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Laboratory studies on effect of fiber content on dynamic characteristics of municipal solid waste
Waste Management xxx (2018) xxx–xxx
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Laboratory studies on effect of fiber content on dynamic characteristics of municipal solid waste P. Alidoust a, M. Keramati b,⇑, N. Shariatmadari a a b
Department of Civil Engineering, Iran University of Science and Technology, Narmak, Tehran 16846-13114, Iran School of Civil Engineering, Shahrood University of Technology, P.O. Box 3619995161, Shahrood, Iran
a r t i c l e
i n f o
Article history: Received 26 April 2017 Revised 29 January 2018 Accepted 16 February 2018 Available online xxxx Keywords: Fiber content Cyclic triaxial test Dynamic behavior Municipal solid waste
a b s t r a c t The dynamic characterization of municipal solid waste (MSW), especially in regions with high seismicity, is of considerable importance in the stability assessment of landfills. Additionally, findings indicated that the response of MSW under dynamic loadings is significantly affected by fibrous material. Therefore, a comprehensive strain-controlled cyclic triaxial testing program was performed on MSW samples retrieved from a landfill in the Kahrizak area, Tehran province. The tests were conducted on fresh MSW specimens (with a diameter of 100 mm) with different percentage of fibers in the consolidated undrained condition. The potential reinforcing capability of fibers and their impacts on changes in the MSW composition were investigated under variations of different factors including confining pressure, loading frequency, Poisson’s ratio, and loading cycles. From the results of the study, increasing fiber content in specimens resulted in improved elastic behavior of MSW under dynamic loadings, irrespective of the test conditions, such that the normalized shear modulus reduction curves shifted to the right, while the damping ratio curves exhibited no specific trend. However, it is necessary to simultaneously consider the impact of fiber contents, confining stress and shear strain on the variation rates of normalized shear modulus reduction values. This trend is attributed to the greater values of stiffness from changing the composition when compared with the one generated by obtained reinforcement within the studied strain range. Given the lack of systematic evaluations on the effect of the fibrous waste materials on the dynamic response of MSW, the results of this study provide additional insight into the seismic analysis of landfills. Ó 2018 Published by Elsevier Ltd.
1. Introduction The design and analysis of waste fills in earthquake prone areas are highly dependent on the reliable estimation of the dynamic properties of Municipal Solid Waste (MSW). Recent worldwide landfill failures and performance problems have resulted in the increasing importance of proper determination of the dynamic properties, including small-strain shear modulus, shear straindependent shear modulus reduction and damping ratio. Several studies reported the importance of the reinforcing effect of the fibrous waste constituents in enhancing the strength of MSW compared to the organic material of MSW (Athanasopoulos et al., 2008; Kavazanjian et al., 1999). It should be noted that the majority of published technical studies do not consider the direct impact of fiber content on the dynamic response of MSW, while several studies only evaluated the direct effect of fibers on the static ⇑ Corresponding author. E-mail address: [email protected] (M. Keramati).
response of MSW. However, the results indicated a substantial effect of fibrous materials on the static behavior of MSW, without any direct evidence to show any substantial effect on the dynamic properties (Shariatmadari et al., 2011, 2010; Zekkos, 2005). Despite numerous advances in the understanding of MSW properties, many of the factors affecting the dynamic response of MSW remain largely unknown. Additionally, in the seismic analysis of landfills, site-specific data are always preferable to general recommendations and experience. This is because waste materials may differ for a number of reasons, including weather conditions and culture (Zekkos et al., 2008). To address the abovementioned problems, this study includes the results of more than 60 cyclic triaxial tests, and is a continuation of previous studies conducted by the authors at the Iran University of Science and Technology (IUST). The aim of the study is to experimentally quantify the effect of fibrous materials on the dynamic properties of MSW, and demonstrate the potential effects of aging on MSW samples. Additionally, bender element (BE) and large-scale oedometer tests were conducted at the IUST
https://doi.org/10.1016/j.wasman.2018.02.038 0956-053X/Ó 2018 Published by Elsevier Ltd.
Please cite this article in press as: Alidoust, P., et al. Laboratory studies on effect of fiber content on dynamic characteristics of municipal solid waste. Waste Management (2018), https://doi.org/10.1016/j.wasman.2018.02.038
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geotechnical research center to evaluate the shear wave velocity and Poisson’s ratio of MSW, respectively.
1.1. Recent studies on dynamic properties of MSW The dynamic response of an MSW landfill subjected to cyclic loading depends to a large extent in the cyclic stress-strain characteristics of MSW. The required properties for seismic response analysis of an MSW landfill by using the equivalent linear method include the following: Shear wave velocity (V s ) or small-strain shear modulus (Gmax ). Strain-dependent material damping ratio (k). Strain-dependent normalized shear modulus reduction (G=Gmax ). The following summarizes the current understanding of the above-mentioned parameters. However, despite several attempts, there is a lack of reliable data on the dynamic properties of MSW (Naveen et al., 2014). The Gmax is one of the basic parameters required for much of the engineering analysis of the geomaterials. Based on the elasticity theory, the Gmax is related to the V s and the mass density (q) of the material as follows (Zekkos et al., 2008):
Gmax ¼ q:V 2s
ð1Þ
The shear wave velocity of MSW has been measured using different laboratory and in situ techniques. The spectral analysis of surface waves (SASW), multichannel analysis of surface waves (MASW), and microtremor analysis method (MAM) are gaining popularity because of their nonintrusive nature and rapid rate of testing, compared to other in situ methods, such as the downhole and crosshole methods (Ramaiah et al., 2015). The in situ methods typically determine a shear wave velocity of MSW materials based on the variation of landfill depth. Following the Northridge earthquake, Kavazanjian et al. (1996) proposed a recommended curve for the shear wave velocity of southern California landfills based on the SASW method (Kavazanjian et al., 1996). Pereira et al. (2002) measured the shear wave velocity using the SASW method in a landfill near Madrid, Spain, and reported a range of 100–250 m/s (Pereira et al., 2002). Khaleghi (2011) performed a series of Continuous Surface Wave System (CSWS) tests and reported on the shear wave velocity profile of the Kahrizak landfill (Khaleghi, 2011). Ramaiah et al. (2015) proposed an empirical linear model for V s of landfills with a maximum depth of 30 m based on statistical analysis of 146 in situ shear wave velocity profiles from 37 MSW landfill sites worldwide (Ramaiah et al., 2015). In addition to the wide use of in situ methods, laboratory methods are becoming increasingly applicable when the assessment of different factors, including waste composition, and confining pressure on shear wave velocity is critical. Many researchers evaluated the shear wave velocity of MSW materials by performing laboratory tests, such as the BE and resonant column (RC). Hossain et al. (2010) reported that the decomposition of waste has an increasing effect on values of small strain shear modulus based on a series of Resonant Column (RC) tests (Hossain et al., 2010). Yuan et al. (2011) estimated V s and subsequently the small-strain shear modulus in a laboratory by using a BE that was installed on a large simple shear apparatus (Yuan et al., 2011). The author reported the significant role of waste composition on V s values. Zekkos, (2005) conducted several tests to compare the laboratory data with field data. He reported that the laboratory-derived values are highly dependent on composition and time under the confinement of specimens (Zekkos, 2005).
The material damping ratio and shear modulus are the parameters required for the seismic response analysis of landfills. The shear modulus describes the stiffness of MSW, while the damping ratio represents the loss of energy during seismic loads. Different factors such as shear strain, number of cycles, and confining pressure can change the values of the parameters. The cyclic shear strain is accepted as the most important and influential of the factors. Because of this dependence, the abovementioned geotechnical parameters for dynamic analysis are typically depicted relative to the shear strain values. The majority of available recommendations on straindependent normalized shear modulus reduction and material damping curves of MSW are based on back analyses of recorded ground motions of the OII1 landfill using different analytical techniques (Zekkos, 2005). Various researchers proposed recommended curves based on back analysis although important differences were observed between the curves (Augello et al., 1995; Elgamal et al., 2004; Idriss et al., 1995; Matasovic´ et al., 1995; Morochnik et al., 1998). However, the necessity of dynamically assessing of MSW worldwide is recognized owing to site-specific characterization of MSW, and these types of data are always preferred to generalized data (the OII landfill is a specific case of landfill). The abovementioned discussion resulted in several laboratory and field test assessments. Large-scale tests, including cyclic triaxial, cyclic simple shear and RC tests, are of great interest. However, there is a lack of field and laboratory data, given the difficulties including health issues associated with testing waste material and sample disturbance. One of the first laboratory studies on the dynamic properties of MSW was conducted based on a large- diameter cyclic direct simple shear testing program by Matasovic´ and Kavazanjian (1998). The test included testing at uniform cyclic shear strains corresponding to 0.1%, 0.3%, 1%, 3%, and 5% at a frequency of approximately 0.1 Hz, on waste recovered from the OII landfill. The data showed good agreement in the dynamic properties of the OII landfill between the laboratory and back-calculated curves (Matasovic´ and Kavazanjian, 1998). Towhata et al., (2004), performed cyclic triaxial and shaking table tests on organic waste bio-treated both with, and without, plastics. The results confirmed that plastic sheets and other fiber components generate shear strength in waste specimens. They also reported that the material damping of specimens without plastic exhibited greater values when compared to containing specimens with plastic (Towhata et al., 2004). Zekkos (2005) conducted more than 80 large-scale cyclic triaxial tests on 25 specimens with varying waste composition, confining stress, loading frequency and density. The author reported that the waste composition is the most critical factor for the stiffness and material damping ratio of MSW. Based on the results, an increase in the larger fraction and fiber materials shifted the normalized shear modulus curve to the right and they exhibited greater elastic behavior (Zekkos, 2005). Towhata and Uno (2008), assessed the influence of confining stress on waste samples collected from a landfill in Japan. The results indicated that the shear modulus increases with increasing confining pressure while the effect of confining stress on the damping ratio was not clear. The author also stated that the dynamic behavior of MSW under strains less than 0.01% is uncertain (Towhata and Uno, 2008). Yuan et al. (2011), performed 15 large-scale cyclic simple shear tests on reconstituted specimens of MSW from the Tri-Cities landfill. The test results clearly showed a strong dependence of shear wave velocity and small strain shear modulus on unit weight as well as a dependence on the composition ratio (Yuan et al., 2011). Ramaiah et al. (2016a, 2016b) and Keramati, et al. (2016) and Keramati et al. (2017a, 2017b also conducted a
1
A landfill site located in Monterey Park, California
Please cite this article in press as: Alidoust, P., et al. Laboratory studies on effect of fiber content on dynamic characteristics of municipal solid waste. Waste Management (2018), https://doi.org/10.1016/j.wasman.2018.02.038
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series of large-scale cyclic triaxial tests to evaluate the influence of different parameters, including confining pressure, loading cycles, loading frequency and percentage of components on the dynamic characteristics of MSW. The results obtained by both researchers indicated that the MSW samples with a greater percentage of fibrous materials (plastics, textiles, paper, wood, and leather) responded more linearly when compared to MSW with a lower percentage of fibrous materials (Keramati et al., 2017b, 2016; Ramaiah et al., 2016b). 2. Testing program This study was conducted to evaluate the effect of fiber contents on the dynamic behavior of MSW. To accurately assess the effects of fibers on MSW, it is beneficial to test the samples with an identified composition as opposed to direct field-sampling of buried MSW. The key point in this method is the fact that waste composition inevitably changes with time owing to the biological degradation of organic components. The percentage of organic content significantly decreases over time while the percentage of fiber material simultaneously increases. Accordingly, a total of 60 straincontrol cyclic triaxial tests were performed on screened MSW samples retrieved from the Kahrizak landfill, located on the outskirts of Tehran. The study was performed using a series of consolidated undrained (CU) strain control triaxial tests on medium sized (with a diameter of 100 mm and a height of 200 mm) reconstituted samples of fresh MSW under isotropic confining stresses of 75 kPa and 150 kPa. The testing program also used a BE device to measure the small-strain shear modulus, and a large-scale oedometer device to determine the Poisson’s ratio of the sampled MSW. The sample preparation methods were in accordance with the procedure specified by Keramati et al. (2016). 2.1. Description of Kahrizak landfill, aradkooh WP2 plant The Kahrizak landfill is located in the south of Tehran, at 54° 510 E, 22° 350 N. The total foot-print area of this landfill is approximately 1200 ha. The site has been in operation for more than 40 y and receives all waste generated from Tehran city, including household waste and sanitary waste. Based on statistical data, the generated waste for the city exceeds 11,000 t per day. The high landfilling percentage of 75% for this site is a result of the failure of the Tehran municipality to provide facilities for proper recycling and lack of basic infrastructure for waste treatment. In the past, waste disposal was performed sporadically in different areas of the landfill, in natural pit or artificial trenches. Within the last decade, given the limitations associated with the environment and landfill size, waste disposal was focused in a specific section, and waste was stored in parallel layers, with a thickness ranging from 2 to 3 m. currently, as shown in Fig. 1, the height of disposed waste has reached approximately 60 m. Additionally, geotechnical studies on the native soil underlying the landfill showed lean clay, with low plasticity, as per the Unified Soil Classification System. 2.2. Material characterization and specimen preparation MSW components worldwide vary over a wide range, because of differences in geographic location, culture, consumption patterns, and existing laws for landfilling waste. However, in general, the dominant constituents are fractions less than 20 mm, paper, soft plastic, wood, and gravel (Zekkos, 2005). To determine the 2
Waste processing
3
constituents of fresh MSW in Tehran, processed fresh waste samples were collected for characterization and testing in a research center at IUST. All waste coming to the Kahrizak landfill was subjected to certain recycling processes, including the removal of large, stiff, and recycled particles. The results of the physical analysis of fresh waste from the Kahrizak landfill are presented in Fig. 2. The significant point of the composition analysis was the high percentage of organic waste, that demonstrates the significant impact of time on variations in waste mass. Because of the high biodegradability of organic matter when compared with the fiber contents, longer time leads to increases in the percentage of fiber in the waste mass (by decreasing the percentage of organic matter). Based on the ASTM D2216 procedure (D2216, 2005), the average moisture content of fresh samples was 141%, thus showing the high percentage of organic content. The samples were then prepared with three fiber contents (by wet weight): 0%, 3%, and 6%. To obtain an accurate waste composition for the fiber content, the materials that could introduce extra fiber content were removed from the waste tray prior to sample preparation. The removed material comprised textiles, large plastic items, and bulk materials like wood and gravel. However, the waste specimens were obtained from a screening station in the Kahrizak landfill, in which a high fraction of the waste was unfit for testing without separating or shredding large constituents. As per ASTM standards for testing of sample using triaxial equipment, the size of the largest particle should be smaller than 1/6th of the sample diameter (ASTM D3999). Zekkos (2005) proposed that large constituents that are soft can be as large as 2/6 of the sample diameter for the reconstituted waste sample to undergo a triaxial compression test (Zekkos, 2005). Based on the sample sizes (100 mm 200 mm for triaxial test samples and 100 mm 100 mm for BE test samples), waste constituents with the greatest softness and stiffness values were 33 mm and 16 mm wide, respectively. The test samples were compacted to reach a unit weight of 9 kN/m3. Based on research conducted by Zekkos et al. (2006), the following equation presents a hyperbolic equation to estimate the MSW unit weight as a function of depth (Zekkos et al., 2006).
c ¼ ci þ
z
a þ bz
ð2Þ
where !i is the near-surface in-place unit weight (kN/m3); z is the depth (m) at which the MSW unit weight ! is to be estimated; and a (m4/kN) and b (m3/kN) are modeling parameters. In this study, the near-surface in-place unit weight was evaluated based on an in situ large-scale method. In this approach, large-scale test pits were excavated and the retrieved waste materials were weighed. The volume of excavated cavity was then filled with water, and the volume of the cavity was determined from the amount of water required. By dividing the measured weight of the excavated MSW by the estimated volume of the cavity, the average near-surface unit weight was calculated as 7.5 KN/m3. The parameter a is a function of the rate of unit weight increase with depth close to the surface while b is a function of the difference in unit weight between that at the surface and at great depth where the unit weight profile becomes approximately constant. These parameters are dependent on the compaction effort and amount of soil in the landfill. Considering the lack of applicable specific density equipment, such as compactors, and a thickness of 2–3 m of fresh waste layer in the Kahrizak landfill, the values of a and b, based on proposed tables by Zekkos et al. (2006), were assumed to be 2.5 and 0.15, respectively (Zekkos et al., 2006). Fig. 3 presents the unit weight profile of fresh MSW from the Kahrizak landfill. Based on Fig. 3, the adopted unit weight multiplied by the related depth results in a 75 kPa confining pressure.
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Fig. 1. General view of Kahrizak landfill area.
Considering the nature of the MSW, the ASTM D4767 standard was used for sample preparation. The ASTM D4767 describes a test procedure for sample preparation and performing a consolidated untrained triaxial compression test for cohesive soils (D4767, 2002). Sample preparation was performed by using an aluminum cylindrical mold, a thin layer of geotextile for better drainage in the consolidation phase, and a significantly thick membrane to avoid puncturing during sample compaction. The waste was accurately weighed and compacted in five layers to reach the desired height.
2.3. Testing equipment
Fig. 2. Physical analysis of MSW materials.
Fig. 3. Unit weight profile of fresh MSW from Kahrizak landfill.
2.3.1. Cyclic triaxial testing device A medium-sized strain-controlled cyclic triaxial apparatus was used to conduct CU compression tests on the MSW samples with varying fiber content. The configuration of the device allowed accurate measurements of the shear modulus at shear strains of approximately 0.08–4%. A data logger, with a recording rate of 60 Hz, was used during the compression phase to record pore pressures and axial loads. The internal pressure sensors were installed on bottom pedestal while the deformation and force sensors were placed on the top cap. The maximum confining stress capacity of the cell was 1000 kPa and a pressure cell with a volume of 2 L and a maximum pressure of 1500 kPa was used to apply the required pore pressure inside the sample. Table 1 presents the accuracies and ranges of the sensors used the in cyclic triaxial device. Owing to the nature of the test materials (MSW), the sample preparation, up to the loading phase, was performed in accordance with ASTM –D4767, and the loading and analyzing stages were conducted in accordance with the ASTM-D3999 (D3999, 1998; D4767, 2002).
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P. Alidoust et al. / Waste Management xxx (2018) xxx–xxx Table 1 Sensor details.
3. Test results
Axial strain sensor
Number Measurement range Measurement accuracy
1 70 mm(Horizontal) ±0.001 mm
Pore water pressure sensor
Number Measurement range Measurement accuracy
2 0–1000 kPa 0.2 kPa
Submersible load cell
Number Measurement range Measurement accuracy
1 ±20 kN 0.0025 kN
2.3.2. Bender element The parameter, Gmax is typically associated with shear strain levels of approximately 0.001%, and is a key parameter in small strain dynamic analyses. To date, most researchers employed in-situ techniques to evaluate the Gmax of the MSW by measuring Vs (with a small-strain range >0.001%). However, few studies for measuring Vs in the laboratory have been conducted. A BE test device comprises of a pair of piezoelectric plates placed in the middle of top and bottom pedestals, a function generator to transmit the desired voltage impulse and an oscilloscope to plot and interpret the received and transmitted motion wave. The tests were performed on saturated samples with heights and widths of 100 mm after the consolidation phase. In the wave propagation method the arrival time of the shear wave, propagated from the source, was measured. The received signal must be carefully analyzed to identify the arrival time of the S wave. Additionally, the arrival time (t) was measured by using simple peak-to-peak and start-to-start time domain analyses (Marjanovic, 2012). Given the tip to tip distance (Ltt ) of the benders, the arrival time is calculated by employing time domain analysis, and the shear wave analysis is computed as follows:
Vs ¼
Ltt t
ð3Þ
The theory of shear wave propagation in an elastic body states that the value of the shear modulus of the MSW from the measurement of shear wave velocity can be determined by Eq. (1). 2.3.3. Large scale oedometer testing device The testing program also included the use of a large-scale oedometer (490 mm 490 mm). The apparatus comprised an oedometer cell, a polyamide porous plate, a displacement transducer, two pressure cells, six pressure transducers, a base plate, and a computerized data acquisition system. Additionally, the loading system involved a hydraulic jack capable of exerting 1 MPa pressure on the specimen. Furthermore, the hydraulic system was computer-controlled to be able to adjust and maintain the desired stress on the specimen. The apparatus can display the atrest lateral earth pressure coefficient (K0) under different vertical and horizontal pressures. Using the elasticity theory, the Poisson’s ratio and the at-rest earth pressure coefficient are related by the following equation:
K0 ¼
m
1m
5
ð4Þ
Owing to the lack of an appropriate leachate collection system in the Kahrizak landfill, a large fraction of waste is in a saturated state. Therefore, to obtain representative parameters of MSW in this landfill, oedometer tests were performed in a saturated condition under 1 bar pressure.
3.1. Triaxial test result A total of 60 cyclic triaxial tests were performed to assess the dynamic behavior of fresh MSW under different conditions. Following the consolidation phase (irrespective of the confining stress), strain controlled cyclic loading was applied at frequencies of 1 Hz and 0.1 Hz, under undrained conditions, on all specimens, and each specimen was loaded up to 15th cycle at an axial strain range from 0.05 to 2.5%. The result of the cyclic triaxial testing system for each cycle exhibits a hysteresis loop that enables the evaluation of the shear modulus and material damping. The influence of different factors, including fiber content, confining stress, loading frequency, and number of cycles is presented in the following sections. 3.2. Shear wave velocity A BE test was performed on MSW samples with varying fiber content and confining stresses. The wave propagation method measures the travel time of the shear wave. Table 2 presents the results of BE tests performed on saturated MSW samples based on Eqs. (1) and (2). As shown, the inclusion of fiber contents resulted in an increase in the Gmax of each sample. This trend is dependent on several factors, including the composition, density and compaction energy of samples. Therefore, it is difficult to distinguish the influence of each parameter individually. However, it should be taken into account that the reinforcement effect of fibers at strains of approximately 104%, and lower, the accepted range for the BE testing, is negligible, and it is not possible for fibers to stretch within the mentioned range. The key factor is thus the change in the elasticity of the constituents. An increase in the fiber content and confining stress increases the elasticity of the waste material, and generates a greater connection between the waste particles. For example, a marginal increase in the shear wave velocity of samples, under a confining pressure of 75 kPa can be related to an increase in the compaction energy to achieve to a target density. This increasing trend is significantly more evident at greater confining stresses because of the increased density of the samples. It must be mentioned that the inclusion of fiber contents reduces the arrival time of the received wave, due to the increased stiffness of the samples, while, at the same time, the bender receives low-voltage waves. The attenuating nature of plastics decreases the voltage of the received wave, without any adverse impact on the shear wave velocity. As can be seen in Fig. 4, despite the low-voltage of a received wave, the data indicates a reduction in the wave arrival time (resulting in an increase in the shear wave velocity) of a sample with 6% fiber, while the sample with 3% fiber exhibits a greater travel time (greater peak-to-peak distance) with a higher voltage. 3.3. Poisson’s ratio Poisson’s ratio is a factor that directly affects the calculation of the shear modulus. Table 3 presents the results of the oedometer tests performed on saturated MSW samples based on Eq. (4). According to the results for samples with 0% fiber content, the average assessed values of the specimens was 0.47 (equivalent to a K0 of 0.9). Although, based on a literature review, it was observed that inclusion of the fibers would lead to a slight increase in Poisson’s ratio value, this was overlooked. The reason for this trend is again the fact that the compressibility of fibers is lower than organic components. Therefore, inclusion of fibers reduces the overall compressibility of waste mass. The resultant data is in
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Table 2 Results of the BE tests on fresh MSW. Prepared unit weight (kN/m3)
Unit weight after consolidation (kN/m3)
Fiber content%
Confining stress (kPa)
Shear wave velocity (m/s)
Gmax (MPa)
9 9 9 9 9 9
10.1 10.6 10.9 11.5 11.7 12.3
0 0 3 3 6 6
75 150 75 150 75 150
96.1 97.9 98.6 100 101.2 120.8
9.34 10.15 10.59 11.53 11.98 17.94
Fig. 4. BE test response on two different samples.
Table 3 Results of oedometer tests on fresh MSW. Sample condition
Fiber content%
Obtained K0
Poisson’s ratio
Average
Saturated Saturated Saturated Saturated Saturated Saturated Saturated Saturated Saturated
0 0 0 3 3 3 6 6 6
0.92 0.88 0.92 0.92 0.92 0.92 0.96 0.96 1
0.48 0.47 0.48 0.48 0.48 0.48 0.49 0.49 0.5
0.47
accordance with the reports of Kavazanjian (2003) because of the high percentage of organic waste and the saturation conditions in this study. Furthermore, the author states that in saturated conditions such as in bioreactor landfills, the value of Poisson’s ratio can exceed 0.4 (Kavazanjian, 2003). In fact, water as a liquid is incompressible; therefore, it is reasonable to expect that as the amount of water, in saturated and undrained conditions, increases, the Poisson’s ratio will also increase. Several researchers worldwide focused on assessing Poisson’s ratio, and their results are summarized in Table 4. The table presents the evaluated Poisson’s ratio, and its related method, for MSW. As can be seen, there is significant scattering in the various measured Poisson’s ratio and its related method, in MSW literature. The reason for this major difference is attributed to many factors such as the decomposition of MSW over time, different constituents of the MSW based on the region or different testing situations including drained or undrained, and, saturated or unsaturated conditions.
0.48
0.49
Table 4 Poisson’s ratio values, from technical literature and this study. No.
Researcher
Method
Values
1 2
Down hole Shear and compressional waves Cross hole Triaxial test
0.49 0.33
3 4
Sharma et al. (1990) Matasovic´ and Kavazanjian (1998) Carvalho et al. (1998) Towhata et al. (2004)
5 6
Zekkos (2005) Naveen et al. (2014)
7 8
Ramaiah et al. (2016a,2016b) Current study
Cyclic triaxial Considered based on partially saturated condition Cyclic triaxial Large scale odeometer
0.3 0.2– 0.29 0.1–0.4 0.3
0.5 0.47
1-(Sharma et al., 1990), 2-(Matasovic´ and Kavazanjian, 1998), 3-(Carvalho et al., 1998), 4-(Towhata et al., 2004), 5-(Zekkos, 2005) 6-(Naveen et al., 2014), 7-(Ramaiah et al., 2016a).
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Fig. 5. Effect of number of cycles on three samples.
4. Discussion 4.1. Effect of number of cycles In this study, dynamic parameters of three samples under different testing conditions, namely confining stress, percentage of fiber contents and axial strain, were evaluated based on loadings
Fig. 6. Effect of number of cycles on the generation of excess pore water pressure.
over 15 cycles (Fig. 5). As observed, shear modulus and material damping experienced slightly increasing and decreasing trends with increases in the number of cycles, respectively. Fig. 5 shows that the shear modulus and damping ratio do not show a significant difference for different numbers of cycles, and the shear modulus and damping ratio are actually independent on the number of cycles. Similar results were reported by Naveen et al. (2014) and Keramati et al. (2017a, 2017b). Naveen et al. (2014) evaluated the effects of the number of cycles on waste with different confining stresses, while Keramati et al. (2017a, 2017b) measured the effect of the number of cycles on waste of different ages. Irrespective of the testing condition, all the results reported a negligible effect of the number of cycles on the shear modulus and damping ratio. It must be noted that the effect of the number of cycles on MSW is significantly less than that of typical soils, and this is attributed to the presence of compressible particles in waste mass. As discussed, based on the results, the shear modulus and damping ratio do not differ significantly with the number of cycles, but, in order to increase the accuracy of the results, all values were evaluated in the tenth cycle (Keramati et al., 2017a; Naveen et al., 2014). Regarding the possible effects of loading cycles on the generation of excess pore water pressure, it is clear that increasing the loading cycle in an undrained condition would lead to a greater excess pore water pressure in samples, and this increase has been
Fig. 7. Hysteresis loops under varying confining stresses and axial strains.
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observed in all the values by means of using effective stress in equations. According to Fig. 6, the resultant analysis could not demonstrate any effective role of fiber contents. Actually, an increase in confining stress values is the main reason of an increase in excess pore water pressure values, while an increase in fiber contents did not change the values significantly. It should also be taken into account that generated excess pore water pressure has higher values under greater shear strains (or equivalent loading cycle), while the majority of the resultant dynamic parameters have similar values within that range. 4.2. Effects of loading frequency To evaluate the effects of loading frequency, samples with 0% fiber were loaded at 1 Hz and 0.1 Hz. Fig. 7 presents an example
of stress-strain hysteresis loops for the above-mentioned frequencies under different confining stresses (75 kPa and 150 kPa) and axial strains. Fig. 8 shows the changes in the shear modulus and damping ratio curves for two samples under 75 kPa confining stress, within the shear strain range of 0.05–2.5%. According to Fig. 8, a 10-fold increase in loading frequency, under the same confining stress, leads to a marginal increase in the shear modulus (that is equivalent to the gradient of the hysteresis loop), while the differences exhibit a decreasing trend with respect to increases in the shear strain. The damping ratio also exhibits a decrease, and the increase in shear strain was identical with respect to changes in the shear modulus. It must be noted that the effect of loading frequency is approximately the same for the specimens under higher confining stresses and different fiber content. The reason for these changes is attributed to the increasing loading rate in the same domain that leads to viscous behavior of the waste (due to the high organic matter content in the waste mass). In this regard, similar results were reported by Keramati et al. (2017a, 2017b) on fresh MSW from the Kahrizak landfill, (Keramati et al., 2017b). Zekkos (2005) reported that the loading frequency does not significantly affect either the normalized shear modulus reduction, or the material damping. However, the normalization of curves can affect the results obtained by Zekkos (2005). 4.3. Effect of confining stress Undrained cyclic triaxial tests were conducted on specimens with 0% fiber content under 75 kPa and 150 kPa confining stresses (at the same frequency and shear strain). Table 5 presents the results of these tests. The variation in cyclic properties is shown in Fig. 9. Based on the results, the application of a two-fold increase in confining stress, from 75 kPa to 150 kPa increases the shear modulus trend, however, the effect of confining stress on the damping ratio is unclear. As can be seen, the increasing trend of shear modulus (with increasing confining stress) decreases at higher shear strains. Specifically, a greater confining stress generates more interaction among MSW constituents and increases the overall density of samples, leading to increased stiffness. Conversely, with respect to higher shear strains the connection of constituents is less, and discreteness is greater. The combination of the above-mentioned factors at higher shear strains reduces the shear modulus values. The results are in accordance with those reported in studies by Ramaiah et al. (2016a, 2016b); Towhata and Uno (2008) and Keramati et al. (2017a, 2017b); Ramaiah et al., 2016b; Towhata and Uno, 2008). 4.4. Effect of fiber contents
Fig. 8. Changes in shear modulus and damping ratio curves under 75 kPa confining stress over range 0.05–2.5% shear strain.
To evaluate the effect of fiber contents on MSW, plastic fibers percentages of 0%, 3%, and 6% (by weight) were added to the fresh samples, and their related cyclic behavior was evaluated under different test conditions. Table 6 shows the effect of fiber contents on MSW samples under 75 kPa confining stress and a frequency of
Table 5 Damping ratio and shear modulus values for samples under different confining pressures. Shear strain%
0.075 0.148 0.37 0.735 1.83 3.63
0-75-1
0-150-1
Variation ratio
Shear modulus (kPa)
Damping (%)
Shear modulus (kPa)
Damping (%)
Shear modulus
Damping
1943.6 1852 1212.32 1005.79 630.55 373.05
5.89 6.68 9.74 15.6 15.99 20.59
2140 1903.47 1327.14 951.85 638 353.12
6.1 7.29 14.19 13.82 21.37 18.61
1.10 1.02 1.09 0.94 1.01 0.94
1.03 1.09 1.45 0.88 1.33 0.9
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Fig. 9. Variation in dynamic properties with respect to changes in confining stress.
1 Hz. Based on the results, increasing plastic percentages result in greater values of the shear modulus (at lower shear strains), while the behavior of the damping ratio is still unclear. However, despite the increase in values when compared with the reference sample, the increasing trend for the shear modulus is not significant between the 3% and 6% samples. This behavior is attributed to the low levels of confining pressure and density of samples that are unable to mobilize the reinforcement of fibers. Additionally, it must be noted that the reinforcing effect of fiber contents decreases to a low level owing to increases in the discreteness of the constituent at higher shear strains (close to the reference sample). An interesting finding with respect to the effect of fiber content was observed at higher confining stresses. Fig. 10 shows the effect of fiber content on the dynamic properties of MSW under different confining stresses, for shear strains from 0.075 to 3.63%. Generally, as observed, an increase in plastic leads to greater values of shear modulus, while the material damping does not exhibit a steady trend. The generated stiffness is related to the growing reinforcement of fibers and sample stiffness by increasing fiber percentages. The important point is the rate of increase in shear modulus, given the simultaneous increases in the fiber contents and confining stress (irrespective of the loading frequency). The greatest increase in reinforcement was obtained when the confining stress increased from 75 kPa to 150 kPa. This resulted in greater values of shear modulus for samples with 3% plastic, under 150 kPa when compared with samples with 6% plastics under 75 kPa. The trend indicated the effect of confining stress on the reinforcing effect of samples with different fiber content percentages. As expected, sample 6-150-1 exhibited the maximum value of shear modulus, because of the presence of two dominant factors at the maximum level.
Fig. 10. Effect of fiber content on dynamic properties of MSW in different situations.
4.5. Normalized shear modulus reduction To compare the results with existing technical literature, the obtained normalized shear modulus reduction and damping ratio of current study are shown in conjunction with the most popular recommendations for MSW curves (Fig. 11). As stated in Section 4.4, the resultant damping ratio in this study does not
Table 6 Effect of fibers content on dynamic properties at a frequency of 1 Hz and confining stress of 75 kPa. Shear strain%
0.075 0.148 0.37 0.735 1.83 3.63
Reference
3% Plastic
6% Plastic
0-75-1
3-75-1
6-75-1
Shear modulus (kPa)
Damping (%)
Shear modulus (kPa)
Damping (%)
Shear modulus (kPa)
Damping (%)
1943.6 1852 1212.32 1005.79 630.55 393.05
5.89 6.68 9.74 12.28 15.99 20.59
2163.3 2100 1910 1362.41 761 594
3.89 5.9 6.48 18.16 16.88 19.04
2767.29 2243.33 1403.94 1006.388 791.3 584
7.65 10.84 15.71 18.95 21.86 28.2
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Fig. 11. Comparison results of previous studies (Augello et al., 1998; Dobry and Vucetic, 1987; Feng et al., 2005; Gunturi, 1996; Idriss et al., 1995; Keramati et al., 2017a; Lee, 2007; Matasovic´ and Kavazanjian, 1998; Ramaiah et al., 2016b; Singh and Murphy, 1990; Yuan et al., 2011; Zekkos et al., 2008).
exhibit a definite trend with increases in the fibrous materials. However, the resulting values are within the range stated in the technical literature and follow the principles of soil mechanics with respect to increases in values with increasing shear strain. Additionally, the normalized shear modulus reduction data falls to the right-side with increases in fibrous material, and this indicates greater elastic behavior. However, it is necessary to simultaneously consider the impact of fiber contents, confining stress and shear strain on the variation rate of normalized shear modulus reduction values. Within common strain ranges for dynamic tests, fiber contents cannot offer sufficient reinforcement in the waste material. On the other hand, the overall stiffness increases with confinement and samples exhibit greater strength. Greater stress generally generates more interaction among constituents, and reduces the discreteness. This behavior creates greater bonding between particles, and increases the elasticity of the entire sample. The importance of shear strain in decreasing the stiffness should also be considered. Typically, applying greater shear strain in MSW will result in greater discreteness in samples. Based on these considerations, the role of a single factor should not be considered the primary contributing factor in the improvement of shear modulus values. It must be noted that the majority of recommended curves include the data of the OII landfill, and this could be the reason for existing differences. However, as clearly observed, the normalized shear modulus and damping ratio of this study are in good agreement with the curve proposed by Singh and Murphy
(1990). The authors assumed that the strength properties of the refuse were more cohesive than frictional and recommended modulus reduction and material damping curves similar to those in peat and clay (Singh and Murphy, 1990). Owing to the greater elastic behavior in this study when compared with the results obtained by Singh and Murphy (1990), the observed dynamic behavior in this study can be taken as similar to that observed in high plasticity index clay.
5. Conclusions Normalized shear modulus and material damping curves of MSW are affected by various factors, such as the test type and procedure, waste composition, drainage conditions, and ranges of shear strain. In this study, the dynamic properties of MSW from the Kahrizak landfill were evaluated with respect to variations of different factors through a comprehensive laboratory testing program. The program included measuring the Poisson’s ratio of MSW by using a large-scale oedometer testing device, measuring the shear wave velocity and smallstrain shear modulus of MSW by BE tests and evaluating the damping ratio and shear modulus of the samples by cyclic triaxial tests. The specimens were reconstituted by adding either 0%, 3% or 6% of plastic fibers (by weight) to assess the effect of fiber contents on the dynamic behavior of MSW. Furthermore, all the tests were consolidated under
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P. Alidoust et al. / Waste Management xxx (2018) xxx–xxx Table 7 Importance of various factors on the dynamic characteristic of MSW.
Fiber contents Loading frequency Confining pressure Loading cycle
G
Gmax
High importance Low importance High importance
High importance — High importance —
a
G/Gmax
D
Important Important
Unclear Low importance Unclear
a
a
a
- - -: Independent. a Negligible.
confining stresses corresponding to75 kPa and 150 kPa, and subjected to up to 15 cyclic loadings. To summarize the results obtained, the impact of each factor on the undrained cyclic behavior of MSW is presented in Table 7. It must be noted that, based on the principles of soil mechanics, increasing shear strain causes significant changes in the dynamic parameters of MSW, and is considered the most effective factor on the normalized shear modulus and damping ratios of MSW. Table 7 clearly indicates the primary influence of the confining pressure and fiber contents. An important point is the necessity of simultaneously considering both factors to accurately illustrate the fiber reinforced behavior of waste mass. Given the inclusion of plastic fiber to waste mass, the efficiency of the fiber reinforcement is observed as dependent on the values of the confining pressure. Increasing stiffness of the samples, from the inclusion of plastic fibers, is the result of changes in the waste composition, as opposed to the creation of reinforcement. This behavior is attributed to the low range of strain for dynamic tests that is unable to create essential tension in fiber particles. Additionally, increasing confining stress leads to increased bonding between particles, and this increases the elasticity of the waste mass. These findings result in higher values of shear modulus and lower values of discreteness in waste samples, while the behavior of damping ratio is unclear. The reason for the discrepancy in values of damping ratio remains unclear, although it may be related to the compressibility of MSW. References Athanasopoulos, G., Grizi, A., Zekkos, D., Founta, P., Zisimatou, E., 2008. Municipal solid waste as a reinforced soil: Investigation using synthetic waste. In: Geocongress 2008: Geotechnics of Waste Management and Remediation. 168– 175. Augello, A.J., Bray, J.D., Abrahamson, N.A., Seed, R.B., 1998. Dynamic properties of solid waste based on back-analysis of OII landfill. J. Geotech. Geoenviron. Eng. 124, 211–222. Augello, A.J., Matasovic, N., Bray, J.D., Kavazanjian Jr, E., Seed, R.B., 1995. Evaluation of solid waste landfill performance during the Northridge earthquake. Geotech. Spec. Publ. 54, 17–50. Carvalho, M. de F., Vilar, O.M., 1998. In-situ tests in urban waste sanitary landfill. In: Proceedings of the 3rd International Congress on Environmental Geotechnics, Lisboa. pp. 95–100. D2216, S.A., 2005. Standard Test Methods for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass. West Conshohocken Am. Soc. Test. Mater. 2216. D3999, S.A., 1998. D3999: Standard Test Methods for the Determination of the Modulus and Damping Properties of Soils Using the Cyclic Triaxial Apparatus. Annu. B. ASTM Stand. D4767, S.A., 2002. Standard test method for consolidated undrained triaxial compression test for cohesive soils. ASTM Int. 2002, ASTM 2. Dobry, R., Vucetic, M., 1987. Dynamic properties and seismic response of soft clay deposits. In: Proc. Intern. Symp. Geotech. Eng. Soft Soils,. 51–87. Elgamal, A., Lai, T., Gunturi, R., Zeghal, M., 2004. System identification of landfill seismic response. J. Earthq. Eng. 8, 545–566. Feng, S., Chen, Y., Kong, X., Zou, D., 2005. Experimental research on dynamic properties of municipal solid waste. Chin. J. Geotech. Eng. Ed. 27, 750. Gunturi, V.R., 1996. Identification and modeling of seismic response of landfills. Troy, NY: Rensselaer Polytechnic Institute. Hossain, M.S., Haque, M.A., Hoyos, L.R., 2010. Dynamic properties of municipal solid waste in bioreactor landfills with degradation. Geotech. Geol. Eng. 28, 391–403. Idriss, I.M., Fiegel, G., Hudson, M.B., Mundy, P.K., Herzig, R., 1995. Seismic response of the operating industries landfill. In: Earthquake Design and Performance of Solid Waste Landfills. ASCE, pp. 83–118.
Kavazanjian Jr, E., 2003. Evaluation of MSW properties using field measurements. In: Proceedings 17th Geosynthetic Research Institute Conference, Hot Topics in Geosynthetics—IV. pp. 74–113. Kavazanjian Jr, E., Matasovic, N., Bachus, R.C., 1999. Large-diameter static and cyclic laboratory testing of municipal solid waste. Procee. Sardinia., 437–444 Kavazanjian Jr, E., Matasovic, N., Stokoe, K.H., Bray, J.D., 1996. In situ shear wave velocity of solid waste from surface wave measurements. In: Proceedings 2nd International Congress on Environmental Geotechnics, Osaka, Japan, pp. 97– 104. Keramati, M., Shariatmadari, N., Karimpour-Fard, M., Saeedanezhad, A., Alidoust, P., 2017a. Effects of aging on dynamic properties of MSW: A case study from Kahrizak Landfill, Tehran, Iran. Scientia Iranica. Nov 14. Keramati, M., Shariatmadari, N., Karimpour-Fard, M., Shahrbabak, M.R.N., 2016. Dynamic behaviour of MSW materials under cyclic triaxial testing: a case of Kahrizak landfill, Tehran, Iran. Iran. J. Sci. Technol. Trans. Civ. Eng. 40, 75–83. Keramati, M., Shariatmadari, N., Sabbaghi, M., Sadegh Abedin, M., 2017b. Effect of confining stress and loading frequency on dynamic behavior of municipal solid waste in Kahrizak landfill. Int. J. Environ. Sci. Technol. 1–8. http://doi.org/10. 1007/s13762-017-1465-1. Khaleghi, M., 2011. In-situ test to determine the effect of aging on shear wave velocity of municipal solid waste (Case study: Kahrizak Landfill). Iran Univ. Sci. Technol. Master Sci. Thesis [in Persian]. Iran University of Science and Technology. Lee, J.J., 2007. Dynamic characteristics of municipal solid waste (MSW) in the linear and nonlinear strain ranges. The University of Texas at Austin. Marjanovic, J., 2012. The study of shear and longitudinal velocity measurements of sands and cohesive soils. Massachusetts Institute of Technology. Matasovic´, N., Kavazanjian Jr, E., 1998. Cyclic characterization of OII landfill solid waste. J. Geotech. Geoenvironmental Eng. 124, 197–210. Matasovic´, N., Kavazanjian Jr, E., Abourjeily, F., 1995. Dynamic properties of solid waste from field observations. In: Proc., 1st Int. Conf. on Earthquake Geotech. Engrg. pp. 549. Morochnik, V., Bardet, J.P., Hushmand, B., 1998. Identification of dynamic properties of OII landfill. J. Geotech. Geoenvironmental Eng. 124, 186–196. Naveen, B.P., Sitharam, T.G., Sivapullaiah, P.V., 2014. Evaluating the dynamic characteristics of municipal solid waste for geotechnical purpose. Curr. Adv. Civ. Eng. 2. Pereira, A.G.H., Sopena, L., Mateos, T.G., 2002. Compressibility of a municipal solid waste landfill, in: Proc., 4th Int. Congress on Environmental Geotechnics. pp. 201–206. Ramaiah, B.J., Ramana, G.V., Kavazanjian, E., Matasovic, N., Bansal, B.K., Kavazanjian Jr, E., Matasovic, N., Bansal, B.K., 2015. Empirical model for shear wave velocity of municipal solid waste in situ. J. Geotech. Geoenvironmental Eng. 142, 6015012. https://doi.org/10.1061/(ASCE)GT.1943-5606.0001389. Ramaiah, B.J., Ramana, G.V., Bansal, B.K., 2016a. Field and large scale laboratory studies on dynamic properties of emplaced municipal solid waste from two dump sites at Delhi, India. Soil Dyn. Earthq. Eng. 90, 340–357. Ramaiah, B.J., Ramana, G. V, Kavazanjian Jr, E., Bansal, B.K., 2016b. Dynamic Properties of Municipal Solid Waste from a Dump Site in Delhi, India, in: Geo-Chicago. pp. 121–130. Shariatmadari, N., Karimpour-Fard, M., Keramati, M., Kolarijani, H.J., Naebi, A., 2011. Fiber content impact on the shear strength of msw materials in direct shear tests. In: Proceedings Sardinia 2009, Thirteenth International Waste Management and Landfill Symposium. Shariatmadari, N., Machado, S.L., Noorzad, A., Fard, M.K., Keramati, M., Kalarijani, S. H.J., 2010. Shearing rate effect on mechanical behavior of MSW materials. In: International Conferences on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics. Missouri University of Science and Technology, San Diego - California, pp. 1–8. Sharma, H.D., Dukes, M.T., Olsen, D.M., 1990. Field measurements of dynamic moduli and Poisson’s ratios of refuse and underlying soils at a landfill site. Geotechnics of Waste fills—Theory and Practice. ASTM International. Singh, S., Murphy, B.J., 1990. Evaluation of the stability of sanitary landfills. Geotechnics of Waste Fills—Theory and Practice. ASTM International. Towhata, I., Kawano, Y., Yonai, Y., Koelsh, F., Koelsch, F., 2004. Laboratory tests on dynamic properties of municipal wastes. In: Proceedings of the 11th Conference in Soil Dynamics and Earthquake Engineering and the 3rd International Conference on Earthquake Geotechnical Engineering. pp. 688–693. Towhata, I., Uno, M., 2008. Cyclic shear tests of municipal waste in large triaxial device for identification of its dynamic properties. Geotech. Earthquake Eng. Soil Dynam. IV., 1–10
Please cite this article in press as: Alidoust, P., et al. Laboratory studies on effect of fiber content on dynamic characteristics of municipal solid waste. Waste Management (2018), https://doi.org/10.1016/j.wasman.2018.02.038
12
P. Alidoust et al. / Waste Management xxx (2018) xxx–xxx
Yuan, P., Kavazanjian, E., Chen, W., Seo, B., 2011. Compositional effects on the dynamic properties of municipal solid waste. Waste Manage. 31, 2380–2390. Zekkos, D., Bray, J.D., Kavazanjian Jr, E., Matasovic, N., Rathje, E.M., Riemer, M.F., Stokoe, K.H., Kavazanjian, E., Matasovic, N., Rathje, E.M., Riemer, M.F., Stokoe, K. H., 2006. Unit weight of municipal solid waste. J. Geotech. Geoenviron. Eng. 132, 1250–1261. https://doi.org/10.1061/(ASCE)1090-0241(2006) 132:10(1250).
Zekkos, D., Bray, J.D., Riemer, M.F., 2008. Shear modulus and material damping of municipal solid waste based on large-scale cyclic triaxial testing. Can. Geotech. J. 45, 45–58. Zekkos, D.P., 2005. Evaluation of static and dynamic properties of municipal solidwaste. University of California, Berkeley.
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Waste Management journal homepage: www.elsevier.com/locate/wasman
Laboratory studies on effect of fiber content on dynamic characteristics of municipal solid waste P. Alidoust a, M. Keramati b,⇑, N. Shariatmadari a a b
Department of Civil Engineering, Iran University of Science and Technology, Narmak, Tehran 16846-13114, Iran School of Civil Engineering, Shahrood University of Technology, P.O. Box 3619995161, Shahrood, Iran
a r t i c l e
i n f o
Article history: Received 26 April 2017 Revised 29 January 2018 Accepted 16 February 2018 Available online xxxx Keywords: Fiber content Cyclic triaxial test Dynamic behavior Municipal solid waste
a b s t r a c t The dynamic characterization of municipal solid waste (MSW), especially in regions with high seismicity, is of considerable importance in the stability assessment of landfills. Additionally, findings indicated that the response of MSW under dynamic loadings is significantly affected by fibrous material. Therefore, a comprehensive strain-controlled cyclic triaxial testing program was performed on MSW samples retrieved from a landfill in the Kahrizak area, Tehran province. The tests were conducted on fresh MSW specimens (with a diameter of 100 mm) with different percentage of fibers in the consolidated undrained condition. The potential reinforcing capability of fibers and their impacts on changes in the MSW composition were investigated under variations of different factors including confining pressure, loading frequency, Poisson’s ratio, and loading cycles. From the results of the study, increasing fiber content in specimens resulted in improved elastic behavior of MSW under dynamic loadings, irrespective of the test conditions, such that the normalized shear modulus reduction curves shifted to the right, while the damping ratio curves exhibited no specific trend. However, it is necessary to simultaneously consider the impact of fiber contents, confining stress and shear strain on the variation rates of normalized shear modulus reduction values. This trend is attributed to the greater values of stiffness from changing the composition when compared with the one generated by obtained reinforcement within the studied strain range. Given the lack of systematic evaluations on the effect of the fibrous waste materials on the dynamic response of MSW, the results of this study provide additional insight into the seismic analysis of landfills. Ó 2018 Published by Elsevier Ltd.
1. Introduction The design and analysis of waste fills in earthquake prone areas are highly dependent on the reliable estimation of the dynamic properties of Municipal Solid Waste (MSW). Recent worldwide landfill failures and performance problems have resulted in the increasing importance of proper determination of the dynamic properties, including small-strain shear modulus, shear straindependent shear modulus reduction and damping ratio. Several studies reported the importance of the reinforcing effect of the fibrous waste constituents in enhancing the strength of MSW compared to the organic material of MSW (Athanasopoulos et al., 2008; Kavazanjian et al., 1999). It should be noted that the majority of published technical studies do not consider the direct impact of fiber content on the dynamic response of MSW, while several studies only evaluated the direct effect of fibers on the static ⇑ Corresponding author. E-mail address: [email protected] (M. Keramati).
response of MSW. However, the results indicated a substantial effect of fibrous materials on the static behavior of MSW, without any direct evidence to show any substantial effect on the dynamic properties (Shariatmadari et al., 2011, 2010; Zekkos, 2005). Despite numerous advances in the understanding of MSW properties, many of the factors affecting the dynamic response of MSW remain largely unknown. Additionally, in the seismic analysis of landfills, site-specific data are always preferable to general recommendations and experience. This is because waste materials may differ for a number of reasons, including weather conditions and culture (Zekkos et al., 2008). To address the abovementioned problems, this study includes the results of more than 60 cyclic triaxial tests, and is a continuation of previous studies conducted by the authors at the Iran University of Science and Technology (IUST). The aim of the study is to experimentally quantify the effect of fibrous materials on the dynamic properties of MSW, and demonstrate the potential effects of aging on MSW samples. Additionally, bender element (BE) and large-scale oedometer tests were conducted at the IUST
https://doi.org/10.1016/j.wasman.2018.02.038 0956-053X/Ó 2018 Published by Elsevier Ltd.
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geotechnical research center to evaluate the shear wave velocity and Poisson’s ratio of MSW, respectively.
1.1. Recent studies on dynamic properties of MSW The dynamic response of an MSW landfill subjected to cyclic loading depends to a large extent in the cyclic stress-strain characteristics of MSW. The required properties for seismic response analysis of an MSW landfill by using the equivalent linear method include the following: Shear wave velocity (V s ) or small-strain shear modulus (Gmax ). Strain-dependent material damping ratio (k). Strain-dependent normalized shear modulus reduction (G=Gmax ). The following summarizes the current understanding of the above-mentioned parameters. However, despite several attempts, there is a lack of reliable data on the dynamic properties of MSW (Naveen et al., 2014). The Gmax is one of the basic parameters required for much of the engineering analysis of the geomaterials. Based on the elasticity theory, the Gmax is related to the V s and the mass density (q) of the material as follows (Zekkos et al., 2008):
Gmax ¼ q:V 2s
ð1Þ
The shear wave velocity of MSW has been measured using different laboratory and in situ techniques. The spectral analysis of surface waves (SASW), multichannel analysis of surface waves (MASW), and microtremor analysis method (MAM) are gaining popularity because of their nonintrusive nature and rapid rate of testing, compared to other in situ methods, such as the downhole and crosshole methods (Ramaiah et al., 2015). The in situ methods typically determine a shear wave velocity of MSW materials based on the variation of landfill depth. Following the Northridge earthquake, Kavazanjian et al. (1996) proposed a recommended curve for the shear wave velocity of southern California landfills based on the SASW method (Kavazanjian et al., 1996). Pereira et al. (2002) measured the shear wave velocity using the SASW method in a landfill near Madrid, Spain, and reported a range of 100–250 m/s (Pereira et al., 2002). Khaleghi (2011) performed a series of Continuous Surface Wave System (CSWS) tests and reported on the shear wave velocity profile of the Kahrizak landfill (Khaleghi, 2011). Ramaiah et al. (2015) proposed an empirical linear model for V s of landfills with a maximum depth of 30 m based on statistical analysis of 146 in situ shear wave velocity profiles from 37 MSW landfill sites worldwide (Ramaiah et al., 2015). In addition to the wide use of in situ methods, laboratory methods are becoming increasingly applicable when the assessment of different factors, including waste composition, and confining pressure on shear wave velocity is critical. Many researchers evaluated the shear wave velocity of MSW materials by performing laboratory tests, such as the BE and resonant column (RC). Hossain et al. (2010) reported that the decomposition of waste has an increasing effect on values of small strain shear modulus based on a series of Resonant Column (RC) tests (Hossain et al., 2010). Yuan et al. (2011) estimated V s and subsequently the small-strain shear modulus in a laboratory by using a BE that was installed on a large simple shear apparatus (Yuan et al., 2011). The author reported the significant role of waste composition on V s values. Zekkos, (2005) conducted several tests to compare the laboratory data with field data. He reported that the laboratory-derived values are highly dependent on composition and time under the confinement of specimens (Zekkos, 2005).
The material damping ratio and shear modulus are the parameters required for the seismic response analysis of landfills. The shear modulus describes the stiffness of MSW, while the damping ratio represents the loss of energy during seismic loads. Different factors such as shear strain, number of cycles, and confining pressure can change the values of the parameters. The cyclic shear strain is accepted as the most important and influential of the factors. Because of this dependence, the abovementioned geotechnical parameters for dynamic analysis are typically depicted relative to the shear strain values. The majority of available recommendations on straindependent normalized shear modulus reduction and material damping curves of MSW are based on back analyses of recorded ground motions of the OII1 landfill using different analytical techniques (Zekkos, 2005). Various researchers proposed recommended curves based on back analysis although important differences were observed between the curves (Augello et al., 1995; Elgamal et al., 2004; Idriss et al., 1995; Matasovic´ et al., 1995; Morochnik et al., 1998). However, the necessity of dynamically assessing of MSW worldwide is recognized owing to site-specific characterization of MSW, and these types of data are always preferred to generalized data (the OII landfill is a specific case of landfill). The abovementioned discussion resulted in several laboratory and field test assessments. Large-scale tests, including cyclic triaxial, cyclic simple shear and RC tests, are of great interest. However, there is a lack of field and laboratory data, given the difficulties including health issues associated with testing waste material and sample disturbance. One of the first laboratory studies on the dynamic properties of MSW was conducted based on a large- diameter cyclic direct simple shear testing program by Matasovic´ and Kavazanjian (1998). The test included testing at uniform cyclic shear strains corresponding to 0.1%, 0.3%, 1%, 3%, and 5% at a frequency of approximately 0.1 Hz, on waste recovered from the OII landfill. The data showed good agreement in the dynamic properties of the OII landfill between the laboratory and back-calculated curves (Matasovic´ and Kavazanjian, 1998). Towhata et al., (2004), performed cyclic triaxial and shaking table tests on organic waste bio-treated both with, and without, plastics. The results confirmed that plastic sheets and other fiber components generate shear strength in waste specimens. They also reported that the material damping of specimens without plastic exhibited greater values when compared to containing specimens with plastic (Towhata et al., 2004). Zekkos (2005) conducted more than 80 large-scale cyclic triaxial tests on 25 specimens with varying waste composition, confining stress, loading frequency and density. The author reported that the waste composition is the most critical factor for the stiffness and material damping ratio of MSW. Based on the results, an increase in the larger fraction and fiber materials shifted the normalized shear modulus curve to the right and they exhibited greater elastic behavior (Zekkos, 2005). Towhata and Uno (2008), assessed the influence of confining stress on waste samples collected from a landfill in Japan. The results indicated that the shear modulus increases with increasing confining pressure while the effect of confining stress on the damping ratio was not clear. The author also stated that the dynamic behavior of MSW under strains less than 0.01% is uncertain (Towhata and Uno, 2008). Yuan et al. (2011), performed 15 large-scale cyclic simple shear tests on reconstituted specimens of MSW from the Tri-Cities landfill. The test results clearly showed a strong dependence of shear wave velocity and small strain shear modulus on unit weight as well as a dependence on the composition ratio (Yuan et al., 2011). Ramaiah et al. (2016a, 2016b) and Keramati, et al. (2016) and Keramati et al. (2017a, 2017b also conducted a
1
A landfill site located in Monterey Park, California
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series of large-scale cyclic triaxial tests to evaluate the influence of different parameters, including confining pressure, loading cycles, loading frequency and percentage of components on the dynamic characteristics of MSW. The results obtained by both researchers indicated that the MSW samples with a greater percentage of fibrous materials (plastics, textiles, paper, wood, and leather) responded more linearly when compared to MSW with a lower percentage of fibrous materials (Keramati et al., 2017b, 2016; Ramaiah et al., 2016b). 2. Testing program This study was conducted to evaluate the effect of fiber contents on the dynamic behavior of MSW. To accurately assess the effects of fibers on MSW, it is beneficial to test the samples with an identified composition as opposed to direct field-sampling of buried MSW. The key point in this method is the fact that waste composition inevitably changes with time owing to the biological degradation of organic components. The percentage of organic content significantly decreases over time while the percentage of fiber material simultaneously increases. Accordingly, a total of 60 straincontrol cyclic triaxial tests were performed on screened MSW samples retrieved from the Kahrizak landfill, located on the outskirts of Tehran. The study was performed using a series of consolidated undrained (CU) strain control triaxial tests on medium sized (with a diameter of 100 mm and a height of 200 mm) reconstituted samples of fresh MSW under isotropic confining stresses of 75 kPa and 150 kPa. The testing program also used a BE device to measure the small-strain shear modulus, and a large-scale oedometer device to determine the Poisson’s ratio of the sampled MSW. The sample preparation methods were in accordance with the procedure specified by Keramati et al. (2016). 2.1. Description of Kahrizak landfill, aradkooh WP2 plant The Kahrizak landfill is located in the south of Tehran, at 54° 510 E, 22° 350 N. The total foot-print area of this landfill is approximately 1200 ha. The site has been in operation for more than 40 y and receives all waste generated from Tehran city, including household waste and sanitary waste. Based on statistical data, the generated waste for the city exceeds 11,000 t per day. The high landfilling percentage of 75% for this site is a result of the failure of the Tehran municipality to provide facilities for proper recycling and lack of basic infrastructure for waste treatment. In the past, waste disposal was performed sporadically in different areas of the landfill, in natural pit or artificial trenches. Within the last decade, given the limitations associated with the environment and landfill size, waste disposal was focused in a specific section, and waste was stored in parallel layers, with a thickness ranging from 2 to 3 m. currently, as shown in Fig. 1, the height of disposed waste has reached approximately 60 m. Additionally, geotechnical studies on the native soil underlying the landfill showed lean clay, with low plasticity, as per the Unified Soil Classification System. 2.2. Material characterization and specimen preparation MSW components worldwide vary over a wide range, because of differences in geographic location, culture, consumption patterns, and existing laws for landfilling waste. However, in general, the dominant constituents are fractions less than 20 mm, paper, soft plastic, wood, and gravel (Zekkos, 2005). To determine the 2
Waste processing
3
constituents of fresh MSW in Tehran, processed fresh waste samples were collected for characterization and testing in a research center at IUST. All waste coming to the Kahrizak landfill was subjected to certain recycling processes, including the removal of large, stiff, and recycled particles. The results of the physical analysis of fresh waste from the Kahrizak landfill are presented in Fig. 2. The significant point of the composition analysis was the high percentage of organic waste, that demonstrates the significant impact of time on variations in waste mass. Because of the high biodegradability of organic matter when compared with the fiber contents, longer time leads to increases in the percentage of fiber in the waste mass (by decreasing the percentage of organic matter). Based on the ASTM D2216 procedure (D2216, 2005), the average moisture content of fresh samples was 141%, thus showing the high percentage of organic content. The samples were then prepared with three fiber contents (by wet weight): 0%, 3%, and 6%. To obtain an accurate waste composition for the fiber content, the materials that could introduce extra fiber content were removed from the waste tray prior to sample preparation. The removed material comprised textiles, large plastic items, and bulk materials like wood and gravel. However, the waste specimens were obtained from a screening station in the Kahrizak landfill, in which a high fraction of the waste was unfit for testing without separating or shredding large constituents. As per ASTM standards for testing of sample using triaxial equipment, the size of the largest particle should be smaller than 1/6th of the sample diameter (ASTM D3999). Zekkos (2005) proposed that large constituents that are soft can be as large as 2/6 of the sample diameter for the reconstituted waste sample to undergo a triaxial compression test (Zekkos, 2005). Based on the sample sizes (100 mm 200 mm for triaxial test samples and 100 mm 100 mm for BE test samples), waste constituents with the greatest softness and stiffness values were 33 mm and 16 mm wide, respectively. The test samples were compacted to reach a unit weight of 9 kN/m3. Based on research conducted by Zekkos et al. (2006), the following equation presents a hyperbolic equation to estimate the MSW unit weight as a function of depth (Zekkos et al., 2006).
c ¼ ci þ
z
a þ bz
ð2Þ
where !i is the near-surface in-place unit weight (kN/m3); z is the depth (m) at which the MSW unit weight ! is to be estimated; and a (m4/kN) and b (m3/kN) are modeling parameters. In this study, the near-surface in-place unit weight was evaluated based on an in situ large-scale method. In this approach, large-scale test pits were excavated and the retrieved waste materials were weighed. The volume of excavated cavity was then filled with water, and the volume of the cavity was determined from the amount of water required. By dividing the measured weight of the excavated MSW by the estimated volume of the cavity, the average near-surface unit weight was calculated as 7.5 KN/m3. The parameter a is a function of the rate of unit weight increase with depth close to the surface while b is a function of the difference in unit weight between that at the surface and at great depth where the unit weight profile becomes approximately constant. These parameters are dependent on the compaction effort and amount of soil in the landfill. Considering the lack of applicable specific density equipment, such as compactors, and a thickness of 2–3 m of fresh waste layer in the Kahrizak landfill, the values of a and b, based on proposed tables by Zekkos et al. (2006), were assumed to be 2.5 and 0.15, respectively (Zekkos et al., 2006). Fig. 3 presents the unit weight profile of fresh MSW from the Kahrizak landfill. Based on Fig. 3, the adopted unit weight multiplied by the related depth results in a 75 kPa confining pressure.
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Fig. 1. General view of Kahrizak landfill area.
Considering the nature of the MSW, the ASTM D4767 standard was used for sample preparation. The ASTM D4767 describes a test procedure for sample preparation and performing a consolidated untrained triaxial compression test for cohesive soils (D4767, 2002). Sample preparation was performed by using an aluminum cylindrical mold, a thin layer of geotextile for better drainage in the consolidation phase, and a significantly thick membrane to avoid puncturing during sample compaction. The waste was accurately weighed and compacted in five layers to reach the desired height.
2.3. Testing equipment
Fig. 2. Physical analysis of MSW materials.
Fig. 3. Unit weight profile of fresh MSW from Kahrizak landfill.
2.3.1. Cyclic triaxial testing device A medium-sized strain-controlled cyclic triaxial apparatus was used to conduct CU compression tests on the MSW samples with varying fiber content. The configuration of the device allowed accurate measurements of the shear modulus at shear strains of approximately 0.08–4%. A data logger, with a recording rate of 60 Hz, was used during the compression phase to record pore pressures and axial loads. The internal pressure sensors were installed on bottom pedestal while the deformation and force sensors were placed on the top cap. The maximum confining stress capacity of the cell was 1000 kPa and a pressure cell with a volume of 2 L and a maximum pressure of 1500 kPa was used to apply the required pore pressure inside the sample. Table 1 presents the accuracies and ranges of the sensors used the in cyclic triaxial device. Owing to the nature of the test materials (MSW), the sample preparation, up to the loading phase, was performed in accordance with ASTM –D4767, and the loading and analyzing stages were conducted in accordance with the ASTM-D3999 (D3999, 1998; D4767, 2002).
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P. Alidoust et al. / Waste Management xxx (2018) xxx–xxx Table 1 Sensor details.
3. Test results
Axial strain sensor
Number Measurement range Measurement accuracy
1 70 mm(Horizontal) ±0.001 mm
Pore water pressure sensor
Number Measurement range Measurement accuracy
2 0–1000 kPa 0.2 kPa
Submersible load cell
Number Measurement range Measurement accuracy
1 ±20 kN 0.0025 kN
2.3.2. Bender element The parameter, Gmax is typically associated with shear strain levels of approximately 0.001%, and is a key parameter in small strain dynamic analyses. To date, most researchers employed in-situ techniques to evaluate the Gmax of the MSW by measuring Vs (with a small-strain range >0.001%). However, few studies for measuring Vs in the laboratory have been conducted. A BE test device comprises of a pair of piezoelectric plates placed in the middle of top and bottom pedestals, a function generator to transmit the desired voltage impulse and an oscilloscope to plot and interpret the received and transmitted motion wave. The tests were performed on saturated samples with heights and widths of 100 mm after the consolidation phase. In the wave propagation method the arrival time of the shear wave, propagated from the source, was measured. The received signal must be carefully analyzed to identify the arrival time of the S wave. Additionally, the arrival time (t) was measured by using simple peak-to-peak and start-to-start time domain analyses (Marjanovic, 2012). Given the tip to tip distance (Ltt ) of the benders, the arrival time is calculated by employing time domain analysis, and the shear wave analysis is computed as follows:
Vs ¼
Ltt t
ð3Þ
The theory of shear wave propagation in an elastic body states that the value of the shear modulus of the MSW from the measurement of shear wave velocity can be determined by Eq. (1). 2.3.3. Large scale oedometer testing device The testing program also included the use of a large-scale oedometer (490 mm 490 mm). The apparatus comprised an oedometer cell, a polyamide porous plate, a displacement transducer, two pressure cells, six pressure transducers, a base plate, and a computerized data acquisition system. Additionally, the loading system involved a hydraulic jack capable of exerting 1 MPa pressure on the specimen. Furthermore, the hydraulic system was computer-controlled to be able to adjust and maintain the desired stress on the specimen. The apparatus can display the atrest lateral earth pressure coefficient (K0) under different vertical and horizontal pressures. Using the elasticity theory, the Poisson’s ratio and the at-rest earth pressure coefficient are related by the following equation:
K0 ¼
m
1m
5
ð4Þ
Owing to the lack of an appropriate leachate collection system in the Kahrizak landfill, a large fraction of waste is in a saturated state. Therefore, to obtain representative parameters of MSW in this landfill, oedometer tests were performed in a saturated condition under 1 bar pressure.
3.1. Triaxial test result A total of 60 cyclic triaxial tests were performed to assess the dynamic behavior of fresh MSW under different conditions. Following the consolidation phase (irrespective of the confining stress), strain controlled cyclic loading was applied at frequencies of 1 Hz and 0.1 Hz, under undrained conditions, on all specimens, and each specimen was loaded up to 15th cycle at an axial strain range from 0.05 to 2.5%. The result of the cyclic triaxial testing system for each cycle exhibits a hysteresis loop that enables the evaluation of the shear modulus and material damping. The influence of different factors, including fiber content, confining stress, loading frequency, and number of cycles is presented in the following sections. 3.2. Shear wave velocity A BE test was performed on MSW samples with varying fiber content and confining stresses. The wave propagation method measures the travel time of the shear wave. Table 2 presents the results of BE tests performed on saturated MSW samples based on Eqs. (1) and (2). As shown, the inclusion of fiber contents resulted in an increase in the Gmax of each sample. This trend is dependent on several factors, including the composition, density and compaction energy of samples. Therefore, it is difficult to distinguish the influence of each parameter individually. However, it should be taken into account that the reinforcement effect of fibers at strains of approximately 104%, and lower, the accepted range for the BE testing, is negligible, and it is not possible for fibers to stretch within the mentioned range. The key factor is thus the change in the elasticity of the constituents. An increase in the fiber content and confining stress increases the elasticity of the waste material, and generates a greater connection between the waste particles. For example, a marginal increase in the shear wave velocity of samples, under a confining pressure of 75 kPa can be related to an increase in the compaction energy to achieve to a target density. This increasing trend is significantly more evident at greater confining stresses because of the increased density of the samples. It must be mentioned that the inclusion of fiber contents reduces the arrival time of the received wave, due to the increased stiffness of the samples, while, at the same time, the bender receives low-voltage waves. The attenuating nature of plastics decreases the voltage of the received wave, without any adverse impact on the shear wave velocity. As can be seen in Fig. 4, despite the low-voltage of a received wave, the data indicates a reduction in the wave arrival time (resulting in an increase in the shear wave velocity) of a sample with 6% fiber, while the sample with 3% fiber exhibits a greater travel time (greater peak-to-peak distance) with a higher voltage. 3.3. Poisson’s ratio Poisson’s ratio is a factor that directly affects the calculation of the shear modulus. Table 3 presents the results of the oedometer tests performed on saturated MSW samples based on Eq. (4). According to the results for samples with 0% fiber content, the average assessed values of the specimens was 0.47 (equivalent to a K0 of 0.9). Although, based on a literature review, it was observed that inclusion of the fibers would lead to a slight increase in Poisson’s ratio value, this was overlooked. The reason for this trend is again the fact that the compressibility of fibers is lower than organic components. Therefore, inclusion of fibers reduces the overall compressibility of waste mass. The resultant data is in
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Table 2 Results of the BE tests on fresh MSW. Prepared unit weight (kN/m3)
Unit weight after consolidation (kN/m3)
Fiber content%
Confining stress (kPa)
Shear wave velocity (m/s)
Gmax (MPa)
9 9 9 9 9 9
10.1 10.6 10.9 11.5 11.7 12.3
0 0 3 3 6 6
75 150 75 150 75 150
96.1 97.9 98.6 100 101.2 120.8
9.34 10.15 10.59 11.53 11.98 17.94
Fig. 4. BE test response on two different samples.
Table 3 Results of oedometer tests on fresh MSW. Sample condition
Fiber content%
Obtained K0
Poisson’s ratio
Average
Saturated Saturated Saturated Saturated Saturated Saturated Saturated Saturated Saturated
0 0 0 3 3 3 6 6 6
0.92 0.88 0.92 0.92 0.92 0.92 0.96 0.96 1
0.48 0.47 0.48 0.48 0.48 0.48 0.49 0.49 0.5
0.47
accordance with the reports of Kavazanjian (2003) because of the high percentage of organic waste and the saturation conditions in this study. Furthermore, the author states that in saturated conditions such as in bioreactor landfills, the value of Poisson’s ratio can exceed 0.4 (Kavazanjian, 2003). In fact, water as a liquid is incompressible; therefore, it is reasonable to expect that as the amount of water, in saturated and undrained conditions, increases, the Poisson’s ratio will also increase. Several researchers worldwide focused on assessing Poisson’s ratio, and their results are summarized in Table 4. The table presents the evaluated Poisson’s ratio, and its related method, for MSW. As can be seen, there is significant scattering in the various measured Poisson’s ratio and its related method, in MSW literature. The reason for this major difference is attributed to many factors such as the decomposition of MSW over time, different constituents of the MSW based on the region or different testing situations including drained or undrained, and, saturated or unsaturated conditions.
0.48
0.49
Table 4 Poisson’s ratio values, from technical literature and this study. No.
Researcher
Method
Values
1 2
Down hole Shear and compressional waves Cross hole Triaxial test
0.49 0.33
3 4
Sharma et al. (1990) Matasovic´ and Kavazanjian (1998) Carvalho et al. (1998) Towhata et al. (2004)
5 6
Zekkos (2005) Naveen et al. (2014)
7 8
Ramaiah et al. (2016a,2016b) Current study
Cyclic triaxial Considered based on partially saturated condition Cyclic triaxial Large scale odeometer
0.3 0.2– 0.29 0.1–0.4 0.3
0.5 0.47
1-(Sharma et al., 1990), 2-(Matasovic´ and Kavazanjian, 1998), 3-(Carvalho et al., 1998), 4-(Towhata et al., 2004), 5-(Zekkos, 2005) 6-(Naveen et al., 2014), 7-(Ramaiah et al., 2016a).
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Fig. 5. Effect of number of cycles on three samples.
4. Discussion 4.1. Effect of number of cycles In this study, dynamic parameters of three samples under different testing conditions, namely confining stress, percentage of fiber contents and axial strain, were evaluated based on loadings
Fig. 6. Effect of number of cycles on the generation of excess pore water pressure.
over 15 cycles (Fig. 5). As observed, shear modulus and material damping experienced slightly increasing and decreasing trends with increases in the number of cycles, respectively. Fig. 5 shows that the shear modulus and damping ratio do not show a significant difference for different numbers of cycles, and the shear modulus and damping ratio are actually independent on the number of cycles. Similar results were reported by Naveen et al. (2014) and Keramati et al. (2017a, 2017b). Naveen et al. (2014) evaluated the effects of the number of cycles on waste with different confining stresses, while Keramati et al. (2017a, 2017b) measured the effect of the number of cycles on waste of different ages. Irrespective of the testing condition, all the results reported a negligible effect of the number of cycles on the shear modulus and damping ratio. It must be noted that the effect of the number of cycles on MSW is significantly less than that of typical soils, and this is attributed to the presence of compressible particles in waste mass. As discussed, based on the results, the shear modulus and damping ratio do not differ significantly with the number of cycles, but, in order to increase the accuracy of the results, all values were evaluated in the tenth cycle (Keramati et al., 2017a; Naveen et al., 2014). Regarding the possible effects of loading cycles on the generation of excess pore water pressure, it is clear that increasing the loading cycle in an undrained condition would lead to a greater excess pore water pressure in samples, and this increase has been
Fig. 7. Hysteresis loops under varying confining stresses and axial strains.
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observed in all the values by means of using effective stress in equations. According to Fig. 6, the resultant analysis could not demonstrate any effective role of fiber contents. Actually, an increase in confining stress values is the main reason of an increase in excess pore water pressure values, while an increase in fiber contents did not change the values significantly. It should also be taken into account that generated excess pore water pressure has higher values under greater shear strains (or equivalent loading cycle), while the majority of the resultant dynamic parameters have similar values within that range. 4.2. Effects of loading frequency To evaluate the effects of loading frequency, samples with 0% fiber were loaded at 1 Hz and 0.1 Hz. Fig. 7 presents an example
of stress-strain hysteresis loops for the above-mentioned frequencies under different confining stresses (75 kPa and 150 kPa) and axial strains. Fig. 8 shows the changes in the shear modulus and damping ratio curves for two samples under 75 kPa confining stress, within the shear strain range of 0.05–2.5%. According to Fig. 8, a 10-fold increase in loading frequency, under the same confining stress, leads to a marginal increase in the shear modulus (that is equivalent to the gradient of the hysteresis loop), while the differences exhibit a decreasing trend with respect to increases in the shear strain. The damping ratio also exhibits a decrease, and the increase in shear strain was identical with respect to changes in the shear modulus. It must be noted that the effect of loading frequency is approximately the same for the specimens under higher confining stresses and different fiber content. The reason for these changes is attributed to the increasing loading rate in the same domain that leads to viscous behavior of the waste (due to the high organic matter content in the waste mass). In this regard, similar results were reported by Keramati et al. (2017a, 2017b) on fresh MSW from the Kahrizak landfill, (Keramati et al., 2017b). Zekkos (2005) reported that the loading frequency does not significantly affect either the normalized shear modulus reduction, or the material damping. However, the normalization of curves can affect the results obtained by Zekkos (2005). 4.3. Effect of confining stress Undrained cyclic triaxial tests were conducted on specimens with 0% fiber content under 75 kPa and 150 kPa confining stresses (at the same frequency and shear strain). Table 5 presents the results of these tests. The variation in cyclic properties is shown in Fig. 9. Based on the results, the application of a two-fold increase in confining stress, from 75 kPa to 150 kPa increases the shear modulus trend, however, the effect of confining stress on the damping ratio is unclear. As can be seen, the increasing trend of shear modulus (with increasing confining stress) decreases at higher shear strains. Specifically, a greater confining stress generates more interaction among MSW constituents and increases the overall density of samples, leading to increased stiffness. Conversely, with respect to higher shear strains the connection of constituents is less, and discreteness is greater. The combination of the above-mentioned factors at higher shear strains reduces the shear modulus values. The results are in accordance with those reported in studies by Ramaiah et al. (2016a, 2016b); Towhata and Uno (2008) and Keramati et al. (2017a, 2017b); Ramaiah et al., 2016b; Towhata and Uno, 2008). 4.4. Effect of fiber contents
Fig. 8. Changes in shear modulus and damping ratio curves under 75 kPa confining stress over range 0.05–2.5% shear strain.
To evaluate the effect of fiber contents on MSW, plastic fibers percentages of 0%, 3%, and 6% (by weight) were added to the fresh samples, and their related cyclic behavior was evaluated under different test conditions. Table 6 shows the effect of fiber contents on MSW samples under 75 kPa confining stress and a frequency of
Table 5 Damping ratio and shear modulus values for samples under different confining pressures. Shear strain%
0.075 0.148 0.37 0.735 1.83 3.63
0-75-1
0-150-1
Variation ratio
Shear modulus (kPa)
Damping (%)
Shear modulus (kPa)
Damping (%)
Shear modulus
Damping
1943.6 1852 1212.32 1005.79 630.55 373.05
5.89 6.68 9.74 15.6 15.99 20.59
2140 1903.47 1327.14 951.85 638 353.12
6.1 7.29 14.19 13.82 21.37 18.61
1.10 1.02 1.09 0.94 1.01 0.94
1.03 1.09 1.45 0.88 1.33 0.9
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Fig. 9. Variation in dynamic properties with respect to changes in confining stress.
1 Hz. Based on the results, increasing plastic percentages result in greater values of the shear modulus (at lower shear strains), while the behavior of the damping ratio is still unclear. However, despite the increase in values when compared with the reference sample, the increasing trend for the shear modulus is not significant between the 3% and 6% samples. This behavior is attributed to the low levels of confining pressure and density of samples that are unable to mobilize the reinforcement of fibers. Additionally, it must be noted that the reinforcing effect of fiber contents decreases to a low level owing to increases in the discreteness of the constituent at higher shear strains (close to the reference sample). An interesting finding with respect to the effect of fiber content was observed at higher confining stresses. Fig. 10 shows the effect of fiber content on the dynamic properties of MSW under different confining stresses, for shear strains from 0.075 to 3.63%. Generally, as observed, an increase in plastic leads to greater values of shear modulus, while the material damping does not exhibit a steady trend. The generated stiffness is related to the growing reinforcement of fibers and sample stiffness by increasing fiber percentages. The important point is the rate of increase in shear modulus, given the simultaneous increases in the fiber contents and confining stress (irrespective of the loading frequency). The greatest increase in reinforcement was obtained when the confining stress increased from 75 kPa to 150 kPa. This resulted in greater values of shear modulus for samples with 3% plastic, under 150 kPa when compared with samples with 6% plastics under 75 kPa. The trend indicated the effect of confining stress on the reinforcing effect of samples with different fiber content percentages. As expected, sample 6-150-1 exhibited the maximum value of shear modulus, because of the presence of two dominant factors at the maximum level.
Fig. 10. Effect of fiber content on dynamic properties of MSW in different situations.
4.5. Normalized shear modulus reduction To compare the results with existing technical literature, the obtained normalized shear modulus reduction and damping ratio of current study are shown in conjunction with the most popular recommendations for MSW curves (Fig. 11). As stated in Section 4.4, the resultant damping ratio in this study does not
Table 6 Effect of fibers content on dynamic properties at a frequency of 1 Hz and confining stress of 75 kPa. Shear strain%
0.075 0.148 0.37 0.735 1.83 3.63
Reference
3% Plastic
6% Plastic
0-75-1
3-75-1
6-75-1
Shear modulus (kPa)
Damping (%)
Shear modulus (kPa)
Damping (%)
Shear modulus (kPa)
Damping (%)
1943.6 1852 1212.32 1005.79 630.55 393.05
5.89 6.68 9.74 12.28 15.99 20.59
2163.3 2100 1910 1362.41 761 594
3.89 5.9 6.48 18.16 16.88 19.04
2767.29 2243.33 1403.94 1006.388 791.3 584
7.65 10.84 15.71 18.95 21.86 28.2
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P. Alidoust et al. / Waste Management xxx (2018) xxx–xxx
Fig. 11. Comparison results of previous studies (Augello et al., 1998; Dobry and Vucetic, 1987; Feng et al., 2005; Gunturi, 1996; Idriss et al., 1995; Keramati et al., 2017a; Lee, 2007; Matasovic´ and Kavazanjian, 1998; Ramaiah et al., 2016b; Singh and Murphy, 1990; Yuan et al., 2011; Zekkos et al., 2008).
exhibit a definite trend with increases in the fibrous materials. However, the resulting values are within the range stated in the technical literature and follow the principles of soil mechanics with respect to increases in values with increasing shear strain. Additionally, the normalized shear modulus reduction data falls to the right-side with increases in fibrous material, and this indicates greater elastic behavior. However, it is necessary to simultaneously consider the impact of fiber contents, confining stress and shear strain on the variation rate of normalized shear modulus reduction values. Within common strain ranges for dynamic tests, fiber contents cannot offer sufficient reinforcement in the waste material. On the other hand, the overall stiffness increases with confinement and samples exhibit greater strength. Greater stress generally generates more interaction among constituents, and reduces the discreteness. This behavior creates greater bonding between particles, and increases the elasticity of the entire sample. The importance of shear strain in decreasing the stiffness should also be considered. Typically, applying greater shear strain in MSW will result in greater discreteness in samples. Based on these considerations, the role of a single factor should not be considered the primary contributing factor in the improvement of shear modulus values. It must be noted that the majority of recommended curves include the data of the OII landfill, and this could be the reason for existing differences. However, as clearly observed, the normalized shear modulus and damping ratio of this study are in good agreement with the curve proposed by Singh and Murphy
(1990). The authors assumed that the strength properties of the refuse were more cohesive than frictional and recommended modulus reduction and material damping curves similar to those in peat and clay (Singh and Murphy, 1990). Owing to the greater elastic behavior in this study when compared with the results obtained by Singh and Murphy (1990), the observed dynamic behavior in this study can be taken as similar to that observed in high plasticity index clay.
5. Conclusions Normalized shear modulus and material damping curves of MSW are affected by various factors, such as the test type and procedure, waste composition, drainage conditions, and ranges of shear strain. In this study, the dynamic properties of MSW from the Kahrizak landfill were evaluated with respect to variations of different factors through a comprehensive laboratory testing program. The program included measuring the Poisson’s ratio of MSW by using a large-scale oedometer testing device, measuring the shear wave velocity and smallstrain shear modulus of MSW by BE tests and evaluating the damping ratio and shear modulus of the samples by cyclic triaxial tests. The specimens were reconstituted by adding either 0%, 3% or 6% of plastic fibers (by weight) to assess the effect of fiber contents on the dynamic behavior of MSW. Furthermore, all the tests were consolidated under
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P. Alidoust et al. / Waste Management xxx (2018) xxx–xxx Table 7 Importance of various factors on the dynamic characteristic of MSW.
Fiber contents Loading frequency Confining pressure Loading cycle
G
Gmax
High importance Low importance High importance
High importance — High importance —
a
G/Gmax
D
Important Important
Unclear Low importance Unclear
a
a
a
- - -: Independent. a Negligible.
confining stresses corresponding to75 kPa and 150 kPa, and subjected to up to 15 cyclic loadings. To summarize the results obtained, the impact of each factor on the undrained cyclic behavior of MSW is presented in Table 7. It must be noted that, based on the principles of soil mechanics, increasing shear strain causes significant changes in the dynamic parameters of MSW, and is considered the most effective factor on the normalized shear modulus and damping ratios of MSW. Table 7 clearly indicates the primary influence of the confining pressure and fiber contents. An important point is the necessity of simultaneously considering both factors to accurately illustrate the fiber reinforced behavior of waste mass. Given the inclusion of plastic fiber to waste mass, the efficiency of the fiber reinforcement is observed as dependent on the values of the confining pressure. Increasing stiffness of the samples, from the inclusion of plastic fibers, is the result of changes in the waste composition, as opposed to the creation of reinforcement. This behavior is attributed to the low range of strain for dynamic tests that is unable to create essential tension in fiber particles. Additionally, increasing confining stress leads to increased bonding between particles, and this increases the elasticity of the waste mass. These findings result in higher values of shear modulus and lower values of discreteness in waste samples, while the behavior of damping ratio is unclear. The reason for the discrepancy in values of damping ratio remains unclear, although it may be related to the compressibility of MSW. References Athanasopoulos, G., Grizi, A., Zekkos, D., Founta, P., Zisimatou, E., 2008. Municipal solid waste as a reinforced soil: Investigation using synthetic waste. In: Geocongress 2008: Geotechnics of Waste Management and Remediation. 168– 175. Augello, A.J., Bray, J.D., Abrahamson, N.A., Seed, R.B., 1998. Dynamic properties of solid waste based on back-analysis of OII landfill. J. Geotech. Geoenviron. Eng. 124, 211–222. Augello, A.J., Matasovic, N., Bray, J.D., Kavazanjian Jr, E., Seed, R.B., 1995. Evaluation of solid waste landfill performance during the Northridge earthquake. Geotech. Spec. Publ. 54, 17–50. Carvalho, M. de F., Vilar, O.M., 1998. In-situ tests in urban waste sanitary landfill. In: Proceedings of the 3rd International Congress on Environmental Geotechnics, Lisboa. pp. 95–100. D2216, S.A., 2005. Standard Test Methods for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass. West Conshohocken Am. Soc. Test. Mater. 2216. D3999, S.A., 1998. D3999: Standard Test Methods for the Determination of the Modulus and Damping Properties of Soils Using the Cyclic Triaxial Apparatus. Annu. B. ASTM Stand. D4767, S.A., 2002. Standard test method for consolidated undrained triaxial compression test for cohesive soils. ASTM Int. 2002, ASTM 2. Dobry, R., Vucetic, M., 1987. Dynamic properties and seismic response of soft clay deposits. In: Proc. Intern. Symp. Geotech. Eng. Soft Soils,. 51–87. Elgamal, A., Lai, T., Gunturi, R., Zeghal, M., 2004. System identification of landfill seismic response. J. Earthq. Eng. 8, 545–566. Feng, S., Chen, Y., Kong, X., Zou, D., 2005. Experimental research on dynamic properties of municipal solid waste. Chin. J. Geotech. Eng. Ed. 27, 750. Gunturi, V.R., 1996. Identification and modeling of seismic response of landfills. Troy, NY: Rensselaer Polytechnic Institute. Hossain, M.S., Haque, M.A., Hoyos, L.R., 2010. Dynamic properties of municipal solid waste in bioreactor landfills with degradation. Geotech. Geol. Eng. 28, 391–403. Idriss, I.M., Fiegel, G., Hudson, M.B., Mundy, P.K., Herzig, R., 1995. Seismic response of the operating industries landfill. In: Earthquake Design and Performance of Solid Waste Landfills. ASCE, pp. 83–118.
Kavazanjian Jr, E., 2003. Evaluation of MSW properties using field measurements. In: Proceedings 17th Geosynthetic Research Institute Conference, Hot Topics in Geosynthetics—IV. pp. 74–113. Kavazanjian Jr, E., Matasovic, N., Bachus, R.C., 1999. Large-diameter static and cyclic laboratory testing of municipal solid waste. Procee. Sardinia., 437–444 Kavazanjian Jr, E., Matasovic, N., Stokoe, K.H., Bray, J.D., 1996. In situ shear wave velocity of solid waste from surface wave measurements. In: Proceedings 2nd International Congress on Environmental Geotechnics, Osaka, Japan, pp. 97– 104. Keramati, M., Shariatmadari, N., Karimpour-Fard, M., Saeedanezhad, A., Alidoust, P., 2017a. Effects of aging on dynamic properties of MSW: A case study from Kahrizak Landfill, Tehran, Iran. Scientia Iranica. Nov 14. Keramati, M., Shariatmadari, N., Karimpour-Fard, M., Shahrbabak, M.R.N., 2016. Dynamic behaviour of MSW materials under cyclic triaxial testing: a case of Kahrizak landfill, Tehran, Iran. Iran. J. Sci. Technol. Trans. Civ. Eng. 40, 75–83. Keramati, M., Shariatmadari, N., Sabbaghi, M., Sadegh Abedin, M., 2017b. Effect of confining stress and loading frequency on dynamic behavior of municipal solid waste in Kahrizak landfill. Int. J. Environ. Sci. Technol. 1–8. http://doi.org/10. 1007/s13762-017-1465-1. Khaleghi, M., 2011. In-situ test to determine the effect of aging on shear wave velocity of municipal solid waste (Case study: Kahrizak Landfill). Iran Univ. Sci. Technol. Master Sci. Thesis [in Persian]. Iran University of Science and Technology. Lee, J.J., 2007. Dynamic characteristics of municipal solid waste (MSW) in the linear and nonlinear strain ranges. The University of Texas at Austin. Marjanovic, J., 2012. The study of shear and longitudinal velocity measurements of sands and cohesive soils. Massachusetts Institute of Technology. Matasovic´, N., Kavazanjian Jr, E., 1998. Cyclic characterization of OII landfill solid waste. J. Geotech. Geoenvironmental Eng. 124, 197–210. Matasovic´, N., Kavazanjian Jr, E., Abourjeily, F., 1995. Dynamic properties of solid waste from field observations. In: Proc., 1st Int. Conf. on Earthquake Geotech. Engrg. pp. 549. Morochnik, V., Bardet, J.P., Hushmand, B., 1998. Identification of dynamic properties of OII landfill. J. Geotech. Geoenvironmental Eng. 124, 186–196. Naveen, B.P., Sitharam, T.G., Sivapullaiah, P.V., 2014. Evaluating the dynamic characteristics of municipal solid waste for geotechnical purpose. Curr. Adv. Civ. Eng. 2. Pereira, A.G.H., Sopena, L., Mateos, T.G., 2002. Compressibility of a municipal solid waste landfill, in: Proc., 4th Int. Congress on Environmental Geotechnics. pp. 201–206. Ramaiah, B.J., Ramana, G.V., Kavazanjian, E., Matasovic, N., Bansal, B.K., Kavazanjian Jr, E., Matasovic, N., Bansal, B.K., 2015. Empirical model for shear wave velocity of municipal solid waste in situ. J. Geotech. Geoenvironmental Eng. 142, 6015012. https://doi.org/10.1061/(ASCE)GT.1943-5606.0001389. Ramaiah, B.J., Ramana, G.V., Bansal, B.K., 2016a. Field and large scale laboratory studies on dynamic properties of emplaced municipal solid waste from two dump sites at Delhi, India. Soil Dyn. Earthq. Eng. 90, 340–357. Ramaiah, B.J., Ramana, G. V, Kavazanjian Jr, E., Bansal, B.K., 2016b. Dynamic Properties of Municipal Solid Waste from a Dump Site in Delhi, India, in: Geo-Chicago. pp. 121–130. Shariatmadari, N., Karimpour-Fard, M., Keramati, M., Kolarijani, H.J., Naebi, A., 2011. Fiber content impact on the shear strength of msw materials in direct shear tests. In: Proceedings Sardinia 2009, Thirteenth International Waste Management and Landfill Symposium. Shariatmadari, N., Machado, S.L., Noorzad, A., Fard, M.K., Keramati, M., Kalarijani, S. H.J., 2010. Shearing rate effect on mechanical behavior of MSW materials. In: International Conferences on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics. Missouri University of Science and Technology, San Diego - California, pp. 1–8. Sharma, H.D., Dukes, M.T., Olsen, D.M., 1990. Field measurements of dynamic moduli and Poisson’s ratios of refuse and underlying soils at a landfill site. Geotechnics of Waste fills—Theory and Practice. ASTM International. Singh, S., Murphy, B.J., 1990. Evaluation of the stability of sanitary landfills. Geotechnics of Waste Fills—Theory and Practice. ASTM International. Towhata, I., Kawano, Y., Yonai, Y., Koelsh, F., Koelsch, F., 2004. Laboratory tests on dynamic properties of municipal wastes. In: Proceedings of the 11th Conference in Soil Dynamics and Earthquake Engineering and the 3rd International Conference on Earthquake Geotechnical Engineering. pp. 688–693. Towhata, I., Uno, M., 2008. Cyclic shear tests of municipal waste in large triaxial device for identification of its dynamic properties. Geotech. Earthquake Eng. Soil Dynam. IV., 1–10
Please cite this article in press as: Alidoust, P., et al. Laboratory studies on effect of fiber content on dynamic characteristics of municipal solid waste. Waste Management (2018), https://doi.org/10.1016/j.wasman.2018.02.038
12
P. Alidoust et al. / Waste Management xxx (2018) xxx–xxx
Yuan, P., Kavazanjian, E., Chen, W., Seo, B., 2011. Compositional effects on the dynamic properties of municipal solid waste. Waste Manage. 31, 2380–2390. Zekkos, D., Bray, J.D., Kavazanjian Jr, E., Matasovic, N., Rathje, E.M., Riemer, M.F., Stokoe, K.H., Kavazanjian, E., Matasovic, N., Rathje, E.M., Riemer, M.F., Stokoe, K. H., 2006. Unit weight of municipal solid waste. J. Geotech. Geoenviron. Eng. 132, 1250–1261. https://doi.org/10.1061/(ASCE)1090-0241(2006) 132:10(1250).
Zekkos, D., Bray, J.D., Riemer, M.F., 2008. Shear modulus and material damping of municipal solid waste based on large-scale cyclic triaxial testing. Can. Geotech. J. 45, 45–58. Zekkos, D.P., 2005. Evaluation of static and dynamic properties of municipal solidwaste. University of California, Berkeley.
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