Influence of Aging on the Mechanical Behavior of ...

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Influence of aging on the mechanical behavior of Municipal Solid Waste A. Nayebi1, N. Shariatmadari2, M. H. Hamzeie Tehrani3, and P. Oskouie4, 1

M. Sc. School of Civil Engineering, Iran University of Science and Technology, P. O. Box 16765-163, Narmak, Tehran, IRAN, email: [email protected] 2 Associated Professor, School of Civil Engineering, Iran University of Science and Technology, P. O. Box 16765-163, Narmak, Tehran, IRAN. PH (98) 21-77240093, FAX (98) 21-77240398; email: [email protected] 3 M. Sc. School of Civil Engineering, Iran University of Science and Technology, P. O. Box 16765-163, Narmak, Tehran, IRAN, email: [email protected] 4 B. Sc. School of Civil Engineering, Iran University of Science and Technology, P.O. Box 16765-163, Narmak, Tehran, IRAN, email: [email protected]

ABSTRACT Study of the mechanical behavior of Municipal Solid Waste (MSW) is accompanied by many complications. Great interest in considering the most parameters involved in this behavior caused the achievement of useful properties of Municipal Solid Waste in recent years. These parameters are waste composition, leachate recirculation, unit weight, moisture, weather condition and aging. However, in the literature, researchers take into account all these parameters but the effect of age is not clearly obtained and its influence is not systematically measured. Impact of this factor on the mechanical behavior of the solid waste is quite complex. Some researchers have made conclusion that, when the waste’s age increases, the strength would be diminished, but there are also some indications that the strength of the solid waste is increased by the growth of age (Machado et al. 2008). In this paper, Large Scale direct Shear tests are applied on samples obtained from Iranian MSW landfills. The results show that mechanical behavior of MSW is significantly relative to the changes of composition with time (aging). As the composition becomes more monotonous the strength increases. INTRODUCTION By considering daily increase in population on earth and growth of production and consumption all over the world, waste production and its issues have become one of the serious subjects that governments are facing today. As the result, waste management or upper, materials cycle management has become one of the most main and important elements of sustainable development. Although, there have been lots of efforts made such as reducing waste production (Revising the consumption pattern), reusing some materials (Reusing the glass) or utilization by recycling (departing and reusing paper as cartoon) in order to reduce buried waste or even after this process, reducing the volume of waste to a very tiny amount by burning or composting, there

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will be need to bury the burned waste or the compost leftover. According to above statements, necessity of secure and hygienic burying of waste is and going to be one of the priorities for human communities. As the result, we should extend available sanitary or provide new landfills. Due to the fact that finding suitable location of landfill site is extremely subtle issue, there are some important points that have to be considered, for instance, some necessities such as living environment, mental impacts on the people living in the vicinity of landfills (if it be close to city) and financial effects in the intervals of transportation and transfer of waste with increase of distance must be considered for choosing the location of new landfill center. All these points have persuaded the city authorities and governments to develop the existing landfills. This type of development may take place in two forms: 1) horizontal and 2) vertical development. Since, vertical development of landfills will be feasible to the highest border allowed by site and design, the investors and policy makers of the waste burial mostly insist on the vertical development. It has resulted in the fact that some of these landfills built with approximately 100 meters height. Although, the development of vertical landfills which hygienic burial has been performed there previously looks appropriate in the first view to environment, but immediate implementation of this task while neglecting complicated properties of waste, can danger the stability (slopes) of landfills. Thanks to mentioned reasons, a secure design contains inevitable needs for prevention of financial, life and environmental losses which are going to occur if rupture takes place. What is presently defined as Waste Mechanics requires extensive completion. Assessment of geometric specifications and thereafter waste behavior is a disputable issue due to changes in the materials present therein. The best method is testing on the real and intact sample. The intact sample cannot be provided; therefore the remolded materials must be placed in the test apparatus by reconsolidation. One of the existing problems is that waste structure will be destroyed by this method and meanwhile the obvious difference in the waste compositions may cause the fact that the sample cannot represent the behavior of the whole waste and also systematic change in ingredients will be problematic to clarify the impact of each of them. Further to all above mentioned, the future changes (age) and their impact on the waste compositions as well as choosing a device with suitable size for sampling and testing are among the important points which should be considered in this research. Although, the effect of biodegradation on the shear strength of the Municipal Solid Waste has been examined by various researchers, but there is no consensus in this regard, in the way that some of them such as Landva and Clark (1986), Turczynski (1988), Md. Sahadat Hossain (2002), Kolsch & Ziehmann (2004) demonstrated the decrease and some others such as Owesis (1993), Van Impe (1998), Machado et al (2008) and Nascimento (2007) demonstrated the increase of shear strength level with age increasing and biodegradation. The amounts of shear strength parameters provided by these researchers are also very different. Considering the fact that there is no specific definition of the waste deterioration level, the problem even becomes worse. The age parameter is quite important but other factors such as waste

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compositions, climatic conditions of the area, percentage of humidity, method of landfill operation, waste permeability and daily cover may also have remarkable impacts on the waste deterioration. Shear Strength. A few methods have been adopted by various investigators in their quest to establish reliable geotechnical strength parameters for municipal solid wastes. Shear strength is perhaps the most important mechanical property of solid waste in landfill engineering. Direct shear test was applied to majority of reported studies because of its simplicity, reliability and adaptability. The waste is typically screened to remove the large particles that would not fit into the standard direct shear equipment. For this purpose the Large Scale Direct Shear apparatus (with dimensions of 300 mm x 300 mm x 150 mm) is used in this research. A total of 36 large-scale direct shear tests were performed in this study. Bio-Degradation. Available information indicates that drained shear strength of degraded MSW is similar to undegraded waste. For long-term behavior and due to the complexity of the waste degradation process, it is difficult to incorporate the time dependent degradation behaviour in the constitutive model. There is not adequate information regarding older waste behaviour. Moreover, there is lack of information about the effect of degradation on mechanical behaviour of MSW in the literature. Fiber. Fiber materials are usually composed of plastic, paper, textile etc. The influence of reinforcement on MSW shear behaviour is currently poorly understood. It was assumed that total shear strength is consisted of friction in shear plane and tensile force in fibers. Testing Procedure. The waste samples used in laboratory tests were fresh, one year and three years old and presented the following average physical composition: wood (0.8%), paper (3.4%), plastic (6.7%), Fabric (0.19%), metal (1.61%), glass (2.1%), rubber (0.80%), PET (0.85%), wet waste (72%), Cardboard (4.75%) and Soil & Trash (6.8%). The water content was determined by drying the specimens at 70º C, over a period of 24 hours, and was expressed as a percentage of wet weight of the material. The average water content for these samples was 74.24%, 149.85% and 86.55% respectively. The initial normal stress (20, 50, 100 and 200 kPa were applied) was then applied and the samples were allowed to consolidate under new load for approximately 24 hours. At this point, shear test was started. The tests were run at a very slow rate to create drained conditions. The strain rate used for direct shear test was 0.8 mm/ min. Tests were performed in accordance with ASTM Method for Direct Shear Tests of Soil under Consolidated Drained conditions. The tests were strain controlled and readings were taken at particular strain interval. However, the samples in many of studies did not undergo shear failure even at large strains. In our tests, we limited the relative displacement to 6 cm, or about 20% strain, in an attempt to remain within the relative operating range of the apparatus.

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RESULTS AND DISCUSSION Shear stresses were plotted against the corresponding applied horizontal displacement (Figure 1).

Figure1. Example results of Large Scale Direct Shear tests on one year old MSW (effect of normal stress). The results indicated increase in shear strength with displacement which is compatible with findings of Gabr and Valero (1995), Thomas et al. (2000), Pelky et al. (2001), Md. Sahadat Hossain (2002) and Karimpour-Fard (2009). As it is observed, with increasing the normal stress, the level of shear strength increases which indicates the frictional nature of these geo-materials. This result is compatible with findings of Mahler & De Lamare Netto (2003), zekkos (2005), Langer (2005) and Karimpour-Fard (2009). As the result of method for sample preparation that material are compacted in horizontal layers, fibers and plastic parts may locate parallel to forced shear plane. As another considerable factor, waste response is probably more dilative at lower normal stresses rather the case for reinforced earth materials (Figure 1). This dilative response is more effective in mobilizing fibrous materials during shear response. However, results show that in higher normal stresses the results tend to be more ductile and compressible which Aburatani et al. (1998) results also support this outcome. Thanks to Figure 2, it can be stated that the old waste have more strength than fresh one. This increasing in strength is completely obvious in normal stresses more than 50 kPa. This can be clarified by some following points: A) With time passing, the bio-degradation rate (related to environmental circumstances) increases. On the other hand, in a long period mass loss due to biodegradation is high; but fiber content during time almost remains constant. It means an increase in fibers content in this period. Since strength of plastic fibers do not change in time so much, shear strength of MSW materials increases (Machado et al., 2008).

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B) As we observed in Figure 2, the fresh sample has more tendency to be compressed rather than samples with one year and three years old. It should be noticed that this material after aging has less free void than fresh state. By this way after a period, we have more monotonous waste (than fresh waste) so friction angle due these wastes were higher than fresh wastes (table 1). Table 1. Effect of the age on the shear strength parameters (in normal stress =100 kPa) Age Cohesion (C) Internal Friction Angle (ɸ) (year) [kPa] [Degrees] Fresh 32.90 5.73 1 35.83 7.60 3 39.91 15.38 Strength increase with bio-degradation is compatible with findings of Owe is (1993), Van Impe (1998), Nascimento (2007), Zhan, T., Y. Chen, et al. (2008) and Machado et al (2008). The reason for part B may be described by conversion of waste materials to soil like materials after a period. But the authors still work to know how metastasis can influence on mechanical behavior of waste materials. CONCLUSION Shear strength parameter is one of the most important subjects in landfill design. Because of this usage, all parameters that can affect shear strength must be considered. Considering that age has most potential to affect shear strength, in this research, we tried to represent this significance by utilizing a large scale direct shear apparatus and testing on samples aged fresh, one year and three years old which were obtained from Iranian landfills. The results indicated that shear strength of MSW is considerably relative to changes of composition with time (aging). This factor (age), also affects compressibility as the result of this fact that components become more monotonous (similar to soil). Waste vertical displacement was decreased because of changing in composition which was resulted by increasing in time. This can be illustrated with bio-degradation causing change in natural shape of waste. Changing in shape can be responsible for increasing in friction angle; this finding is compatible with Zhan, T., Y. Chen, et al. (2008).

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Figure 2. Effect of the age on the shear strength and waste behaviour with different normal stresses (a = 20 kPa, b = 50 kPa and c = 200 kPa).

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