Giri Reddy 2014 Slope Stability

8/16/2019 Giri Reddy 2014 Slope Stability

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Waste Management & Research2014, Vol. 32(3) 186–197© The Author(s) 2014Reprints and permissions:sagepub.co.uk/journalsPermissions.navDOI: 10.1177/0734242X14522492wmr.sagepub.com

Introduction

In recent years, bioreactor landfills have emerged as a successfulmeans 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 themoisture and result in fast biodegradation of MSW due toenhanced microbial activity (Barlaz et al. , 1992; Chugh et al. ,1998; Reinhart et al. , 2002). Horizontal trenches (HTs), verticalwells, and/or drainage blankets are employed as leachate recir-culation systems to recirculate leachate in bioreactor landfills.Constant injection pressure for a specified time period is neededto add moisture to the landfill (Xu et al. , 2012). However, highinjection pressures in leachate recirculation systems near theside 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 ofthe landfill slope.

Engineered landfill design and operation should consistof a careful assessment of landfill slope stability. Often,

these landfills are constructed near highly populated areas,which further increases the risk associa ted with landfill slopefailure. Slope stability analyses of conventional landfills

based on the geotechnical properties of MSW and underlyingsoils (i.e. unit weight, shear strength) have been reported inthe 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 operationshave been getting more attention. For example, Koerner andSoong (2000) studied numerous landfill slope failures toanalyse the developed pore-water pressures due to leachate

Slope stability of bioreactor landfillsduring leachate injection: Effects ofheterogeneous and anisotropicmunicipal solid waste conditions

Rajiv K Giri and Krishna R Reddy

AbstractIn bioreactor landfills, leachate recirculation can significantly affect the stability of landfill slope due to generation anddistribution 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 reci rculation on the physical stabili ty of landfil l slope. In thisstudy, a numerical two-phase flow model (landfill leachate and gas as immiscible phases) was used to investigate the effectsof heterogeneous and anisotropic nature of MSW on moisture distribution and pore-water and capillary pressures and theirresulting impacts on the stability of a simplified bioreactor landfill during leachate recirculation using horizontal trenchsystem. The unsaturated hydraulic properties of MSW were considered based on the van Genuchten model. The strengthreduction technique was used for slope stability analyses as it takes into account of the transient and spatially varying pore-water and gas pressures. It was concluded that heterogeneous and anisotropic MSW with varied unit weight and saturatedhydraulic conductivity significantly influenced the moisture distribution and generation and distribution of pore fluid

pressures in landfill and considerably reduced the stability of b ioreactor landfi ll s lope . It is recommended that heterogeneousand anisotropic MSW must be considered as it provides a more reliable approach for the design and leachate operations in

bioreactor landf ills.

KeywordsBioreactor landfill, capillary pressure, leachate recirculation, moisture distribution, municipal solid waste, pore-water pressure,slope stability

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, IL60607, USA.Email: [email protected]

2492 WMR 0010.1177/0734242X14522492WasteManagement andResearchGiri and Reddy

Original Article


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Giri and Reddy 187

injection that resulted in slope failures. Kavazanjian andMerry (2005) reported the Payatas landfill failure due to ele-vated levels of leachate that led to excessive pore-water

pressure in the landfilled MSW. Xu et al. (2012) carried outa single-phase flow modelling to determine the effects of

pressurized leachate in jec tion on the stabili ty of a simplified bioreactor landfil l slope.

However, these studies only considered the effects of pore-water pressure and neglected the pore gas (air) pressure. Also, theMSW was considered to be homogeneous, which does not repre-sent the true nature of landfilled waste, as the MSW is found to

be heterogeneous in nature with hydraulic conductivity, unitweight, and shear strength parameters varying with depth due tooverburden stress and degree of decomposition (DOD) (Reddyet al. , 2011). Using a numerical two-phase flow model, Reddyet al. (2013) showed that gas pressures can significantly exceedliquid pressure in a typical bioreactor landfill configuration dur-ing 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 liquidand coupled gas pressures generated due to leachate injectionusing HTs for heterogeneous and anisotropic MSW (HTAW)conditions.

In this study, a numerical two-phase flow modelling was usedto determine the effects of heterogeneous and anisotropic natureof MSW under elevated injection pressure on the moisture distri-

bution, 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 modelvalidation has been presented elsewhere (Giri and Reddy, 2013) based on previously published studies using a single-phase flowmodel and stability analyses for a simplified bioreactor landfillconfiguration incorporating homogeneous and anisotropic MSW(HAW).

MethodsNumerical two-phase flow and slopestability modelThe pores of unsaturated MSW were assumed to be filled withtwo immiscible fluids: namely the landfill leachate and landfillgas. The two-phase flow model incorporated modelling the flowof these two immiscible fluids (i.e. leachate considered as wet-ting fluid and landfill gas considered as nonwetting fluid). Thecapillary pressure is a function of leachate degree of saturationand can be represented using the model of van Genuchten (1980).The flow of leachate and landfill gas was described by Darcy’slaw, whereas relative permeability of each fluid is based on lea-chate saturation by the empirical laws of van Genuchten function(ITASCA Consulting Group, 2011). In the numerical two-phaseflow model, the governing equations of unsaturated MSW are

given by the linear momentum balance and the fluid mass bal-ance laws and are represented as:

ρ ρ ρ ρ = + +d L L G Gn S S ( ) (equation 1)

n S K

P t

S t

q x

L

L

L L i L

i

∂∂

+ ∂∂

= − ∂∂ (equation 2)

n S K

P t

S t

q x

G

G

G G iG

i

∂∂

+ ∂∂

= − ∂∂ (equation 3)

where n is porosity, S L is leachate (liquid) saturation, S G is gassaturation, P L is pore liquid pressure, P G is pore gas pressure, ρ L and ρG are fluid densities, ρd is matrix dry density, K L and K G areliquid and gas bulk modulus, respectively, and qi

L 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 formula-tions including governing equations related to the two-phaseflow model are explained elsewhere (ITASCA Consulting Group,2011, Reddy et al. , 2013).

Concurrently, slope stability analyses were performedusing FLAC, wherein the strength reduction technique wasadopted to compute factor of safety (FOS; Dawson et al. ,1999). The Mohr–Coulomb failure criterion was combinedwith the strength reduction approach for stability analyses. Inthis approach, the FOS calculation was performed by succes-sively reducing the shear strength parameters (cohesion andfriction angle) of MSW until the slope reached on the vergeof failure. Further information on the modelling approach andits successful application is presented elsewhere (Giri andReddy, 2013).

Landfill configurationsA two-dimensional bioreactor landfill, 175 m wide and 50 mdeep with a side slope of 3:1 (horizontal/vertical), was created inFLAC to investigate the effects of HTAW under pressurized lea-chate addition. Figure 1a shows the landfill cell, known as basescenario, wherein the leachate was injected through a HT with acontinual injection pressure of 49 kPa (i.e. equivalent to a 5-mwater column head). The landfill model was considered to becompletely filled with a homogeneous and anisotropic waste(HAW) throughout its entire depth for the numerical two-phaseflow model validation (Figure 1a). The wet zone contour repre-sented the extent of moisture surrounding the HT due to leachateinjection. The top boundary was extended to a width of 25 maway from the side slope. A 0.3-m-thick leachate collection-and-removal system, consisting of free draining granular soil, wasassumed 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 leachatecollection-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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188 Waste Management & Research 32(3)

SEEP/W and SLOPE/W, respectively, to evaluate pore-water pressures and their impact on slope stability analysis. The con-ceptual model did not consider the effects of a landfill cover sys-tem because this study was mainly focused on pressurizedleachate injection and flow through landfilled waste when biore-actor landfill is in active state.

Temperature effects, mechanical compression, and infiltra-tions were not included in the model. In bioreactor landfills, vari-ous biochemical processes and waste degradation generatesignificant amount of heat that result in long-term elevated wastetemperatures (Yeşiller et al. , 2005). The relatively high waste tem-

peratures (~30–45°C) affect the process of landfilled wastedecomposition that further varies the waste physical (e.g. voidratio, deformation) and geotechnical properties (e.g. unit weight,shear strength) as well as hydraulic properties (e.g. moisture con-tent, saturation, pore pressure, leachate infiltration). The effects ofelevated temperatures on the aforementioned waste properties areassumed to be even more prominent in bioreactor landfills thanconventional dry landfills and should be considered for futureresearch studies. However, the present study did not account forthese coupled thermo-hydro-bio-mechanical interactions.

To investigate the effects of HTAW conditions, the 50-m-deeplandfill model was divided into 10 different layers, each layer

having a depth of 5 m. The unit weights, saturated hydraulic con-ductivities, and shear strength of MSW varied with depth and areexplained in the next section. Figure 1b depicts a landfill con-figuration taking HTAW scenarios into account.

Material propertiesLimited data are available on shear strength of MSW and furtherresearch is needed to accurately predict the variation in shearstrength properties of MSW with depth during leachate recircula-

tion (Reddy et al. , 2009a). Variation in unit weight of MSW withdepth was given using the relationship proposed by Zekkos et al. (2006):

γ γ α β = +

+i

z

z (equation 4)

where γ is unit weight at depth z, α and β are modelling parame-ters for typical MSW, and γi is near surface in-place unit weight.

The saturated hydraulic conductivity of MSW decreases withdepth due to the increase in normal stress caused by overlying

MSW and this can be expressed by the relationship proposed byReddy et al. (2009b):

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


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Giri and Reddy 189

k k p

v va

= +

−

0

1

5 3

σ

.

' (equation 5)

where k v0 is initial saturated hydraulic conductivity at zero nor-mal stress (10 –2 cm s –1), k v is saturated hydraulic conductivityunder effective overburden of σ′, and pa is atmospheric pressure

MSW is heterogeneous and anisotropic in nature. Therefore,to investigate this condition systematically, the bioreactor landfillcell (Figure 1b) was modelled for three different HTAW condi-tions with varying geotechnical properties with depth, and theresults were compared with simplified HAW:

• HAW: For this condition, the MSW properties were assumedto be the same for entire depth of the landfill and were directlyadopted from Xu et al. (2012). Unit weight ( γ), cohesion ( c),friction angle ( φ), vertical saturated hydraulic conductivity(k v), and anisotropy ( a; k h/k v) 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 ofMSW were taken to be exactly the same as that of HAWacross the landfill cell. However, the saturated hydraulic con-ductivity for each layer was varied and calculated using equa-tion 5, depending on the estimated overburden stress at thecentre 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 hydraulicconductivity of MSW were varied with depth due to overbur-den stress. The unit weight of the MSW at the mid depth(25 m) of the landfill cell was exactly the same as that ofHAW (i.e. γ=15 kN m –3) and unit weights for rest of the layerswere varied with depth using equation 4. Saturated hydraulicconductivity of each waste layer decreased with depth andwas calculated using equation 5. Shear strength and anisot-ropy (value 10) of MSW were constant throughout the land-fill cell (Table 1).

• HTAW-3: Landfilled MSW, in the presence of moisture, under-goes microbial decomposition and this causes change in the geo-technical 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 typi-cal state of degradation, a linear variation in the DOD and geo-technical properties of the MSW with depth was considered. It isassumed that the topmost layer had the geotechnical propertiesof fresh MSW (DOD=0 %), while the bottommost layer wasnearly 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 estimatedand are summarized in Table 1.

Unsaturated hydraulic parameters were kept constant for allMSW cases and followed the values given by Breitmeyer andBenson (2011) to evaluate the effect of two-phase flow (Table 2).Unsaturated hydraulic properties of the MSW were not variedwith respect to the depth because: (1) very little published infor-mation 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 impacton the key design parameters at steady-state conditions (Hayderand Khire, 2005; Reddy et al. , 2013).

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

Layer Depth(m)

HTAW-1 HTAW-2 HTAW-3

γ (kNm–3)

kv (cms–1)

φ (°) c (kPa) γ (kNm–3)

kv (cms–1)

φ (°) c (kPa)

DOD(%)

γ (kNm–3)

kv (cms–1)

φ (°) c (kPa)

10 (Top) 0–5 15 1.9×10–3 35 15 12.6 2.4×10–3 35 15 0 14.9 1.9×10–3 35.0 15.09 5–10 15 1.9×10–4 13.5 2.9×10–4 10 17.2 1.3×10–4 33.9 16.68 10–15 15 3.9×10–5 14.1 5.9×10–5 20 17.9 2.1×10–5 32.9 18.17 15–20 15 1.1×10–5 14.6 1.7×10–5 30 18.4 5.2×10–6 31.8 19.76 20–25 15 4.2×10–6 14.9 5.9×10–6 40 18.7 1.7×10–6 30.8 21.25 25–30 15 1.8×10–6 15.1 2.4×10–6 50 19.0 6.6×10–7 29.7 22.84 30–35 15 8.9×10–7 15.3 1.1×10–6 60 19.2 3.0×10–7 28.7 24.33 35–40 15 4.7×10–7 15.4 5.7×10–7 70 19.4 1.5×10–7 27.6 25.92 40–45 15 2.7×10–7 15.6 3.1×10–7 80 19.5 7.8×10–8 26.6 27.41 (Bottom) 45–50 15 1.6×10–7 15.7 1.8×10–7 90 19.7 4.4×10–8 25.5 29.0

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

Unit weight (kN m –3) 7.8Inverse of air-entry pressure α (kPa–1) 1.18Saturated moisture content φ s 0.41Residual moisture content φ r 0.03van Genuchten steepness parameter ‘ n’ 1.33van Genuchten ‘ m’ 0.25


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Initial and boundary conditions andmodel input parametersMechanical boundary conditions were applied by fixing the basein both horizontal and vertical directions, so that the lateral andvertical deformations of the landfill at the base were zero. Thelateral deformation was restrained on the right side boundary of

the model, whereas the side slope was free to move in both direc-tions and the top boundary was free to move only in the verticaldirection. Hydraulic boundary conditions were taken into consid-eration 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 theliquid as long as the liquid pressure (water pressure) was nega-tive: the gas pressure was taken as zero at boundary nodes wherethe condition was not met (ITASCA Consulting Group, 2011).The right-side boundary and the bottom of the landfill modelwere 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-removalsystem to represent the drainage layer. The pore gas pressureswere fixed to be zero initially at all grid points in order to estab-lish initial mechanical equilibrium. The initial pore-water pres-sure was calculated based on the initial gas pressure by default.Thereafter, the gas pressures were set to vary for different flowconditions (ITASCA Consulting Group, 2011). The initial wastesaturation of 40% at all grid points and an initial porosity of 40%at all zones were considered.

Model simulationsTwo-phase flow modelling, presented in this study, was validated

based on the published studies using single-phase flow modellingand slope stability analysis under simplified conditions (e.g.HAW). The effects of continuous elevated injection pressures onthe stability of bioreactor landfill slope were modelled and theresults were compared with the single-phase flow study in termsof: (1) FOS vs. time, and (2) flow rate vs. time. For the validation

purpose, continuous injection pressures of 49, 98, and 147 kPawere considered. In addition, the sensitivities of MSW geotech-nical properties on the stability of landfill slope were analysedand validated with a two-phase flow model; these results are pre-sented elsewhere (Giri and Reddy, 2013).

All simulations were carried out using different continualinjection pressures to examine transient leachate distributionuntil the steady state was reached or the injection time period forwhich the landfill slope design became unacceptable (i.e.FOS


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Giri and Reddy 191

years of continuous leachate additions, since the saturatedhydraulic conductivities in successive deeper layers of the MSWfor all HTAW were significantly lower than that of HAW.Furthermore, the leachate tended to migrate more laterally thanvertically downward and tried to seep out through side slope, pri-

marily due to high saturated hydraulic conductivity in lateraldirection ( k h=10 k v).

This study aimed at achieving a wetted area of MSW corre-sponding to 60% or higher degree of saturation, as recommended

by Alternative Landfill Technologies Team (2006). Figure 4shows the evolution of wetted area of MSW with injection timefor different MSW conditions. The wetted area increased withincrease in injection pressure due to more migration of leachatethroughout the MSW. For continuous leachate injection with arelatively high pressure of 196 kPa, the wetted area after 30 days’injection was 44 and 59% higher in HAW and HTAW-1, respec-

tively, and 42% lower in HTAW-3 compared with HTAW-2. Thiscould be attributed to larger migration of leachate in successive

deeper layers of MSW in HTAW-1 and HAW than in HTAW-2(Figure 3). However, for HTAW-3, the lateral spread of leachatewas limited and found to be lower than in HTAW-2, due to lowersaturated hydraulic conductivity of HTAW-3 than HTAW-2. Thisled to a limited rate of leachate migration in the MSW and

resulted in a smaller wetted area. In addition, for shorter injectionduration, there would be large differences in the wetted areas fordifferent MSW conditions, and it is necessary to account forHTAW-2 (a realistic field condition) for various leachate opera-tions and designs in bioreactor landfill. A similar trend wasobserved for other three injection pressures (49, 98, 147 kPa)during a 4-week injection period for different MSW conditions.

Figure 5 summarizes the maximum wetted area for differentMSW conditions using different injection pressures after 10 years.For any injection pressure, the maximum wetted area was in thefollowing order: HTAW-1>HAW>HTAW-2>HTAW-3. This sig-

nified the presence of higher wetted area (moisture distribution)under HTAW-1 and HAW for different injection pressures and

Figure 2. Model validation: effects of different injection pressures for homogeneous-anisotropic MSW for (a) factor of safetyvs. time and (b) outflow rate vs. time.


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indicated that, as the injection pressures were lowered, the wettedareas 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 land-fills incorporating HTs, as the simplified HAW accounted for thelarger distribution of leachate in landfills. In addition, HTAW-1resulted in a higher wetted area with respect to HTAW-2, whereasHTAW-3 lowered it because of the DOD varying with depth.

Pore fluid pressuresThe generation and distribution of pore-water and capillary

pressures and the degree of saturation for different waste condi-tions are shown in Figures 6 and 7. The results are shown for the

first 30 days of injection. These observations were made at alocation 5 m left of the HT and at an elevation of 30 m from the

base. The results show that, due to continuous leachate recircu-lation over time, the degree of saturation increased from theinitial 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 graduallywith time as the leachate was continuously injected and becamezero once the MSW was fully saturated. However, the pore-water pressure increased continually due to leachate injection.

For the high injection pressure of 196 kPa over 30 days, thedeveloped pore-water pressure at 5 m left of the trench wasapproximately 8% lower in HAW than in HTAW-2, as the rela-tively lower hydraulic conductivity of the successive deeper lay-ers of the MSW in HTAW-2 resulted in generation of higher

pore-water pressures than in simplified HAW. However, thedeveloped pore-water pressure in the case of HTAW-1 wasapproximately 18% higher than HTAW-2, since the saturatedhydraulic conductivities of HTAW-1 in the deeper layers of theMSW were lower than HAW, and HTAW-2 and, therefore,

resulted in a maximum pore-water pressure. Furthermore, thedeveloped pore-water pressure in the case of HTAW-3 wasapproximately 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 49kPa, during 4 weeks, the pore-water pressure was estimated to be48% lower in HAW, while 37% higher in HTAW-1, when indi-vidually compared with respect to HTAW-2 (Figure 7).Furthermore, pore-water pressure was 133% lower in HTAW-3than in HTAW-2. Therefore, the results imply that it is critical toconsider the effects of real landfilled waste conditions (i.e.

HTAW-2), otherwise the developed pore-water pressures wouldsignificantly be lower in simplified HAW. In addition, as theinjection pressure reduced (from 196 to 49 kPa), the relativedifference in pore-water pressure increased considerably acrossdifferent MSW cases, so the effect of heterogeneity and anisot-ropy of MSW must be assessed.

Similarly, the capillary pressures were also affected under dif-ferent waste conditions. For example, with injection pressure of49 kPa over 2 weeks, the developed capillary pressure in HAWwas 28.5% lower than in HTAW-2. In addition, the capillary

pressure in HTAW-3 was higher by approximately 21% than inHTAW-2. Similar relationships were observed for other injection

pressures. However, as the injection pressure and durationincreased, the capillary pressure subsequently reduced due to thecontinual increase in leachate saturation.

Figure 8 summarizes the maximum developed pore-water pressure for different MSW conditions after 10 years of continualinjection. For all injection pressures, the maximum developed

pore-water pressure was in the following order: HTAW-1>HTAW-2>HTAW-3>HAW. In the case of HAW, the pore-water pressureswere significantly reduced and found to be the lowest amongst allMSW conditions due to the attainment of steady-state conditionsat 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,

Figure 3. Saturation contours (leachate distribution) after10 years with continuous injection pressure of 196 kPa for (a)HAW, (b) HTAW-1, (c) HTAW-2, and (d) HTAW-3.


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Giri and Reddy 193

in real field conditions, pore-water pressures would be muchhigher than in the case of simplified HAW. Furthermore, thedeveloped pore-water pressure was highest in HTAW-1 due torelatively low permeability of deeper layers of the MSW. Also, the

difference in maximum developed pore-water pressure increasedwith decrease in injection pressures (from 196 to 49 kPa) for allMSW conditions.

Slope stability analysesThe stability of the bioreactor landfill slope was evaluated interms 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 tothe varied geotechnical properties of MSW.

As shown in Figures 9 and 10, increase in injection pressureduring 10 years resulted in lowered FOS for all MSW condi-tions, primarily due to excessive pore fluid pressures. Theinfluence of pore fluid pressures was minimal in HAW due toattainment of steady-state flow conditions and this led to rela-tively higher values of FOS during leachate injection. Hence, itcan be interpreted that bioreactor landfill designs incorporatingHAW would be unreliable and nonconservative, as the valuesof FOS computed for different injection pressures were 4–15%higher in HAW than in the case of real field conditions

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.

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


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(i.e. HTAW-2). Maximum pore fluid pressures were observedin HTAW-1 due to relatively low hydraulic conductivities ofdeeper layers of the MSW. These higher pore pressures conse-quently yielded significantly lowered FOS for HTAW-1, espe-

cially at a high injection pressure of 196 kPa, for which thedesign 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 thanHTAW-2 due to higher developed pore pressures and resultedin unaccepted landfill designs (i.e. FOS


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Giri and Reddy 195

heterogeneous-anisotropic nature of MSW during leachateinjection.

When estimating slope stability under leachate injection:

• Steady-state flow was possible only in the case of homoge-neous and anisotropic MSW (HAW), wherein the time toreach steady-state flow varied with injection pressures,with higher injection pressure resulting in a relativelyfaster attainment of the steady state. The steady-state flowin HAW led to relatively lower pore fluid pressures andhigher FOS during leachate recirculation. Thus, simplifiedHAW represented the nonconservative and unreliabledesign approach and leachate operations in bioreactorlandfill.

• Steady-state flow was not achieved in the case of heterogene-ous and anisotropic waste (HTAW) for different injection

pressures during 10 years of continuous injection, because, inthe case of HTAW, the spread of leachate was more in thelateral direction than in the vertical downward direction, andhence, the leachate tried to seep through the side slope oflandfills.

• Neglecting waste heterogeneity in depth (i.e. simplified

homogeneous MSW) resulted in overestimation of the MSWwetted area (moisture distribution) by 5–40%, underestima-tion of the developed pore-water pressure by 8–48% for

Figure 8. Maximum developed pore-water pressure fordifferent MSW conditions under different injection pressuresafter 10 years of continuous leachate injection.

Figure 9. Evolution of factor of safety with injection time under different MSW conditions for different injection pressures: (a)196 kPa; (b) 147 kPa; (c) 98 kPa; (d) 49 kPa.


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shorter duration (i.e. 30 days) and by 22–57% for longerduration (i.e. 10 years) for different injection pressures (49– 196 kPa), and consequently overestimation of the values ofFOS for bioreactor landfill slope by 4–15% over 10 years. Inaddition, the capillary pressures for different MSW condi-tions were highest at the beginning of the leachate injection(15–20 kPa) and decreased to zero over time due to continu-ous leachate injection.

• Neglecting the variations in MSW saturated hydraulic con-ductivity and unit weight resulted in overestimation of theMSW wetted area and developed pore-water pressures by6–59% and 5–16%, respectively, for different range(s) ofcommonly employed injection pressures (49–196 kPa) over10 years. Subsequently, the values of FOS were underesti-mated by 2–10%.

• Neglecting the different levels of waste degradationresulted in underestimation of the MSW wetted area and

pore-water pressures by 27–42% and 43–133%, respec-tively, for different range of continuous injection pressuresover 10 years. The stability of landfill slope was overesti-mated by 3–10%.

Overall, it is concluded that heterogeneous and anisotropic

nature of MSW greatly influences the distribution of leachate, thegeneration and distribution of pore fluid pressures in landfilledwaste, and the stability of bioreactor landfill slope during con-tinuous leachate injection. Additionally, it is recommended thatheterogeneous and anisotropic MSW with varying unit weightand saturated hydraulic conductivities with depth must be con-sidered for designs and leachate operations as it provides a safer,critical, and more reliable approach than simplified homogene-ous and anisotropic MSW.

Declaration of conflicting interest

The authors declare that there is no conflict of interest.

FundingThis project was supported by the US National Science Foundation(grant CMMI 0600441).

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Figure 10. Factor of safety for different MSW conditionsunder different injection pressures after 10 years (baseline:no leachate injection).


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