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Shear strength and creep settlement properties of municipal solid waste at the Chong Qing Landfill, China

By Crystal Hart,2014-09-09 11:28
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Shear strength and creep settlement properties of municipal solid waste at the Chong Qing Landfill, China

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    Shear strength and creep settlement properties of municipal

    solid waste at the Chong Qing Landfill, China

    ZHAO Yanru

    5 (Department of Civil Engineering, ChongQing University, ChongQing 400045)

    Abstract: This paper presents experimental results on geotechnical properties of municipal solid waste

    (MSW) at different biodegradation phases. The shear strength of fresh and landfilled municipal solid

    waste was determined by large direct shear tests (LDS). For fresh samples, the cohesion was increased

    from 0kPa to 19.89kPa, and the friction angle decreased from 27.84?to 14.41? as biodegradation 10 degree increased from 0 to 63.83%.. For the triaxial tests under the consolidation drained (CD)

    condition, the shear strength of cohesion and friction angle for degraded samples presents a continuous

    increasing when the defined axial strain is increased from 5% to 20%, and the cohesion is vary from

    35.90kPa to 66.42kPa, the drained friction angle ranged from 29? to37?. A narrow range for organic

    content and temperature that better for biodegradation and creep settlement were found in the creep 15 tests, and it was ranged from 21.9% to 36.47%, 22? to 41?, respectively. The test results are useful for

    assessing the stability and creep settlement of landfills located in ChongQing city, in China.

    Keywords: Municipal solid waste; Shear strength; Biodegradation; Creep settlement; Leachate

    0 Introduction

    20 Increasingly affluent life styles and population, continuing industrial and commercial grow in

    und the world over the past decade has been accompanied by rapid increases in many countries aro

    both municipal and industrial solid waste production. As the economical disposal way of MSW,

    52%,90%,95%,54.3%and 83%of urban wastes production are landfilled into regulated centers,

    respectively, in Korea, Poland , Taiwan, USA, and China. (L.Di Palma et al., 2002;US EPA,2009?

    25 Tony L.T.Zhan,2008). However, with the continuous increasing of height and volume in the

    landfills, significant landfill slides have been triggered. For example, the largest Maine Slop

    failure in the United states happened in the 1996, march 9(Eid et al.,2000); and The Payatas

    landfill slide in Quezon City, Philippines, it was triggered by the extremely heavy rains from two

    typhoons, and at least 278 people were killed in this disasters(Merry.S.M,et al., 2005). 30 Consequently, assessing the stability of landfill is becoming one of the major concerns for

    engineering designers. And the Shear strength is required as the primary factors for calculation.

     Numerous studies has been conducted on it at field and laboratory tests.~Edil et al.,1990?Landva

    and Clark, 1990?Howland and Landva,1992?Jessberger and Kockel, 1993?Wall, D.K.&

    Zeiss,1995?Grisolia et al.,1995?Houston et al.,1995?Kavazanjian et al.,1995?Kockel and

    35 Jessberger,1995?Edincliler et al.,1996?Van Impe,1998? Thomas et al.,1999? Pelkey et al.,

    2001? Caicedo et al.,2002?Vilar and Carvalho, 2004?Gomes et al., 2005?Itoh et al., 2005?

    Feng, 2005?O.M.Vliar, 2005?T.L.T.Zhan et al.,2008; Dixon et al., 2008? Reddy,2008?

    Reddy,2009a?Reddy, 2009b?Reddy, 2011??.It is believed that the degradation of organic matter

    in the waste could resulted in the changes of the particle sizes and composition of MSW, and the 40 shear strength properties and moisture content in the waste could be also changed (Reedy et

    al.,2011).However, shear strength values reported in previous literatures varied widely (the

    cohesion varied from 0.5kPa to 71kPa, and inter friction angle ranged from 17.8? to 34?), and little

    studies focused on determining the change in properties of MSW due to the biodegradation under

    aerobic and anaerobic condition, therefore, further studies are needed to better understand the

     45 effect of decomposition on the shear strength. However, it is difficult to identify the shear strength

    Brief author introduction: Zhao Yanri, ph.d. E-mail: zhaoyanru54022@163.com

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    properties because of its heterogeneous nature and biodegradation degree, and it has been

    becoming one more important thing for engineering designers to evaluate the stability of landfill

    by using the suitable shear strength value of MSW at different biodegradation levels.

    Information on the creep settlement is also required to evaluate the total settlement and 50 stability in the landfill, Numerous researches have been studied on it (Sowers,1973?Yen and

    Scanlon,1975?Tan et al., 1991?Rao and Oweis, 1977? Gabr et al., 2000?Hossain and Gabr.,

    2005?Hettiarachchi et al., 2003,2005,2009?Marques, 2001,2003?Elagroudy, 2008?Bareither,2008;

    Machado, 2002, 2008?Sivakumar and Babu, 2009?).An additional factor is the change of

    proportion of creep settlement in the total settlement due to the organic degradation. Coduto and 55 Huitric.(1990) pointed out that the secondary compression settlement can be enlarged by the creep

    settlement due to the biodegradation. Pump.(1998), M.S wati and Kurian Joseph.(2008) indicated

    that the secondary settlement caused by the natural biodegradation reached to 40% ,and even

    reached to 49% of total settlement by using the recirculation of leachate. However, little research

    has been conducted on investigation the effect of biodegradation and temperature on the creep 60 settlement due to the biodegradation in the landfill.

    This paper describes a comprehensive laboratory study on Chong Qing landfill in China. The

    borehole samples collected at the different depth are shredded at the laboratory before the tests,

    and the shear strength properties of MSW samples were estimated using a series of larger direct

    shear tests (LDS) at the different biodegradation phases. The effect of organic content on shear 65 strength of MSW due to the biodegradation was also identified in this study. Through the three

    axial shear test under consolidation drained (CD) condition, the correlation between shear strength

    of landfilled sample with the different shear strain of axial (5%, 10%, 15%, 20%) were clarified,

    and the effect of temperature on the degradation was clarified too. Unless stated otherwise, creep

    settlement tests were performed under the no precipitation condition during tests, and all of the 70 experiments were carried out in accordance with the standard procedures established by the China

    Society of standard for soil test method (GB-T/50123-CSTM 1999).

    1 Sample collection and characterization

    Fresh and landfilled sample were prepared, which collected from the shallow of 1-3 meters

    layers and the depth of 25m, respectively, in Chong Qing landfill. Composition of the MSW was 75 determined and grouped into different friction (organic friction, inorganic friction, cinder and

    others).

    During the shear tests, The fresh and landfilled of shredded samples were prepared and all the

    samples were shredded before the drained direct shear tests. The shredded sample was dried and

    gradation was determined using sieves (size of sieves: 100,50,20,10,5mm) analysis in accordance 80 with (GB-T/50123-CSTM 1999), the typical gradation of MSW was shown in fig 1. It is observed

    that approximately 95.3% particle sizes of shredded MSW are less than 10 mm.

    In order to investigate the effect of degradation on the shear strength of fresh MSW, and 45

    fresh shredded samples (organic content is 45.95 %) and 28 shredded partially degradable samples

    were prepared for the lager direct shear test.

    85 Table 1* Typical components of MSW of research areas Category Organic Inorganic (I) Other Cooking andWooBrickTextile Plasti type Waste type Bones PaperMetalGlass Cinder s c Garden wa ste d and tile 2.19 17.21 30.48 Percent(%) 22.82 1.55 5.39 2.84 11.82 1.53 1.16 3.01 3.57 30.48 Total (%) 45.95 2

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     Table 2* Range and representative value of MSWs natural water content Cooking andBrickWaste type Textiles Plastic Wood MetalGlass CinderBones Paper Garden and tile wa ste

    Percent(%) 50~80 5~20 4 ~10 6~15 1~4 15~40 2~4 6~12 1~4 6~12 Typical Percent670 15 6 10 2 20 3 8 2 ~%? *Table 1 and 2 dates were afforded by eco-environment science research institutes of Chong Qing. 90

     The effect of moisture content on the shear strength of the landfilled MSW(older than 10 years) was conducted through the Drained Large direct shear test (DLDS).In order to avoid the particle sizes effect, landfilled samples were shredded and reconstructed at different moisture contents.

    the borehole samples were initially subjected to a confining pressure of During the LDS tests, 95

     40kPa and a vertical pressure of 30kPa and rest for 24 h before the tests. Latterly, the shearing was done at a low constant strain rate (0.1mm /min) under different normal stress conditions (50, 100,

     200, 300, 400kPa). The shear strength was defined at 15% horizontal deformation and was used to establish the Mohr-coulomb shear envelopes. The tests stopped when the horizontal placement

     exceeds 6mm. The LDS tests were repeated for the other shredded landfilled samples.100 100 90 80 70 60 50 40 30 Percent Finer(%) 20 10 0 100 10 1 0.1 0.01 Waste Particle Size(mm) Fig.1 Grain size distribution curve of landfilled waste Creep settlement tests were performed to investigate the changes of settlement properties of

    MSW due to the physics-chemical reaction and biodegradation. A special apparatus with a 105

     dimension of 500mm and 500mm in diameter (Fig.2) was prepared in the tests and which is completed sealed to simulate the anaerobic condition, and no precipitation was used during the tests. Five groups of shredded fresh MSW (each group have five samples ) with varying moisture contents (10%,20%,35%,50%,65%,100 %)were prepared and then compacted into odometer (the

    sample is 480mm inside diameter and 480mm longer). Creep test was performed under the normal 110

    temperature environment. During the 350 days surveyed, the values of leachate production

    (ml/days) and creep settlement (mm/days) were recorded elaborately.

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     Orential apparatus vertical Pressure Settlement observation plate Dialgage Magnetic cushion

     Support plate Transmite load Testing can Specimen Temperature probe Leachate collection layer

     Leachate

     Funnel Fig 2 Compressive and Creep Settlement Test Apparatus 115 2 Testing methods Tchobanoglous, et al.,(1993) Pointed out that the biodegradation could be completed during one or two years, and the drained direct shear tests are considered to be suitable way to achieve the shear strength parameters, and the value of shear strength should be better used for analyzing the

    stability of MSW (Hossain et al.,2009). A larger shear box (300 inside diameters and 200mm 120

     height) were used during the tests in order to eliminate the effect of shredding and test apparatus size on specimens.

     2.1 Drained LDS tests for the shredded fresh Sample

    Drained LDS tests were performed on the fresh shredded specimens. The dry density of 3125 MSW is varied from 610 to 630 Kg/m, and the organic content of specimen is 45.95%. the tests were performed according to (GB-T/50123-CSTM 1999).The fresh sample were initially subjected to a confining pressure of 40kPa and a vertical pressure of 30kPa, and it saturated by

     carbon dioxide before taking the tests .Each month, one group of five degraded samples were taken to shear at a constant shear strain rate (0.1mm/min) and sheared under the normal stress

    of 50,100, 150,200,250,300,400kPa. Based on the Mohr-coulomb failure criterion, the shear 130

     strength at 15% horizontal deformation was selected to determine the shear strength properties in order to make a comparison with other literatures. 2.2 Drained LDS tests for the shredded Landfilled Sample Drained LDS tests were performed to study the effect of moisture contents on the shear

    strength (cohesion and the angle of internal friction) of landfilled MSW. The borehole samples 135

     were collected (at the depth of 25m and older than 10 years) and reconstructed with different moisture contents (25.2%, 26.7%, 29.2%, 30.6%, 33.6%, 37.2%, 41.1%). Specimens were

     compacted in the larger direct shear boxes ( 300 mm inside diameters and 200mm height) in 3laboratory. The average dry unit weight of specimens is 947Kg/m, and the proportion of particle

    sizes less than 10mm approximately 95.3% (Fig.1). Shear tests were performed at a constant strain 140

     rate of 4mm under three different normal stress conditions: 100, 200, 300kPa. Based on the Mohr-coulomb failure criteria, the shear strength (cohesion and inter friction angle) was obtained

     at 15% Horizontal deformation. 2.3 Consolidated drained (CD) shear strength

    Triaxial testing under CD condition was performed (accordance with GB-T/50123-CSTM 145

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     1999) to study the changes of shear strength properties of landfilled MSW at the different axial strain level. The borehole was drilled by using a bucket auger that was 110mm in diameters and

     600mm long, and the borehole samples were collected from a depth of 25m by using a inside diameter of approximately 96mm and 200mm height of torque(the filled age was determined more

    150 than 10 years old). The borehole samples were carefully transferred to the laboratory and placed in

     the triaxial chamber. No trimming was used to avoid disturbing the structure of each sample. The 3. Before the tests, The sample was average dry unit weight of the specimens was 1022Kg/m initially subjected to confining pressure of 30kPa, and then four specimens were consolidated under 100,200,300,400kPa confining pressure and sheared at a constant strain rate of 1mm/min.

    Based on the Mohr-coulomb failure criteria, the shear strengths (cohesion and inter friction angle) 155

     were determined at 5%,10%,15%,20% axial strain, respectively. The tests‘ results will be further studied later in this paper. 2.4 Static Creep testing Creep settlement plays an important role in secondary compression settlement in the landfill

    after it was closed. In addition, with the growth of organic matters in the waste, the value of creep 160

     settlement grows due to the biodegradation and which represented an increasing proportion of total settlement. (Watts et al,(2002). However, because of the variations of the physics-chemical reaction and biodegradation and effect of temperature in the waste, and all of these made it difficult to accurately predict landfill settlements.

    165 Machado et al.,(2002) pointed out that it is essential to acknowledge the creep settlement

     properties under the co-effect of stress, biodegradation and temperature for predicting the total settlement of waste. Haandel et al., (1994) take the leachate production as a calculate parameters to assess the biodegradation degree. Numerous previous literatures have been conducted and mainly focused on analysis the composition of leachate, gas production and the quality changes of

    MSW. However, little research has been conducted on the co-effect of stress, biodegradation and 170

     temperature on the creep settlement .In order to investigate the creep settlement properties under the anaerobic condition, shredded fresh samples at different organic content (at:10%,20%,35%, 50%, 65%,100%) were prepared and then were compacted into the specially designed apparatus. A special confining pressure of 40kPa and an initial vertical pressure of 30kPa were given for all

    the samples during the tests, the leachate production, creep settlement and the volume change 175 were measured carefully.

     2.5 Temperature effect Significant amounts of heat are generated in municipal solid waste due to decomposition and

     phy-chemical reaction in the waste (Yesiller et al.,2003; J.L. Hanson,2005). Numerous literature

     180 were reported about the temperature distribution in the landfill .(Rigo and Cazzuffi, 1991?By T. G. Townsend,1996? J.L. Hanson, 2005?J.J.Bowders and C.M.Mitchell,2005? Yesiller et al.,

     2003,2005) and previous studies concluded that the temperature was varied from 15 to 60? at a large ranged of depth in the landfill. Rigo and Cazzuffi (1991) concluded that the high temperature

     could accelerate biodegradation of organic matters in the waste. However, little researches have been done to study the mutual effect on temperature and biodegradation systematically. Therefore, 185

    Special apparatus were used in the creep tests (Fig.2) and the correlation between temperature and

    organic degradation was identified. Through temperature sensors fixed in the body of specimens,

    the change of temperature with the varying time was recorded elaborately during the creep

    settlement tests.

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    190 3 Results and discussion

     3.1 Drained direct shear strength result of shredded fresh MSW Drained shear direct test were performed during 9 months to determine the shear strength

     properties at different biodegradation phase. The shear strength parameters calculated from Fig.3 are summarized in Table 3, The Mohr-Coulomb shear strength envelopes for direct tests at

    different months are presented in Fig.4. 195

     Fig.3 shows (t=5months) the strain-stress curves exhibited a typical strain-hardening behavior of MSW, and the shear stress increased continuously with horizontal displacement

     without reaching any peak values, The similar trend could be observed for other samples at different biodegradation conditions. Table 3 shows that the cohesion of MSW was increased from

    0kPa to 21.50kPa and the friction angle decreased from 27.84?to 21.69?due to the biodegradation. 200

    An increase in cohesion and decrease in friction angle was clearly observed through the tests, and

     the tests results were consistent with the work reported by Edincliler (1996). 80 σ(50kPa σ(100kPa (200kPa(400kPaσσσ(300kPa 70

    60

    50

    40

    30 Shear Stress(kpa) 20

    10

    0 0 2 4 6 8 Horizontal Displacement(mm)

     Fig.3 Direct shear test results for shredded fresh MSW at t=5 months

     205 250T=0 T=1 T=2 T=3 T=4 T=5 T=6 T=7 T=8 225 200 175 150 125 100 75 Shear Stress (kPa) 50 25 0 0 100 200 300 400 500 Normal Stress (kPa)

     Fig 4 Mohr-Coulomb failure criteria for fresh shredded MSW at different times(T= months.) The composition of samples after each test (every month) was determined and found that

    210 organic content was decreased from 45.95% to 16.6% during 9months of biodegradation. This

     finding was consistent with the results reported by Reddy, et al.,(2011). The comparison results between current and previous literatures were shown in Fig.5. M.A.Gabr.(2007)and M.S.Hossai

     n.(2009) take the extent of refusing decomposition for characterized by the cellulose plus hemi-cellulose to lignin ratio(C+H)/L, and finding the measured friction angle decreased from 32

    to 24?as (C+H)/L decreased from 1.29 to 0.25. With biodegradation, the proportion of 215 un-degraded composition like plastic and fiber was increasingly play a predominant role in the waste and acted as a reinforcement during the shearing test, which resulted in a strain-hardening

     behavior of MSW (Klosch, 1995,1997?Machado et al.,2002?Reddy,et al.,2009b).However, the current tests results were contrary to literatures (Langer,2005; Hossain,2002; Howland and

    Landva,1992,Tony Z.T,2008), Analyzing the results we concluded that the wide variation in shear 220

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    strength may be contributed to the composition and biodegradation level of MSW. And the current

    tests results only representative the variation of shear strength due to the biodegradation in the

    short-term (350days). The change of shear strength with the biodegradation after one or two years

     need enhance research to identify. 60 60 Reddy(2011)-Cohesion 55 55Reddy(2011)-Friction angle 50 50Current-Cohesion Current-Friction angle 45 45 40 40

    35 35 30 30 25 25

    C20 20 ohesion(kPa) 15 15

    10 10 5 5 0 0 0 10 20 30 40 50 60 70 80 90 100 Degree of Decomposition(%) 225Fig. 5 Variation of cohesion and friction angle with degree of Decomposition The effect of organic content on the shear strength of the fresh MSW was also studied in the tests, the fresh shredded samples were prepared at different initial organic contents (10%, 20%,

    230 35%,50%,60%),and the test was performed under the normal stress at 50,100,150,200,250,300,

     400kPa. The shear strength was defied at the 15% horizontal deformation and the shear strength parameters were calculated based on the Mohr-Coulomb failure criteria from Fig.6 and it was

     presented in Table 3. Based on the analysis of the date shown in Table 3, the cohesion of shredded fresh MSW was calculated and it varied from 4.90kPa to 18.58kPa, the friction angle decreased

    235 from 24.44 to 14.12?.

     Fig 7 shows that neither the cohesion nor the inter friction angle demonstrated any Friction Angle(?) correlations with the organic content, and no correlation between shear strength with the moisture

     content was found (Reedy, 2009a). Pelkey et al.,(2001) point out that the waste is a complex matters and the Shear strength is co-effected by the biodegradation ,stress level, inter structure and

     240other factors. 200 Organic Content(%) 180 10% 160 20%140 35% 50%120 65%100 80 60 Shear Stress(kPa) 40 20 0

     0 100 200 300 400 500

    Normal Stres s (kPa) Fig 6 Mohr-Coulomb failure criteria for fresh shredded MSW at different times(T= months.)

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     28 28 24 24

     20 20

     16 16

    12 12 Cohension (kPa) 8 8 Friction angle (degree)

    4 4 Cohension Friction angle 0 0 0 10 20 30 40 50 60 70 80 90 100 Organic content (%)

    Fig 7 Variation of fresh shredded MSW at different initial Organic Content 245

     3.2 Drained direct shear strength result of Landfilled sample Fig.8 shows the direct shear test results for landfilled MSW at an in-situ moisture content of 25.2%, Similar trends were observed from the tests conducted with samples at other moisture

    content. The landfilled specimens exhibited a strain-hardening and contractive behavior when the 250 horizontal deformation well in excess of 15% of the apparatus diameters. In the absence of specimens reaching any peak strength, shear strength at 15% horizontal deformation was used to establish the Mohr-Coulomb shear strength envelopes(Fig.9), and the shear strengths of samples at different water content (wet gravimetric content: 25.2%?26.7%?29.2%?30.6%?33.6%?37.2%?

    41.1%)are presented in Table 3. It is concluded from Table 3 that the cohesion of landfilled MSW 255

     is very small and varied in a narrow ranged from 0kPa to 8.4kPa and the drained friction angle ranged from 36.38 to 41.38?. Neither the cohesion nor the drained friction angle demonstrated any

     correlation with the moisture content. This finding is consistent with previous results reported by Reddy et al. 2009a).Through analysis the composition of landfilled MSW, we found that the

    organic content is close to zero after 10 years degradation. 260

     250 100kPa 200kPa 200 300kPa 150

     Stress(kPa) 100 Shear 503 Moisture Content=25.2%,Dry Density=950kg/m

    0 0 1 2 3 4 5 6 7 Horizontal Displacement (mm)Fig 8. Direct shear test results for unshredded landfilled MSW Wet Gracimetric Moisture Content 300 25.2% 26.7% 29.2% 30.6% 250 33.6% 37.2% 41.1% 200 150 100 50 Shear stress(kPa) 0 0 100 200 300 400 Normal stress(kPa)

     Fig 9.Mohr-Coulomb failure envelope for shredded landfilled

    265 MSW at different moisture content under the large Direct tests

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     Table 3 Drained shear strength properities of specimens on the larger direct shear testing Time Organic Cohension Friction angle Gravimetric moisture Source (month/years) content(%) kPa (degrees) content (%) 0 45.9 0 14.413 1 41.2 0 27.84 2 40.4 6.396 27.664 Current study: Fresh shredded MSW,Appratus size 300mm diamater,shear strength defined at 15% 3 38.7 11.566 27.856 strain,the initial orgnanic content is 45.95%,mositure content is 43.17%,Dry density =620KN/m3, not tested4 32.3 19.889 26.262maximum particle size is less than 30mm. 5 26.3 11.934 23.432 6 22.8 18.509 22.9627 18.4 21.497 21.688 8 16.6 19.365 24.83 57.5 1 35 83 40.2 16 34 68.8Fresh to Reddy et al.(2011) Fresh synthetic MSW, 63.5mm diameter shear box,shear strength defined at 15% Partialy 38.9 18 29 77.93. strain,initial moisture content is 50%,Bulk unity weight varied from 11.2 to 16.2KN/m28.6 34 29 84.3 degraded 15.5 40 28 53.9 2.22 36.38 25.2 2.38 39.28 26.7 6.29 41.38 29.2 Current study: Landfilled shredded MSW,Appratus size 300mm diamater,shear strength defined at 15%3. ?10 years 00 41.6 30.6strain,the orgnanic content is 0,Dry density =947kN/m 3.01 39.26 33.6 3.64 40.42 37.2 8.4 41.33 41.1 12 32 44Reddy et al. (2009b) partially decomposition shredded MSW, 63.5mm diameter shear box,shear strength 63 31 60defined at 15% strain, maximum particle size less than 40 mm and approximately 80% of the waste 1.5 years 41.8 34 35 80 consisting of particles ranging from 10 to 20 mm. 56 32 100Gabr et al.(2007) 100mm diameter and 50mm height shear box,the Initial stages of decomposition 24 days R=1.29 32 shredded sample is composited by paper,partially decomposed accelerated methane production phase 53 days R=0.73 27 refuse, and plastics .the particle size of fresh paper particle decelerated methane production phase 84 days R=0.38 25 size is 10 mm by 20 mm.shear strength defined at 10% strain, Stable methane production phase R=(C+H)/L. 127 days R=0.25 24 46 30 44 64 26 60 Reddy et al.(2009a) Fresh shredded MSW, 63.5mm diameter shear box,shear strength defined at 15% Fresh 55.1 32 28 80 strain.31 30 100 10% 4.9 24.437 23.08 20% 18.58 16.977 35.78 35% 7.97 19.829 39.83 Current study:Fresh shredded MSW,Appratus size 300mm diamater,shear strength defined at 15% strain, Fresh 50% 4.53 23.098 51.59 65% 13.628 14.122 56.95

    Caicedo et al.~2002?,unshredded waste,,900mm diameter diameter shear box,shear strength 1 years 78 23 67 defined at 6.7% strain. Reddy et al. (2008) 0.9m diamaterand,and 1.5m long,the incoming waste component is 70% MSW,17% contrustion and demolition debris,11% soils,and 2% others.the waste components included 8% Fresh 24 47 29 43.7 papers,11% plastics,12% wood,4% glass, 4% metal and others.shear strength defined at 15% strain, 3 Dry density of landfill msw was 515Kg/m. R=0.5 32 0.55 Hossain et al.(2009),100mm diameter shear box,Shredded and processed MSW.R=Maxmium particle Freshsize/shear box size. R=0.25 27 0.55 Gabr and Valero (1995),63.5 mm diameter, 23mm thick,33% Ash, soil and rock, 23% textiles, 13% no 20-39 3plastics, 10% metals,Dry unit weight=10-12.1kN/m,shear strength defined at 10% ,20% strain 15-30 years 0respective. 0-28 no Landva and Clark (1990),Shredded sample,434×287mm,20-55% Paper products,5-42% food waste,4-20% Fresh 0 23 34 garden waste,6-15% metal,2-15% plastic. Kavazanjian et al. (1995),457mm diameter shear box,Shredded sample,waste recover from the depth of 35m?and also conducted the direct shear simple testing(460mm) ,particle larger than 100mm 11-35 Years 0 43 31 7.5%-41.2% was removed,unit weight:Ground face:10-13kN/m3,depth 30m:13-16kN/m3,depth 60m:14-17kN/m3,shear strength defined at 15% strain.

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270 3.3 Shear strength Based on Triaxial (CD) tests In geotechnical engineering, CD tests results might be considered as a suitable way to analysis and evaluate the stability of landfill, because it can be better simulate the drained situation in the waste. Four groups of unshredded ladfilled samples (Each group have four samples and the 3 average dry unit weight is 1022kN/m?and the average moisture content is 41%) were collected

    from a depth of 25m and the consolidated drained (CD) triaxial tests were performed under the 275

     confining pressure at 100,200,300,400kPa. Fig.10. shows the representative stress-strain tests results, the peak value of shear strength

     was not gained even the horizontal deformation well in excess of 20% axial strain and the strain-stress curves of CD tests exhibited a typical strain-hardening behavior of landfilled MSW.

    Similar trends were also observed by other researchers (Grisolia et al.,1995; Machado et al.,2002; 280

     Feng, 2005; O.M.Vliar et al.,2005; T.L.T.Zhan et al.,2008). Currently, no standard cut-off displacement value is appropriate to define MSW shear

     strength, and it is customary in geotechnical engineering to define strength at 10% to 15% axial strain in the event of continuous shear strength gain (T.L.T.Zhan et al.,2008). To identify a

    possible correlation between shear strength with shear strain in the waste of different fill age, CD 285

    tests were performed and the results were compared with the published literatures. Based on the

    Mohr-coulomb failure criteria, The shear strength (cohesion and inter friction angle) in this tests

    was determined at 5%,10%,15%,20% axial strain, respectively. And the comparison of previous

     literature with current tests results was shown in Table 4. 1600 Initial effective confining pressure 1400 100(Kpa) 200(Kpa) 1200 300(Kpa) 400(Kpa) 1000 800 600 400 Deviator Stress (kPa) 200 0 0 2 4 6 8 10 12 14 16 18 20 22 Axial strain (%) 290Fig.10 Stress-strain relationships obtained from five samples under

     drained CD test for the landfilled MSW

     5%Current tests,Machodo et al,(2002),Feng(2005) 10%Currenttests,Fiveauthors15% Currenttests,Feng(2005) 70 20%Current tests,Machado et al.(2002),O.M.Vilar(2009) 25%Grisolia et al.(1995)60 30%,O.M.Vliar 50 40 30 Cohension(kPa) 20 10 0 0 5 10 15 20 25 30 35 40 45 Friction angle(degree)295 Fig.11 The comparison between Triaxial compression under CD condition

    test result for landfilled MSW and previous literatures

    300

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