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ISSN: 2157-7587
Hydrology: Current Research

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The System Dynamics Modeling of Geo-Synthetic Clay Landfill Liners

Ojoawo SO*
Department of Civil Engineering, Ladoke Akintola University of Technology, Ogbomoso, Nigeria
Corresponding Author : Ojoawo SO
Department of Civil Engineering
Ladoke Akintola University of Technology
P.M.B 4000, Ogbomoso, Nigeria
Tel: 234-803-391-6883
E-mail: [email protected]
Received October 14, 2012; Accepted December 03, 2012; Published December 05, 2012
Citation: Ojoawo SO (2012) The System Dynamics Modeling of Geo-Synthetic Clay Landfill Liners. Hydrol Current Res S12:005. doi: 10.4172/2157-7587.S12-005
Copyright: © 2012 Ojoawo SO. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Abstract

The effectiveness of geo-synthetic clay (GC) material as landfill liners using the system dynamics modeling
approach is the focus of this paper. Study area is the Local Government Area (LGA) in Ogbomoso land of Oyo State, Nigeria. These five LGAs include the urban (Ogbomoso North and Ogbomoso South) and rural (Oriire, Ogo Oluwa and Suurulere). The properties of focus on the material were hydraulic conductivity, porosity, thickness and placement slope. A mathematical model was developed by applying the governing equations coded in Visual Basic Computer programming Language. This model was validated with field data collected on the study area. The interrelationship of the properties and the breakthrough times for GC liner was found through the STELLA 9.1.4 software application. It was discovered from this research that GC liners were more effective in urban than rural LGAs. The GC liner is therefore recommended for use as landfill liner in the urban LGAs of the study area.

Keywords
System dynamics; Geo-synthetic clay; Liner; Landfill; Ogbomoso land
Introduction
Geosynthetic Clay Liner (GCL)s was introduced in 1986 as barrier systems for waste containment sites [1]. According to GRI, GCL is a factory manufactured geo synthetic hydraulic barrier consisting of clay supported by geo textiles or geo membranes, or both, that are held together by needling, stitching, or chemical adhesives. GCLs are also called Geo synthetic barriers-Clay (GBR-C). They are an interesting juxtaposition of polymeric materials and natural soils. In other words, they are rolls of factory fabricated thin layers of bentonite clay sandwiched between two geo-textiles or bonded to a geo membrane [2]. They are of thick bentonite double layers and of high swelling capacity. Water is absorbed until crystal sheets dissociate and form a gel with thioxotropic properties. The main forms of GCLs are geo textile-encased and geo membrane-supported.
A major application of GCL is in limiting the escape of landfill leachate or gases in the construction of solid waste storage, heap leach pads, and disposal site base liners and to inhibit the ingress to groundwater [3]. The use of GCL could be exclusive or together with geo membrane, clay, concrete and bitumen as a composite. When compared with clay it has lighter volume, light weight and is convenient to install. The GCL is versatile, cost-effective, and thinner than Compacted Clay Liner (CCL) s with less air space within landfill. According to USEPA [4], GCL technology offers some unique advantages over conventional bottom liners and covers. GCLs for example, are fast, easy to install, have low hydraulic conductivity (i.e. low permeability) and have the ability to self-repair any rips or holes caused by the swelling properties of the bentonite from which they are made. GCLs are cost-effective in regions where clay is not readily available. A GCL liner system is not as thick as a liner system involving the use of compacted clay, enabling engineers to construct landfills that maximize capacity while protecting area ground water. Bentonite is extremely absorbent, granular clay formed from volcanic ash. Bentonite attracts positively charged water particles; thus, it rapidly hydrates when exposed to liquid, such as water or leachate. As the clay hydrates it swells, giving it the ability to “selfheal” holes in the GCL. In laboratory tests on bentonite, researchers demonstrated that a hole up to 75 millimeters in diameter will seal itself, allowing the GCL to retain the proper-ties that make it an effective barrier system [4].
The barrier systems provided by GCL technology is with low hydraulic conductivity (i.e., low permeability), which is the rate at which a liquid passes through a material. Laboratory tests demonstrate that the hydraulic conductivity of dry, unconfined bentonite is approximately 1×10-6 cm/sec. When saturated, however, the hydraulic conductivity of bentonite typically drops to less than 1×10-9 cm/sec. The quality of the clay used affects a GCL’s hydraulic characteristics. Sodium bentonite, a naturally occurring compound in silicate clay formed from volcanic ash, gives bentonite its distinct properties. Additives are used to enhance the hydraulic properties of clay containing low amounts of sodium bentonite. Hydraulic performance also relates to the amount of bentonite per unit area and its uniformity. The more bentonite used per unit area, the lower the system’s hydraulic conductivity. The sealing effect and containment of moisture in landfill covers and liners containing GCLs are largely influenced by moisture retention, swelling, and the self-healing behavior of bentonite used in GCLs. The method of binding of GCLs also significantly affects the behavior [5].
Depending on the particular configuration of the barrier system, GCL technology can provide considerable shear strength (i.e., the maximum stress a material can withstand without losing structural integrity). In particular, a geotextile-backed GCL, with bentonite affixed via stitch-bonding, provides additional internal resistance to shear in the clay layer. Needle-punching yields an even stronger, more rigid barrier. In addition, needle-punching requires the use of a nonwoven geotextile on at least one side. These GCL configurations provide enhanced interface friction resistance to the adjoining layer, an important consideration for landfill slopes [6].
Two principal divisions of modeling are statistical forecasting and system dynamics. In statistical forecasting model equations are developed following observation such that the model output matches available historical data as closely as possible and mostly achieved through regression analysis. System dynamics models are casual mathematical models [7]. Here, the underlying premise is that the structure of a system gives rise to its observable and thus predictable behavior [8,9]. A fundamental step in any system dynamics modeling project is to determine the system structure consisting of positive and negative relationships between variables, feedback loops, system archetypes, and delays [10,11]. The projection then follows where future system states are replicated from this model.
A system dynamics project consists of the following phases [10,12,13]: Problem definition, system conceptualization, model formulation, model evaluation/testing, policy analysis and implementation. These phases are logically pursued in an iterative fashion [14]. The primary purposes for development of system dynamics models are improved system understanding, the development of a certain tools for analyzing and evaluating strategies and policies, and the testing of theories [10,12,15].
Unlike the CCL, natural environmental stresses like freeze/thaw and desiccation do not affect the GCLs. The shortcomings of GCLs however include low shear strength, less attenuation capacity, faster diffusive breakthrough and susceptibility to puncture failure.
This present paper models geosynthetic clay liner (GCL)s with the system dynamics methodology to determine its effectiveness as a landfill leachate containment material. The selected case study is the five (5) Ogbomoso LGAs. Ogbomoso lies approximately on Longitude 4°15’ East, Latitude 8°07’ North and situated in the transitional zone between rain forest and savannah region [16]. The urban LGAs (North and South) and the rural ones (Oriire, Ogo-Oluwa and Suurulere) were all studied. The respective population figures of the 5 LGAs according to the National Population Commission are 198, 720; 100, 815; 150, 628; 65, 184; and 142, 070 [17].
Methodology
Properties of GCL studied
The properties studied and employed in data validation as presented in table 1 [5,18] on GCL were:
(i) Hydraulic conductivity
(ii) Porosity
(iii) Thickness
(iv) Maximum slope
Modeling equations
There were three (3) main governing equations employed in the model, these are:
(a) Leachate generation equation, as given by Safari and Baronian [19]
N cells
LQnT(nΔt)=W4(t)–Wg(t)+Σ LQn ( i(n–i+1)Δt )
i=1 (1)
where
LQnT=Accumulative amount ofleachat generated from the system
nΔt=No of waste cells at the given time
W4=Overall mass of water entering or leaving the dumpsite
Wg=Total water loss due to degradation
LQn=Overall leachate quantity generated from a single cell
n & i=Counters
t=Breakthrough time of the liner
d=Thickness of the liner
α’=Effective porosity
K=Coefficient of permeability and
h=Hydraulic head
(b) Equation for the Breakthrough time, t according to Kadlech and Knight, [20] is
t=d2 α’/K(d+h) (2)
where
d=thickness of the liner (m)
α’=effective porosity
K=coefficient of permeability (m/s)
(c) Leackage rate through the liners qi, as again given by Kadlech and Knight [20], is determined from this relationship:
qi=K[1+y cosΦ ]d (3)
where
K=coefficient of permeability (m/s)
d=liner thickness (m)
Φ=the liner slope (measured in angles)
y=the leachate depth over liner (m)
The developed computer program and simulation
Equation coding was achieved using the Visual Basic computer language. The main elements of the model were well defined and quantified as variables. Hydraulic conductivity, porosity, liner’s thickness and slope were some of the variables considered. The relationships were linked mathematically and the system dynamics structures applied in developing the source codes. The parameters and the initial values for the State Variables (Stocks) were specified and the model then became definitively determined through the program. The stock flow diagram of the system was designed using STELLA 9.1.4 software and simulation package. System dynamics principles were applied in the determination of the interrelationships of leachate and Geo-synthetic Clay Liner (GCL)s retention ability. Simulation was performed on these to predict the results for the next 50 years using year 2006 data [21] as initial values in the stocks of the flow diagram. Figure 1 shows the STELLA flow diagram of the model where the causal loops indicating the linkage of leachate generation and variables with the breakthrough time were displayed.
Validating the model
In order to compare the model results with historical data and to check whether the model generates plausible behavior there is need for its validation. Validation of the developed model was carried out by applying it in the assessment of practical problems of managing leachate pollution with GCLs in the study area. Table 2 (17,21,22) summarizes the validation data employed.
Results and Discussion
The results of the simulation for 100 years performance of the studied geo-membranes depicting the behavioral patterns one year after the other are as presented in figures 2-6.
As shown in figure 2 the breakthrough times (i.e. the times it would take the modeled quantity of leachate to penetrate the GCL) for Ogbomoso North and South LGAs are about 8 days. The respective durations for GCL in Ogbomoso South, Oriire, Ogo Oluwa and Suurulere LGAs as depicted in figures 3-6 are approximately 6, 5 and 6 days. The summary of these results are presented in table 3. It was further observed that the breakthrough times of the rural LGAs (Oriire,Ogo Oluwa and Suurulere) were similar and generally lower than those of the Urban LGAs (Ogbomoso North and South). This trend could be attributed to the fact that leachate toxic parameter concentrations of the rural LGAs are usually higher than those of the urban areas and as such penetrates the GCL more easily.
The GCLs of urban LGAs (North and South) therefore have the highest retention capability for the leachate volume simulated for 100years in the study area. The reason for this trend may be the less toxicity nature of the leachate found in the area compared with those of the rural areas [21]. The GCLs of Ogo Oluwa LGA in the rural area however, recorded the lowest breakthrough period. The GCL of this LGA is therefore considered as the weakest amongst the studied ones.
Conclusion
From this study it could be concluded that the effectiveness of the GCLs in the studied LGAs are in the order Ogo Oluwa<Suurulere<Oriire<Ogbomoso South<Ogbomoso North. The longest breakthrough period discovered for the application of GCL in Ogbomoso land, Nigeria was 8 days. The GCL is therefore recommended for use as landfill liner in the urban LGAs of the study area.
Acknowledgment
The author appreciates the efforts of Mrs. Olubunmi T. Ojoawo in the type setting and arrangement of the manuscript.
References






















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Table 1   Table 2   Table 3

 

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