m 3 /s per meter) 1e-4 8e-5 6e-5 4e-5 2e-5 49 kpa - Present Study 98 kpa - Present Study 147 kpa - Present Study 49 kpa - Xu et al. (2012) 98 kpa - Xu et al. (2012) 147 kpa - Xu et al. (2012) Time (day) Effects of Leachate Injection under Elevated Pressures Figure 6 shows the effects of continuous leachate injection under different elevated pressures for HAW conditions. The evolution of FOS calculations with time is studied for different elevated injection pressures of 49 kpa, 98 kpa, 147 kpa, 245 kpa, 294 kpa, and 390 kpa. It is found out that the landfill slope failure takes place after total flow duration of 161 days for an injection pressure of 245 kpa, 118 days for an injection pressure of 294 kpa, and 21 days for an injection pressure of 390 kpa, respectively, in HAW conditions. Whereas, the landfill slope is stable for relatively smaller injection pressures of 49 kpa, 98 kpa, and 147 kpa, and the FOS after total flow duration of 10 years were computed to be 3, 1.98, and 1.94, respectively. 11

12 Figure 6. Evolution of FOS with Time for Elevated Injection Pressure of 49 kpa, 98 kpa, 147 kpa, 245 kpa, 294 kpa, and 390 kpa kpa 98 kpa 147 kpa 245 kpa 294 kpa 390 kpa Time (Day) Effect of HTAW Conditions and High Injection Pressure This study investigates HTAW condition under high injection pressure. The results are compared with HAW condition to evaluate the effects of HTAW in landfill. The Figure 7 predicts the evolution of wetted area of MSW (the area where the degree of liquid saturation 60%) with time for the HAW and HTAW conditions at a continuous injection pressure of 390 kpa for a time period of 30 days. In case of HTAW, the maximum wetted area of 234 m 2 was achieved after 30 days of continuous leachate injection, which is about 5% of the total landfill area. Similarly, the maximum wetted area was found to be 284.5m 2 at the end of 30 days for the HAW, which is about 6% of the total landfill area. The wetted area is greater in case of HAW than HTAW condition because the saturated hydraulic conductivity of HAW near the trench location (kv = 1 x 10-5 cm/s) is estimated to be higher than that of heterogeneous-anisotropic waste (kv = 1.8 x 10-6 cm/s). 12

13 Figure 7. Evolution of Wetted Area with time for HTAW and HAW Conditions under a High Injection Pressure of 390 kpa HTAW Conditions kpa Pressure HAW Conditions kpa Pressure Wetted Area (m 2 ) Time (day) Figure 8 shows the evolution of liquid saturation, generation and distribution of pore-water and pore-gas pressures with time at three different locations in the landfill model. Figure 8(a) shows the effect of continuous leachate injection of 390 kpa at a point located 4m directly below the horizontal trench. In this case, the maximum liquid pressure (pore water pressure) at the end of the 30 days is about 28 kpa, while the gas pressure which peaked at 41 kpa drops to less than 1 kpa after 30 days. As the degree of liquid saturation of the waste increases, the gas pressure decreases. Figure 8(b) shows the pore water and gas pressures profile at 8 m left of the horizontal trench system. The liquid pressures are higher than those generated at 4 m below the trench. Even, the time to reach 100 % liquid saturation is lesser than the previous case. This is due to the fact that, in HTAW condition, the leachate spreads more laterally than vertically down, due to higher waste anisotropy in the lateral direction. For 30 days continuous injection, the maximum pore water and gas pressures generated are about 80 kpa and 1 kpa, respectively. Similarly, Figure 8(c) reports the generation and distribution of pore water and gas pressures at a location, situated 10 m right of the horizontal trench. In this case, the maximum pore water and gas pressures after 30 days of continuous leachate injection are estimated to be 54 kpa and less than 1 kpa, respectively. It is worth mentioning that the transient behavior of pore fluid pressures at all the three selected locations is caused due to a very high injection pressure of 390 kpa. 13

14 Figure 8. Evolution of Pore Water Pressure, Pore Gas Pressure, and Saturation with time for HTAW Conditions at Locations: (a) 4 m directly below the horizontal trench; (b) 8 m left of the horizontal trench, and (c) 10 m right of the trench. Figure 8a Figure 8b Figure 8c 14

15 Figure 9 shows the comparison of factor of safety with time for HAW and HTAW conditions due to the leachate injection at a very high pressure of 390 kpa for a total period of 30 days. The baseline factor of safety for HTAW is calculated to be 2.1, whereas a FOS of 5 is reported for HAW conditions. With time and increase in moisture, the distributions of pore fluid pressures are significantly increased near the side slope in both waste conditions. It is found that, in case of HTAW, the slope failure (FOS < 1.5) takes place after about 19 days, while it takes about 21 days to occur slope failure in HAW conditions. Figure 9. Evolution of with Time for HAW and HTAW Conditions with an Injection Pressure of 390 kpa HTAW Conditions HAW Conditions Time (day) CONCLUSIONS In this study, a numerical two-phase flow model was validated based on the published studies using single-phase flow modeling and slope stability analysis under simplified conditions (e.g., homogeneous MSW). Subsequently, the numerical two-phase flow modeling was used to predict the moisture distribution, generation and distribution of pore-water and pore-gas pressures, and their impacts on stability of bioreactor landfill slopes in HTAW conditions under a high injection pressure of 390 kpa. In order to simulate the HTAW conditions, the waste is divided into ten different layers with varying unit weights and saturated hydraulic conductivities. Unsaturated hydraulic properties of the MSW are given based on a study carried out by Breitmeyer and 15

16 Benson (2011) for MSE dry unit weight of 7.8 kn/m 3. In addition, the effects of leachate injection under different elevated pressures (i.e., 49 kpa, 98 kpa, 147 kpa, 245 kpa, 294 kpa, and 390 kpa) were evaluated for HAW condition. Following conclusions are drawn from this study. The stability of simplified bioreactor landfill slopes is greatly influenced by continuous leachate injection under elevated pressures. It is concluded that the slope failure takes place after a total duration of 161 days for an injection pressure of 245 kpa, 118 days for an injection pressure of 294 kpa, and 21 days for an injection pressure of 390 kpa, respectively, in HAW conditions. HTAW conditions significantly influenced the degree of saturation, leachate distributions, maximum wetted area, and pore fluid (water and gas) pressures in the simplified bioreactor landfill configuration. Factor of safety (a measure of physical stability) is reduced when the leachate is continuously injected into HTAW at a relatively high pressure and this scenario led to a side slope failure after 19 days. Similar failure scenario was obtained in the HAW conditions after a flow period of three weeks. REFERENCES 1. Koerner, R. M.; Soong, T. Y. Leachate in landfills: the stability issues. Geotextiles and Geomembranes, 2000, 18(5), van Genuchten, M. Th. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil. Sci. Soc. Am. J., 1980, 44, pp Peaceman, D.W. (1977). Fundamentals of Numerical Reservoir Simulation. Elsevier Scientific Publishing Company New York, NY, U.S.A. 4. Lu, N.; Likos, W.J. Unsaturated Soil Mechanics; John Wiley & Sons Inc., Hoboken, New Jersey, U.S.A, ITASCA Consulting Group. FLAC-Fast Lagrangian Analysis of Continua, ITASCA Consulting Group Manuals, Minneapolis, Minnesota, U.S.A. 6. Reddy, K.R.; Kulkarni, H.S.; Khire, M.V. Two Phase Modeling of Leachate Recirculation Using Vertical Wells in Bioreactor Landfills. Journal of Hazardous, Toxic and Radioactive Waste, ASCE, Kulkarni, H.S. Optimization of Leachate Recirculation Systems in Bioreactor Landfill; PhD Thesis, Dept. of Civil and Materials Engineering, University of Illinois, Chicago, Dawson, E. M.; Roth, W. H. Slope Stability Analysis with FLAC. In FLAC and Numerical Modeling in Geomechanics, Proceedings of the International FLAC Symposium on Numerical Modeling in Geomechanics, Minneapolis, Minnesota, 1999, pp C. Detournay and R. Hart, eds. Rotterdam: A. A. Balkema. 9. Dawson, E. M.; Roth, W. H.; Drescher, A. Slope Stability Analysis by Strength Reduction. Géotechnique, 1999, 49 (6), Richards, K.S.; Reddy, K.R. Slope Failure of Embankment Dam under Extreme Flooding Conditions: Comparison of Limit Equilibrium and Continuum Models; In Geo-Frontiers, Austin, Texas,

17 11. Xu Q.; Tolaymat, T.; Townsend, T. G. Impact of Pressurized Liquids Addition on Landfill Slope Stability. Journal of Geotechnical and Geoenvironmental Engineering, 2012, 138 (4), Zekkos, D.; Bray, J.; Kavazanjian, E.; Matasovic, N.; Rathje, E.; Riemer, M.; and Stokoe, K. Unit Weight of Municipal Solid Waste J. Geotech. Geoenviron. Eng.; 2006, 132(10), Reddy, K.R.; Hettiarachchi, H.; Parakalla, N.; Gangathulasi, J.; Bogner, J.; Lagier, T. Hydraulic conductivity of MSW in landfills. Journal of Environmental Engineering, 2009, 135 (8), Stoltz, G.; Tinet, A.; Staub, M.; Oxarango, L.; Gourc, J. Moisture Retention Properties of Municipal Solid Waste in Relation to Compression. J. Geotech. Geoenviron. Eng., 2012, 138(4), Breitmeyer, R.J.; Benson, C.H. Measurement of unsaturated hydraulic properties of municipal solid waste. In Proceedings, Geofrontiers 2011, ASCE Geotechnical Special. 16. Kazimoglu, Y.K.; McDougall, J.R.; Pyrah, I.C. Unsaturated hydraulic conductivity of landfilled waste. In Proc. 4th Intl. Conference on Unsaturated Soils, Carefree, Arizona, 2006, pp KEYWORDS Municipal solid waste, bioreactor landfill, leachate injection, pore water pressure, pore gas pressure, slope stability. 17