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Deformation Characteristics of MSW
Materials
Mehran Karimpour-Fard
Visiting researcher, Ph.D.
Federal University of Bahia
Sandro Lemos Machado
Associate Professor
Federal University of Bahia
ABSTRACT Abstract: Deformation properties of geo-materials, are considered as the necessary
information to perform sound and precise stability analysis both in overall or partial aspects.
In the case landfill engineering, since Municipal Solid Waste, MSW, are the main material
shaping the body of these constructions, having a proper background of the compressibility
and stiffness of these materials to execute the stability analyses in design procedure is
necessary. The compressibility characteristics of waste are of special concern when designing
interim and final closure covers for landfills. Also stiffness modulus is an important
mechanical property of waste which governs the deformation behaviour and could affect the
partial stability of lining system. In this paper the results large scale oedometer and triaxial on
MSW samples collected form Metropolitan Center Landfill, MCL, have been employed to
estimate the compressibility characteristics of these materials and their stiffness modulus and
Poisson ratio. Besides field monitoring of settlement in waste fills in MCL, has been
performed as an alternative method to estimate short and long term compression properties.
KEYWORDS: MSW; Deformation; Compressibility; Stiffness; Poisson ratio
INTRODUCTION Deformation characteristics of Municipal Solid Waste, MSW, as the main material in landfill
construction, have been the subject of many research studies during recent years.
It is necessary to predict the long-term settlement of MSW for the final cover design as well
as end-use and post-closure facility design. In addition, estimation of settlement is needed to
assess the stability of leachate and gas collection pipes, drainage systems, landfill storage
capacity, and the overall landfill operating costs. Excessive settlement may cause fracture in the
cover system and may also cause damage to the drainage and leachate/gas collection pipes.
Improving the efficiency of waste placement and assessment of interaction between side slope
barrier systems are some other typical targets of such researches.
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Evaluation the mechanisms related to different aspects of deformation in MSW materials is
the first step in finding a proper method to mitigate these deformations which could be helpful in
landfill’s development programs.
The mechanism of compression in waste has been described by various authors (Van Impe
and Bouazza 1996, Gasparini et al. 1995, Wall and Zeiss 1995, Coduto and Huitric 1990, Morris
and Woods 1990, Sowers 1973). The settlement occurred in waste fills is a combination of two
mechanisms, primary and secondary compression (Dixon and Jones 2005). Primary settlement is
caused by distortion, bending, crushing and particle orientation which referred to physical
deformation and consolidation which is attributed to dissipation of excess pore water pressure
generated during the loading in porous media. Since this mechanism will occur in a period of a
few days to a few weeks, in literature referred as short-term deformations. In other side secondary
compression which includes all creep effects both mechanical deformations under constant stress
and those relating to degradation is time dependent and may continue for years. This part of
deformation referred as viscous or long term deformation.
This paper embraces the results of several large oedometer tests performed on the waste
materials with different ages and settlement records in Metropolitan Center Landfill, MCL,
located in Salvador, Brazil. Besides the results of several large scale triaxial tests performed on
MSW samples with different composition in consolidated-drained manner have been used to
estimate the Poisson ratio of these materials.
A comparison has been presented between achieved results and the values reported in literature.
MATERALS AND EXPERIMENTAL PROGRAM To monitor the settlement of waste fills, MCL, located around 20 Km from the Salvador,
third biggest city in Brazil, was chosen. Several bench marks were installed on the top of different
cells and their displacement were recorded in monthly basis, using surveying technics (Figure 1).
To perform large oedometer and triaxial tests, MSW samples were collected from this
dumpsite transported to geo-environmental laboratory of Federal university of Bahia, Salvador,
Brazil.
Figure 1: Metropolitan Center Landfill
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The average fresh MSW sample’s composition could be visualized in Figure 2. Average
percentage of plastics which referred as the fibrous elements in the MSW samples was about
20%. This value could be considered high compared to the reported values in literature. Taking
into account textiles and rubbers as reinforcement elements, the fiber content of the waste reaches
to about 25%. Furthermore, paper and cardboard can act as reinforcement elements (at least in
short term analysis) but because of the high moisture content of MSW in this region, it is
expected these materials lose their strength quickly.
Figure 2: Average composition of fresh MSW samples used in laboratory tests
As could be observed also, the paste fraction which referred as the easily degradable part of
MSW is about 35%. Assuming papers and cardboards as the materials which could be
decomposed easily especially in the case of MSW samples with high level of water content,
almost 50% of MSW sample’s components are susceptible for quick biodegradation.
Total Volatile Solids (TVS) of MSW showed averagely a 55% of organic content in MSW
samples which supports above statements. It is also worthy to not that the moisture content of
fresh MSW materials averagely was around 100%. This value reduced with the aging process and
was about 70% for 4 years old samples.
In Figure 3, the employed large triaxial and oedometer could be observed. The utilized
triaxial apparatus was able to test the samples with diameter of 20 cm and 40 cm height with
loading rates of 1 to 1000 mm/min and using a loading frame with a capacity up to 300 kN.
The large oedometer apparatus with dimensions of 548 mm x 497 mm, was able to apply vertical
compression tests up to 700 kPa.
It should be mentioned that the operations both in triaxial and oedometer apparatus was
controlled by a servo control system using a set of related software.
COMPRESSIBILITY AND SETTELEMENT The MSW compressibility was evaluated both in field and laboratory. In filed, employing
bench marks installed at the top of the cover layers of cell with different depth, settlements were
recorded systematically each month.
In laboratory also using the large scale oedometer apparatus which explained in last section,
different MSW samples with different age were tested and their results were compared with
achieved results of field measurements.
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Figure 3: A view of utilized large apparatus in this research
Monitoring of Field Settlements
In Figure 4 the settlement records of some benchmarks ( shown in figure 1) are presented.
The benchmarks were installed 28 months after the end of the landfilling process. The average
initial height of waste was about 26.5 meters. The measured vertical strain values ranged from 3.5
to 4.6%. These values are compatible with the fact that the benchmarks were installed more than
2 years after the end of the landfilling process.
According to Oweis & Khera (1986), the settlement in a waste fill could be approximated by
the following equation:
2
22
1
0
logloglog)(t
tC
t
tC
p
pCt
i
c (1)
o
cc
e
CC
1 (2)
oe
CC
1
(3)
where: Cc and, C'c are the compression and normalized compression indexes; p0 and p are the
primary and secondary effective stress and the load increment, C1 and C2 are the coefficients of
short and long-term secondary compression and C'1 and C'2 are the normalized form of the pre-
mentioned indexes. ti, t2 and t represent the end of the initial settlement period, the time at which
the slope of strain-time curve changes and the elapsed time, respectively. e0 is the MSW initial
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void ratio. The normalized form of the presented coefficients is usually preferred due to some
difficulties in the MSW void ratio determination.
The values of short and long term secondary compression indexes, C'1 and C'2, could be
observed in the table enclosed to Figure 4. According to the estimated values the long term
secondary compression index, C'2 , averagely is almost 10 times higher than C'1. This high
difference indicates on the paramount effect of biodegradation in settlement of waste fills which
according to Figure 4 occurs after three years.
Figure 4: Time dependent settlement in waste fills from different bench marks and their relevant
time dependent compression indexes
Plate load tests
Oliveira (2000) presents the results obtained in three plate load tests (PLT) performed in the
MCL. The plates were located just above the MSW and for leveling purpose a thin layer of fine
sand was placed between the plate and the waste. The used plate had a diameter of 79.9 cm. The
hydraulic jack had a capacity of 24 Mg and a ram with a traveling course of 12 cm. The applied
load to the plate was controlled using a manometer attached on the pumping device of jack. These
tests were performed in accordance to ASTM D1196 method (Standard method for non repetitive
static plate load test of soil in flexible pavements). The MSW age varied from 3 months (load
tests PLT-03) to about 2.5 years (PLT-01 & PLT-02).
Fig. 15 shows the obtained results of the plate load tests performed. The stress-displacement
curves PLT-02 and PLT-03 presented similar behavior, while the PLT-01 curve presented a
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smaller strength. This can be associated, however, to some problems in the loading system and
hydraulic jack reported by Oliveira (2000).
Figure 5: Plate load test results performed in Metropolitan Landfill
The loading process was conducted up to 20cm of settlement. Assuming the depth of
influence of the load test as equal twice the plate diameter, the maximum average axial strain of
the waste was about 13%. No evidence of general shear failure or strength peak was observed.
According to Sowers (1968), failure modes in shallow foundations embedded on sanitary landfills
with a compacted soil cover could be punching shear or rotational shear. Punching shear failure
occurs when the width of the foundation, B, is relatively small compared to the thickness of the
soil cover. However, when the thickness of the soil cover is relatively small compared to the
foundation width and when the strength of the soil cover is low, rotational shear failure may
occur. He suggested that the allowable bearing capacity of shallow foundations for light
residential or office buildings over sanitary landfills should not be greater than about 20-40 kPa
but these values could be increased by increasing the thickness of the compacted soil cover to
values higher than 1.5 times the foundation width. The results of plate load tests reported by Van
Impe (1998) and Santos et al (1998) also indicate a punching shear failure in MSW materials;
however Santos et al (1998) showed that with increase the thickness of soil cover beneath the
plate, the failure mechanism could be changed from punching shear failure to local failure.
According to the experimental results, the failure mode could be classified as punching shear,
since the settlement is high compared to the applied load. In this situation the bearing capacity is
better estimated based on a tolerable settlement for design purposes, which is commonly fixed in
25 mm. This gives us values of allowable vertical stress of about 25 to 45 kPa.
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The results of the plate load tests must be modified to predict the behavior of foundations
with different widths and constructed on different soil layers. Terzaghi & Peck (1948) proposed
the following equation for the modification of plate load test results in the case of foundations on
granular soils:
3.0
3.0
Bb
bBSS
p
p
pf
(4)
where: Sf, Sp, B and bp are permissible settlement of foundation in mm, settlement of plate in
mm, width of foundation in m and width of plate in m, respectively.
Assuming a maximum settlement of 25 mm in a foundation with a width of 1.5 m, the
bearing capacity of foundation varies from 20 to 35 kPa. These values are consistent with the
values suggested by Sowers (1968). However, since the plate was located directly on the waste,
the bearing capacity may be increased using appropriated layers of compacted soil layers.
Laboratory Confined Compression Tests
Confined compression tests were performed in a oedometer with nominal dimensions of 497
mm height and 548 mm diameter. Figure 5 presents the results of 4 typical compression curves.
Three fresh waste and one 4 years old samples were used. It can be noted that not only the
primary compression index but also the swelling index seem to be decreased with the waste age.
The compression indexes in the case of the fresh waste samples are similar; however the rebound
indexes are significantly different.
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Figure 6: Confined compression curves achieved from different MSW samples
Fig. 7 compares the obtained results to the primary compression index with others published
in the technical literature.
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Figure 7: Measured primary compression index comparing reported values in literature
As can be observed, the results obtained using the MCL waste samples fit in the range
suggested by Sowers (1973) and are also are compatible with the results presented by Gabr and
Valero (1995). According to Landva et al. (1984), the value of the normalized primary
compression index, C'c, varies from 0.2 to 0.5 which is in agreement with the results achieved in
this study.
Estimation of constrained modulus of MSW is another way to estimate the waste fill primary
settlement. Waste placement can be considered to be a one-dimensional compression problem
(e.g. waste is placed over a large area in relation to the thickness of the deposit). An increment of
vertical effective stress, v, produces an increase in vertical strain, εv. A constrained modulus
D, can be defined as follow:
v
vD
(5)
Using this parameter and based elastic theory, elasticity modulus of MSW also might be
calculated as follow:
1
211DE (6)
Where, ν is Poisson’s ratio. Figure 8 shows the MSW constrained moduli values, D,
calculated using the results presented in Fig. 5 and some results reported in the literature. The
MSW constrained modulus appears to be linearly dependent of the applied vertical stress and its
values varied from 100 to 5500 kPa, depending on the value of vertical stress.
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The MSW’s time dependent compression properties were evaluated. Since the compression
test of MSW samples performed in multi stage manner, for each step of loading, the secondary
compression index were estimated. It should be noted that each loading stage took a time around
two weeks. Figure 9 shows the variation of the MSW void ratio in each stage of loading for the
test performed on 4 years old sample. As can be observed, after an initial period of stabilization,
the experimental results became approximately parallel.
Figure 8: Measured primary compression index comparing reported values in literature
Figure 9: Variation of MSW’s void ratio with time in different loading stage of 4 years old
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sample’s compression test
Figure 10 compares achieved secondary compression both from field and laboratory
compression tests to those reported in literature.
As could be observed, the achieved laboratory secondary compression index, however lies in
the proposed range by Sowers (1973) but rather is consistent with lower bound of this range.
Table 1 shows a comparison between field and laboratory results of normalized secondary
settlement indexes. Values presented in the Table 2 correspond to average values.
Figure 10: Measured primary compression index comparing reported values in literature
The values represented in Table 1 shows difference between short secondary compression
indexes achieved from laboratory and field measurement which could be due to non-homogeneity
of waste materials and maybe due to compaction level of MSW materials in field which supposed
to have lower initial weight. As stated earlier because of relatively short loading period in
laboratory tests, the long-term secondary compression index could not be estimated.
Table 1: Properties of materials Large compression laboratory test
Field re
cords 4 years
old
Fresh N
o.1
Fresh N
o.2
Fresh N
o.3
C1 0.0126 0.0154 0.0180 0.0150 0.02
C2 # # # # 0.19
POISSON RATIO OF MSW MATERIALS Poisson’s ratio, ν, and the at-rest lateral earth pressure coefficient, K0, are two properties that
affect engineering analyses. However, in Municipal Solid-Waste large scatter has been reported
in both measured values of Poisson’s ratio and the lateral earth pressure coefficient. Using
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elasticity theory, Poisson’s ratio and the at-rest earth pressure coefficient are related by the
following equation:
10K (7)
This equation is commonly used to estimate the Poisson’s ratio from measurements of the at-
rest lateral earth pressure coefficient, and vice-versa.
To achieve the elasticity modulus from the results of large compression test based on the
elasticity theory, as it was stated, having a proper values for Poisson ratio is necessary.
To estimate the Poisson ratio there are different ways: geophysical approaches like down-
hole and cross-hole (Sharma et al., 1990, Houston et al., 1995, Carvalho and Vilar, 1998,
Matasovic and Kavazanjian, 1998), triaxial tests (Jessberger and Kockel, 1995, Towhata et al.,
2004, Zekkos, 2005) , in-situ tests like pressuremeter (Dixon et al., 1999) and using large
instrumented compression cell (Landva et al., 2000, Dixon et al., 2004, Singh & Felleming,
2008).
In this research the results of consolidated-drained triaxial tests were employed to estimate
this factor using following equation:
a
v
22
1 (7)
where εv and εa are volumetric and axial strain, respectively.
Figure 11: Measured Poisson ratio in this study accompanying
with values reported in the literature
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In Figure 11 the results of this research have been presented and compared to those reported
in literature. As could be observed the scatter of values reported for this factor is considerable.
Based on the results of this research the Poisson’s ratio varies from 0.35 to 0.49. As the fiber
content of samples increases, this factor also increases which supports the finding of Zekkos
(2005) in this regard. Also the results indicated that the higher the unit weight of samples before
shearing stage, the higher the level of Poisson ratio. This finding also is in agreement with results
Zekkos (2005).
CONCLUSIONS In stability analyses of soils construction or each constriction which is built be geo-materials,
deformation properties are among information which is necessary to perform a sound and precise
stability analysis. The factors like elasticity modulus and Poisson ratio are the main factors to
perform stress-strain analyses.
Besides for post-closure purpose and construction of end-use facility, estimation long-term
deformation is vital. Estimation of this factor also is necessary to assess the stability of leachate
and gas collection pipes, drainage systems, landfill storage capacity, and the overall landfill
operating costs.
In this way, several bench marks were installed at the top of several waste fills in MCL, and
their displacement were recorded in monthly basis.
Also several large scale oedometer and triaxial tests were performed to achieve
complementary information.
The results showed that normalized long-term compression index could be 10 times higher
than short-term compression indexes since the effect of biodegradation is more pronounced in
long terms.
The normalized short term compression index achieved from large scale oedometer are also
lower than those achieved from field records. It could be due to could be due to non-homogeneity
of waste materials and maybe due to compaction level of MSW materials in field which supposed
to have lower initial weight.
The achieved primary compression index lie in the proposed range by Sowers (1973), and it
seems that aging process decreases this factor.
Depending on the vertical stress level, constrained modulus linearly varies from 100 to 5500
kPa.
The analyses of large scale CD triaxial tests, showed that Poisson ratio of MSW materials
varies from 0.35 to 0.49 depending on the level of fiber content and unit weight. Increasing the
unit weight and fiber elevates the Poisson ratio. This finding is compatible with Zekkos (2005)
results.
REFERENCES 1. Antoine, M. M. and J. M. Bradford (1982) “Parameters for Describing Soil
2. Beaven, R. P. and Powrie, W. (1995) “Hydrological and Geotechnical Properties of
Refuse Using a Large Scale Compression Cell,” Proceedings Sardina 95, Fifth
International Landfill Symposium, 2, 745-760.
Vol. 17 [2012], Bund. A 2022
3. Carvalho, M, F., (1999) “Mechanical Behavior of Municipal Solid Waste,” PhD Thesis.
Sao Carlos, SP, Brazil: University of Sao Paulo; [in Portuguese].
4. Carvalho, M.de F., Vilar, O.M. (1998) “In situ Tests in Urban Waste Sanitary Landfill,”
Environmental Geotechnics,” Editor Seco e Pinto, 1998, Balkema, Rotterdam, 121- 126.
5. Castelli, F. and Maugeri, M. (2008) “Experimental Analysis of Waste Compressibility,”
Geo congress 2008: Geotechnics of Waste Management and Remediation, New Orleans,
Louisiana, March 9-12, 208-215.
6. Coduto, D. P., and Huitric, R. (1990) “ Monitoring Landfill Movements Using Precise
Instruments,” Geotechniques of Waste Fills – Theory and Practice, ASTM STP 1070, A.
Landva and G.D. Knowles, Editors, ASTM, Philadelphia, P.A., 358-369.
7. Dixon, N., Jones, D. R. V. (1998) “ Stress States in, and Stiffness of, landfill waste.
Geotechnical Engineering of Landfills, Thomas Telford, London, 19-34.
8. Dixon, N., Jones, D.R.V. and Whittle, R.W. 1999 “Mechanical Properties of Household
Waste: In-situ Assessment Using Pressuremeter,” Proc. 7th International Waste
Management and Landfill Symposium, Sardinia, 453-460.
9. Dixon, H., Ng’ambi, S. C., Jones, D. R. V., Connell, A. K. (2000) “The Role of Waste
Deformations on Landfill Steep Side Wall Lining Stability,” Proceedings Wastecon 2000,
September, Cape Town, 2, 379-388.
10. Dixon, N., Ng’ambi, S., Jones, D.R.V., (2004). Structural performance of a steep slope landfill lining system. Proceedings of the Institution of Civil Engineers Geotechnical
Engineering, 157, 115-125.
11. Dixon, N and Jones, D.R.V. (2004). Engineering properties of municipal solid waste,
Geotextiles and Geomembranes journal, 23 (2005) 205–233
12. Gabr, M.A. and Valero, S.N. (1995), Geotechnical properties of municipal solid waste,
Geotechnical Testing Journal, Vol. 18, 241-254.
13. Gasparini, P.A., Saetti, G.F. and Marastoni, M. (1995). Experimental research on MSW
compaction degree and its change with time. Proc. 5th International Waste Management
and Landfill Symposium, Sardinia, 833-842.
14. Gotteland, P., Gachet, C., Vuillemin, M., (2001). Mechanical study of a municipal solid
waste landfill. Proceedings Sardinia ’01, 8th International Waste Management and
Landfill Symposium, Cagliari, Italy, 425-433.
15. Houston, W.N., Houston, S.L., Liu, J.W., Elsayed, A. and Sanders, C.O. (1995). In-situ
testing methods for dynamic properties of MSW landfills. Geotechnical Special
Publication, ASCE, 54: 73-82.
16. Jessberger, H.L. and Kockel, R. (1993). Determination and assessment of the mechanical
properties of waste. Waste disposal by landfill. Green '93. R.W. Sarsby (edit). 313-322.
Rotterdam- Balkema.
17. Landva, A.O., Clark, J.I., Weisner, W.R. and Burwash, W.J. (1984). Geotechnical
engineering and refuse landfills. Proc. 6th National Conference on Waste Management,
Vancouver, Canada, 1–37.
Vol. 17 [2012], Bund. A 2023
18. Landva, A. O., Valsangkar, A. J., Pelkey, S.G., (2000). Lateral earth pressure at rest and
compressibility of municipal solid waste. Canadian Geotechnical Journal, 37: 1157-1165.
19. Matasovic, N., Kavazanjian, E. Jr. (1998). Cyclic characterization of OII landfill solid
waste. Journal of Geotechnical and Geoenvironmental Engineering, 124(3): 197-210.
20. Morris, D. V. and Woods, C. E. (1990). Settlement and Engineering considerations in
Landfill and final cover design. Geotechniques of Waste Fills – Theory and Practice,
ASTM, STP 1070, A. Landva and G.D. Knowles, Eds., Philadelphia, P.A., 9-21.
21. Oweis, I.S., Khera, R., (1986). Criteria for geotechnical construction on sanitary landfills.
In: Fang, H.Y. (Ed.), International Symposium on Environmental Geotechnology, vol. 1.
Lehigh University Press, Bethlehem, PA, pp. 205–222.
22. Powrie, W and Beaven R.P. (1999). Hydraulic properties of household waste and
applications for Landfills. Proceedings Institution of civil Engineers, Geotechnical
Engineering, 137, 235-247.
23. Powrie, W., Hudson, A.P. and Beaven, R.P., (2000). Development of sustainable landfill
practices and engineering landfill technology. Final report to the Engineering and
Physical Sciences Research Council, (Grant reference GR/L 16149)
24. Sharma, H.D., Dukes, M.T., Olsen, D.M. (1990). Geotechniques of Waste Fills – Theory
and Practice, ASTM, STP 1070, A. Landva and G.D. Knowles, Eds., Philadelphia,
P.A., 57-70.
25. Singh, M. K. and Fleming, R. F.,(2008). Estimation of the mechanical properties of
MSW during degradation in a laboratory compression cell. Geo congress 2008:
Geotechnics of Waste Management and Remediation, New Orleans, Louisiana, March 9-
12, 200-207
26. Sowers, G. F. (1973). Settlement of waste disposal fills. Proc. 8th International
Conference on Soil Mechanics and Foundation Engineering, Moscow, 2, A. A. Balkema,
207-210
27. Towhata, I., Kawano, Y., Yonai, Y. and Koelsh, F. (2004). "Laboratory tests on dynamic
properties of municipal wastes." 11th Conference in Soil Dynamics and Earthquake
Engineering and 3rd International Conference on Earthquake Geotechnical Engineering,
Vol.1, 688-693.
28. Van Impe, W.F. and Bouazza, A. (1996). Densification of domestic waste fills by
dynamic compaction. Canadian Geotechnical Journal, 33: 879-887.
29. Wall, D.K. and Zeiss, C. (1995). Municipal landfill biodegradation and settlement.
Journal of Environmental Engineering, 121(3): 214-224.
30. Zekkos, D. P. (2005). Evaluation of static and dynamic properties of municipal solid-
waste. A dissertation submitted in partial satisfaction of the requirements for the degree
of Doctor of Philosophy in Geotechnical Engineering – University of California,
Berkeley
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