This study investigates the integration of recycled concrete aggregate (RCA) in landfill cover systems as a sustainable alternative to natural aggregates. The research primarily examines the interaction of RCA with geomembranes by determining the interface friction angle between RCA drainage layers and geomembranes through large-scale direct shear testing. A Finite Element Method (FEM) model of a municipal solid waste landfill in Este, Italy, was developed and validated against the analytical two-wedge theory for veneer shear failure in layered systems. The results demonstrate that RCA outperforms traditional aggregates as a drainage layer material. It also highlights the limitations of commercial FEM software in identifying weak interfaces in landfill cover systems.
1 Introduction
The development of cover systems for Municipal Solid Waste (MSW) landfills has advanced from simple soil coverings to intricate, multi-component structures designed to effectively regulate infiltration and mitigate landfill gas (LFG) emissions. Cost considerations and resource availability have posed challenges to the use of typical landfill cover materials, such as gravels, sands, and clays, in recent years. The use of alternative materials, such as those obtained from waste, has garnered interest due to their potential to save expenses, conserve natural resources, and improve sustainability by reusing waste streams. Furthermore, this falls in line with international environmental regulations that give priority to minimizing waste, promoting recycling, harnessing energy, and conserving resources. Nevertheless, these materials must satisfy certain requirements for hydraulic, shear strength, and compatibility to be considered as feasible alternative materials. Prior studies on the use of alternate materials in landfill cover systems have mostly focused on the hydraulic efficiency of materials, disregarding a thorough investigation of veneer slope stability. Several researchers have investigated the appropriateness of using materials such as recycled asphalt pavement [1], tire-derived aggregates [2], glass culets [3], recycled concrete aggregates [4], and steel slags [4, 5] as drainage components in landfill cover systems. Significantly, previous research has extensively examined the failure of veneer shear at geosynthetic interfaces in traditional landfill covers [6‐13]. However, there is a dearth of studies focussing on the interaction between geosynthetics and alternative waste materials.
In this context, the present study aims at investigation of the feasibility of use of recycled concrete aggregates (RCA) as an alternative material for the drainage layers in landfill final cover systems. The first phase involves the determination of fundamental geotechnical properties of RCA, followed by the assessment of the interface friction angle between RCA and HDPE geomembrane through large-scale direct shear tests. To showcase the practicality of employing RCA within the drainage layer, a numerical model of a municipal solid waste landfill located in Este, Italy, was developed utilizing Finite Element Method (FEM) software, GeoStudio. The validation of this model was achieved by applying the analytical two-wedge theory of veneer shear failure within layered strata, as introduced by Koerner and Soong (2005) [14]. An investigation on the use of recycled asphalt pavement (RAP) aggregates, employing identical methodology, has also been conducted by the authors [15]. While RAP aggregates demonstrated favourable results as a potential alternative material for drainage, it must be noted that most of the RAP produced during road construction and demolition activities is commonly reintegrated on site.
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2 Materials and Methods
2.1 Description of Site
The MSW landfill under investigation in this study is owned by Società Estense Servizi Ambientali (SESAS p.a.), a limited liability company operating in Italy that specializes in waste collection and treatment. The landfill is situated in the municipality of Este, which is in the southwestern region of Padova Province. The landfill in its entirety encompasses a surface area of 13 ha and a volume of approximately 1.5 million cubic metres [16].
2.2 Sample Procurement and Preparation
The RCA for the present study were obtained from the structural laboratory of the Department of Civil Engineering at the Indian Institute of Technology Roorkee (IITR). Several pre-existing concrete cubes from the laboratory, originally cast for determining the characteristic compressive strength of concrete in various projects, were manually crushed using a hand rammer. The concrete cubes did not conform to a predetermined strength specification. Rather, a varied composition was used to reflect the inherent heterogeneity of RCA typically obtained from construction and demolition (C&D) waste processing facilities. For compatibility with the large-scale direct shear test, particles over 20 mm were excluded from the sample through sieving, and those under 4.75 mm were omitted to prevent material washout. The fundamental geotechnical characteristics of the RCA were determined at the geotechnical laboratory IITR (Table 1). The shear strength parameters of RCA were taken from literature [17].
Table 1
Properties of recycled concrete aggregates
Parameter
Unit
Value
Test Procedure
Specific gravity (Gs)
–
2.55
ASTM D 854
Maximum bulk density (γmax)
kPa
12.8
ASTM D 4253
Minimum bulk density (γmin)
kPa
11.9
ASTM D 4253
Hydraulic conductivity (k)
cm/s
1.4 × 10−2
ASTM D 2434
Internal friction angle (ϕ)
Degree
40
–
Cohesion (c)
kPa
0
–
The HDPE geomembrane (GM) utilized in the studies was obtained from Megaplast India Pvt. Ltd. and its properties (provided by the manufacturer) are presented in Table 2. The GM had a thickness of 2 mm and was supplied in rolls with a width of 1 meter. The GM was divided into sheets of 35 cm × 55 cm to be positioned on the lower shear box, which has an overall size of 35 cm. To secure the GM in position, 10 cm overhangs were maintained on both sides, positioned perpendicular to the shearing direction.
Table 2
Properties of geomembrane
Parameter
Unit
Value
Test Procedure
Material
–
HDPE
–
Thickness
mm
2
ASTM D 5199
Density
kg/m3
940
ASTM D 792
Tensile Strength
kN/m
57
ASTM D 6693
Puncture resistance
N
675
ASTM D 4833
Tear resistance
N
249
ASTM D 1004
2.3 Experimental Program
To assess the functionality of the drainage layer in combination with the geosynthetic membrane (GM), the shear strength of the interface between RCA and GM was evaluated using the guidelines outlined in ASTM D5321-12. For this purpose, a direct shear apparatus with a shear box of size 30 cm × 30 cm × 30 cm was utilized. The GM interface was set over the top of the lower half of the shear box, which was first filled with standard sand as filler material. This was done by pouring sand in 4 layers and tamping each layer 25 times by a hand tamper. The 35 cm × 55 cm GM sheet was then firmly secured over the top of the lower box using a custom-made clamp designed to hold the GM through reaction force against the outer box as shown in Fig. 1a. The upper box was then placed over the GM and properly aligned with the lower box. The upper box was filled with RCA in four layers and tamping each layer by 25 blows of hand rammer, to achieve a relative density of 95%. After assembly of the direct shear apparatus, the sample was subjected to shearing at standard loads of 50 kPa, 175 kPa, and 275 kPa, with a strain rate of 1 mm/min. The displacements were measured LVDTs, while the associated shear load was recorded using a data logger. During testing, it was observed that the shearing resulted in stripping of cement mortar from the aggregates a shown in Fig. 1b. This suggests a need for selecting RCA with minimal adhered cementitious material to ensure the optimal performance and durability of the drainage layer. Further testing may be necessary to identify suitable aggregates for drainage applications.
Fig. 1
(a) Geomembrane fastened to lower box using a fabricated clamping device (b) Upper box filled with RCA
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2.4 Numerical Program
A numerical analysis of the failure of the cover slope was conducted for the case study of Este landfill, Italy, to illustrate the practicality of utilizing RCA as a drainage medium in conjunction with geosynthetics. The landfill simulation was conducted using the SLOPE/W module of the widely accessible FEM program GeoStudio. The Morgenstern-Price approach was used to analyse the factor of safety (FOS) as it considers both force and moment equilibrium. Below are the specifics of the simulation:
Landfill Geometry and Material Properties
The research utilized a 2-D plane strain geometry, focussing specifically on the steepest cross section of the landfill. The analysis was performed on the left half section of the geometry because of the structural symmetry. Figure 2 depicts the landfill geometry, including the slope that was examined for potential collapse. The numerical simulation utilized material characteristics reported by Trivellato (2014) [16], which are outlined in Table 3.
Fig. 2
Geometry of Este MSW landfill taken up in this study [15]
Table 3
Properties of the materials used in numerical model
The software GeoStudio facilitates the application of geosynthetics as reinforcements with primary purpose of determining their pull-out resistance. However, this approach fails to consider the influence of the interface friction angle on the FOS. Consequently, the GM was constructed using two techniques. The initial approach involved representing the GM as reinforcement, following the conventional protocol. In the second approach, the GM layers were represented as thin layers with thicknesses significantly less than the cover layers (10 mm) using Mohr-Coulomb model, and the interface adhesion and friction angle were taken as the shear strength properties [18].
Modelling of Landfill Cover Cross-Section
The landfill cover slope S was analysed in for four cases as depicted in Fig. 3.
Case I: Landfill analysed in its original form.
Case II: Gravel from the drainage layer replaced by RCA.
Cases III: GM was added to the analysis below the drainage layer as reinforcement.
Case IV: GM was added as a thin layer (10 mm) below the drainage layer.
Fig. 3
Cover system profiles for numerical model cases
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Modelling the Slip Surface
The analysis of the landfill cover slope was conducted using two distinct methods (Fig. 4): (i) the block specified slip method, which generates slip surfaces that are parallel to the cover slope, indicative of translational slope failure. This method involved positioning right and left point grids at the slope’s crest and toe, respectively, with the software identifying the most critical slope along the cover system. (ii) For calculating the FOS along the actual interface between RCA and geomembrane, a fully specified slip surface parallel to the slope was used, accompanied by a tension crack line based on the approach outlined by Krahn (2004) [19].
The validation of the numerical model was performed using an analytical approach for veneer failure analysis, as described by Koerner & Soong (2005) [14]. This method considers a linear potential failure surface for veneer cover soils, where the cover soil slides in relation to the interface having the lowest friction angle in the section below. The FOS is determined as the ratio of total resisting forces to destabilizing forces. Considering that the slope is relatively shorter as compared to those typically assumed for infinite slope analysis, the evaluation incorporates the consideration of a passive soil wedge at the toe of the slope. Figure 5 illustrates the forces on the cover slope and the identified failure surface. For comprehensive details, readers are encouraged to consult Koerner and Soong (2005) [14]. To validate the model, failure interfaces within the numerical model were identified for each scenario, followed by the derivation of analytical solutions tailored to these specific failure interfaces.
Fig. 5
Free body diagram of forces on cover soil. (After Koerner & Soong (2005) [14])
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3 Results and Discussion
The shear strength parameters for RCA were derived from available literature, showing a range of 40–50° [17]. A sensitivity analysis was conducted to establish a specific value for use in the simulation. The internal friction angle of RCA was conservatively varied from 35° to 45°. As can be seen in Fig. 6a, the FOS consistently exceeded 1.5 across the entire range. Based on this, a friction angle of 40° was selected for subsequent analyses. Further, the FOS was found to be insensitive to variations in the interface friction angle of the geosynthetic material when it was specified as a reinforcement in the software. This indicates that the program will produce inaccurate results if the geosynthetic membrane is incorporated in as per conventional method.
Fig. 6
(a) Sensitivity Analysis for internal friction angle of RCA aggregates and interface friction angle of GM (b) Results of large-scale direct shear test
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The agreement between the outcomes of the analytical and numerical investigations, which are presented in Table 4, provides strong support for the numerical model. It is critical to acknowledge that a steeper landfill cover slope (1:1.6) was utilized in this specific instance, deviating from the recommended range of 1:3 to 1:5. As a result, the study is mostly concerned with comparing several cases as opposed to identifying particular FOS values.
Table 4
Results of analytical and numerical analysis
Slip surface specification method
Case
Factor of safety
Numerical Results
Analytical Results
Error
Block specified method
Case 1
1.171
1.161
0.8%
Failure interface
Gravel-clay
Gravel-clay
Case 2
1.72
1.65
4%
Failure interface
RCA-clay
RCA-clay
Case 3
1.72
1.65
4%
Failure interface
GM
RCA-clay
Case 4
1.46
1.22
16%
Failure interface
GM
GM
Fully specified slip surface
RCA-GM interface
1.39
1.22
16%
The FOS for Case I, i.e. original cover system with gravel was 1.171 while on replacing the drainage layer material with RCA in Case II, the FOS increased to 1.72. Hence, the RCA layer demonstrated superior shear strength compared to the gravel layer, primarily due to a higher internal friction angle. Additionally, RCA exhibited favourable drainage properties with a hydraulic conductivity of 1.4 × 10−4 m/s. These positive hydraulic characteristics underscore the potential of RCA to contribute to improved slope stability in landfill cover systems.
In order to evaluate the efficacy of alternative cover systems utilizing geosynthetics, GM was integrated into the model for Cases III and IV. The shear strength characteristics of the RCA-GM interface, as determined by large-scale direct shear experiments, are illustrated in Fig. 6b. A negligible intercept of the curve was obtained, which was fixed to zero by linear curve fitting, yielding a Pearson’s r value of 0.999 (R-Square = 0.9994). The angle of contact friction (δ) was calculated to be 23.2°.
Case III exhibited no sensitivity to FOS when GM was incorporated as reinforcement, while in Case IV, the FOS value decreased from 1.72 to 1.48 due to formation of a weak interface in the cover system. This, in addition to the results of sensitivity analysis highlights that, the software exhibits a limitation in recognizing the effects of a weak interface along the geosynthetic in Case III. The software demonstrates a constraint in identifying the consequences of a weak interface along the geosynthetic in Case III, as evidenced by this and the sensitivity analysis results. Syed and Mishra (2023) [18] explain that in order to address this concern and guarantee precise evaluation of slope stability, it is crucial to incorporate the geomembrane as a thin layer beneath the drainage layer.
It is worth mentioning that the analytical techniques employed in this research were specifically designed to compute the Factor of Safety (FOS) at the interfaces between soil and geosynthetic materials. In contrast, the block failure slip surface method enables the software to independently identify the most critical slip surface along a cover slope. To facilitate a significant comparison, the numerical analysis was used to identify the materials at the top and bottom of the slip surface in each case, which was then used to substitute the properties of the failure interface for each case. The results obtained from this comparative analysis indicated that the analytical model and the numerical model exhibited a significant level of agreement.
4 Conclusion
The study delves into the integration of recycled concrete aggregate (RCA) in landfill cover systems, examining its potential as a sustainable alternative to natural aggregates. Through geotechnical tests and numerical simulations, the performance of RCA in drainage layers and its interaction with geomembranes were assessed. Following are the conclusions drawn from the study:
The geotechnical tests conducted on recycled concrete aggregates (RCA) have demonstrated their suitability for use as a drainage materials in cover systems. With a hydraulic conductivity of 1.4 × 10−4 m/s, the material possesses the desirable permeability characteristics for effective drainage layers in various applications. However, it is recommended that aggregates with minimum cementitious mortar adhered should be used to ensure the optimal performance and durability of the drainage layer.
RCA aggregate layer outperforms the gravel layer in terms of shear strength mainly on account of higher value of internal friction angle. There was a 47% increase in the FOS value from of Case I and Case II.
The interface friction angle between RCA and GM was experimentally determined to be 23.2°. Upon numerical simulation, this resulted in the formation of a weak interface in the cover system.
Sensitivity analysis and numerical analysis result for Case III revealed that the software is insensitive to changes in interface properties of geomembrane when applied as a reinforcement.
On the introduction of the GM as a thin layer in the cover system, the FOS experienced a decline from 1.71 to 1.46. This was supported by similar outcomes obtained from the analysis of a fully specified slip surface along the RCA-GM interface.
Analytical methods for determination of FOS at soil-geosynthetic interfaces were utilized to obtain FOS values for soil-soil interfaces with good agreement.
Acknowledgements
The authors are thankful to the management at Megaplast Pvt. Ltd. for providing the geomembrane specimens and their friendly cooperation. Additionally, we acknowledge Mr. Nishant Varshney, an intern under the SPARK Program at IIT Roorkee, for his invaluable contributions to the experiments.
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