SETTLEMENT AND CHARACTERISTICS OF WASTE AT A
MUNICIPAL SOLID WASTE LANDFILL IN MELBOURNE
Samuel T. Yuen1and John R. Styles1
ABSTRACT
This paper presents the findings of a settlement investigation conducted at a municipal solid waste landfill
in Melbourne. As part of the study, data related to the characteristics of the landfill in terms of its waste
composition, moisture content, density/porosity, daily covers, and biodegradation were also collected. The
settlement data were fitted to an empirical (Sowers) model to determine long-term secondary settlement
parameters related to the known landfill characteristics. They were then compared to values reported in the
literature.
INTRODUCTION
Compared with soils, the settlement mechanisms in municipal solid waste (MSW) landfills are complex
(Sowers 1973, Edil et al. 1990, Wall and Zeiss, 1995). The waste fill has an inherent heterogeneity and
anisotropic material properties that are more difficult to characterise than soils. These include
compaction/density, daily cover, moisture content, composition, and biodegradation. The variations in these
properties together with the unsaturated nature of most landfills have imposed certain limitations on the use
of classical soil mechanics approaches to predict landfill settlements. Settlement prediction based on
monitoring and observational procedures thus becomes a viable tool. While the literature has reported some
field settlement observations, few studies attempted also to relate them to the above important associated
landfill properties.
This paper presents the findings of a settlement investigation conducted at a municipal solid waste landfill
in Melbourne. This investigation formed part of a larger study conducted on a fully instrumented full-scale
cell at the Lyndhurst Sanitary Landfill located 35 km south-east of the city. The objective of the project was
to evaluate the full-scale landfill behaviour (including settlement), enhanced biodegradation promoted by
leachate recirculation, and landfill hydrology. A full description of the research project was given by Yuen
(1999).
The settlement of the landfill has been monitored for about four years since final capping in January
1996. Data related to the characteristics of the landfill in terms of its waste composition, as-placed moisture
content, density/porosity, and daily
covers were collected. Changes in
moisture content caused by infiltration
N
Recirculation Section
Control Section
and leachate recirculation were
obtained based on an associated
hydrological evaluation. The production
of landfill gas, leachate composition,
and in-situ temperature were also
Control B:
Control A:
SP5 SP6 SP7 SP8 SM2
monitored to reflect the progress of
SP1 SP2 SP3 SP4 SM1
Test A:
biodegradation. The settlement data
SP9 SP10 SP11 SP12 SM3
were then fitted to an empirical
(Sowers) model to determine settlement
(N.T.S.)
parameters. This paper provides a
summary of the settlement investigation
Figure 1 - Location Plan showing settlement plates
and related findings.
1
Department of Civil & Environmental Engineering, University of Melbourne, Parkville, Victoria 3052, Australia
DESCRIPTIONS OF LANDFILL
Size Of Landfill Cell
The full-scale experimental cell covers a footprint area of approximately 180m x 75m (about 1.5
hectares). The thickness of MSW fill varies from 10m to 15m according to the surface landscape. Based on
survey data, the as-constructed volume is 180,365 m3 (excluding liner and cap). Total tonnage as recorded at
the weighbridge is 100,824 tonnes. In plan, the cell is divided into two sections of roughly equal area (Figure
1). The western half has been designated as the control section (i.e. dry landfilling) and the eastern half as the
leachate recirculation section (i.e. wet landfilling).
Cell Design
The cell comprises a minimum 1m thick side and base liner of compacted clay with a specified hydraulic
conductivity of less than 1x10-9 m/s.
Upon completion of filling, a 1m thick final cap was laid which was made up of a 300mm topsoil layer, a
200mm sand drainage layer, and a 500mm compacted clay layer.
The leachate collection system comprises a 300mm thick gravel drainage layer immediately above the
liner, with collector pipes draining into a header pipe then into a leachate collection sump. To enable both
leachate quantity and quality from the control and recirculation sections to be monitored separately, each of
the two sections has its own separated collection system.
An integrated leachate recirculation system comprising sub-surface horizontal infiltration trenches and deep
vertical injection wells was employed in the recirculation section with an aim to promote biodegradation.
Daily/Interim Covers
Due to licensing requirements as well as operational needs to control litter, birds and odour during filling,
daily cover was used in a manner similar to other operational cells. The licence requires a 150mm layer of
earth material as daily cover during waste disposal. In addition, each completed vertical lift (of 2m) should
be covered by an interim cover of 300mm earth material. This requirement was applied to the experimental
cell. A record of all cover material was maintained which reveals that the daily/interim cover material (in this
case a clayey sandy silt) occupies about 15% of the total waste volume.
CHARACTERISTICS OF WASTE
B y D ry M a s s
Metal
G lass
3%
1%
5%
G ar d en /fo o d
In er t (b u ild in g
23%
/d aily co ver s /
r esid u al)
47%
Moisture Content
T extlle
200%
A T1
150%
A T2
A T3
A T4
100%
A T5
A C1
A C2
50%
0%
Figure 2 – Waste Composition
m
11
to
12
m
11
m
to
9m
10
9t
o
10
D ep th
8t
o
8m
7t
o
7m
6t
o
5t
o6
m
3m
5m
4t
o
12%
3t
o4
m
P ap er
2t
o
9%
1t
o2
m
P lastic
Figure 3 - Moisture Content with Depth
Waste Composition
Waste composition information was determined by collecting continuous waste samples from seven
augered holes immediately after final capping. The samples were dried to determine their moisture content
(described next) prior to sorting. The composition as sorted, expressed on a dry mass basis, is presented in
Figure 2. It can be noted that the inert waste (including daily/interim cover soil) takes up a significant
proportion due to its higher dry density than other components.
Moisture Content
The variations of moisture content (dry mass basis) with depth immediately after final capping are plotted
in Figure 3. These values were determined based on the auger samples collected for the above waste
composition sorting. The mean value and standard deviation of moisture content are 55% and 38%
respectively.
Compaction/Density/Porosity
The waste was compacted in vertical layers by a Caterpillar 826C landfill compactor with an operating
mass of 32 tonnes. Based on mass and volume records, the in-situ bulk density (with daily/interim earth
covers accounted) was calculated to be 0.83 tonne/m3.
From the bulk density and the mean moisture content of 55% (above), the dry density and porosity of the
MSW were also calculated: 0.54 tonne/m3 and 0.55 respectively. The literature suggests a porosity range
between 0.5 to 0.6 for MSW (e.g. Korfiatis et al., 1984; Oweis et al., 1990; Zeiss and Major, 1992). In this
case 0.55 reflects that the MSW in the experimental cell is reasonably well-compacted.
MONITORING OF LANDFILL SETTLEMENT
The plan in Figure 1 shows three clusters
of settlement monitoring points, two in the
Control A
Control B
Test A
Approx.
SM 1
SM 2
SM 3
control section and one in the recirculation
18m AHD
Final Cap
section. Each cluster is composed of a series
of five settlement plates installed on top of
liner at 4m AHD (Australian Hard Datum),
Layer
L4
at approximately 6m AHD, 8m AHD, 10m
Approx.
SP 4
SP 8
SP 12
AHD, and on surface of final cap at 18m
10m AHD
AHD respectively (Figure 4). Instead of just
Approx.
L3
SP 3
SP 7
SP 11
8m AHD
monitoring the cell surface subsidence, the
Approx.
SP 2
SP 6
SP 10
L2
6m AHD
settlement data collected at various levels
Approx.
SP 1
SP 5
SP 9
provides
additional
information
on
L1
4m AHD
subsidence behaviour along a vertical
Liner
profile.
Figure 4 - Settlement Plate Schematic Cross-Section
The monthly monitoring results for the
period up to July 1999 are plotted in Figure
5. The minor local fluctuations along the cumulative curves reflect the accuracy of the level survey
(+10mm).
ANALYSIS OF SETTLEMENT DATA
An empirical approach based on a logarithmic function (e.g. Yen and Scanlon, 1975), power law (e.g.
Edil et al., 1990) or a hyperbolic relationship (Ling et al., 1998) has often been used to express landfill
settlement rate in conjunction with an observational procedure. However, the model proposed by Sowers
(1973) is the most widely used approach for settlement prediction, which considers primary and secondary
consolidations separately. The primary settlement component is stress dependent, which occurs rather
quickly, usually within 30 to 90 days after the landfill is completed (Sowers, 1973, Wall and Zeiss, 1995).
Secondary settlement is the non-stress dependent long-term creeping settlement. This component accounts
for the majority of the total landfill settlement and can take place over many years. The Sowers model
assumes that the portion of the settlement curve corresponding to secondary settlement is linear with respect
to the logarithm of time as expressed by equations (1) and (2) (Sowers 1973). This assumption has been
supported by many subsequent researchers (e.g., Moore and Pedler, 1977; Morris and Woods, 1990; Wall
and Zeiss, 1995). This paper focuses on the use of the Sowers model to analyse the more important nonstress dependent long-term secondary settlement.
Jun-97
Apr-97
Aug-99
Oct-96
(e) S ettlem en t P lates at ap p p r o x. 18m AH D
1.20
(o n su r face o f fin al cap )
C o n t ro l A (S M 1 )
C o n t ro l B (S M 2 )
0.60
Te s t A (S M 3 )
0.40
0.20
0.00
Figure 5 - Settlement Monitoring Results
0.25
C o n t ro l B (S P 8 )
0.20
Te s t A (S P 1 2 )
0.15
0.10
0.05
0.00
Apr-99
Feb-99
Dec-98
Oct-98
Aug-98
Jun-98
Apr-98
Feb-98
Dec-97
Oct-97
Aug-97
Jun-97
Apr-97
Feb-97
Dec-96
Oct-96
Aug-96
Jun-96
Apr-96
Feb-96
Jun-99
C o n t ro l A (S P 4 )
Aug-99
0.40
Jun-99
(d ) S ettlem en t P lates at ap p p r o x. 10m AH D
Aug-99
Apr-99
Feb-99
Dec-98
Oct-98
Aug-98
Jun-98
Apr-98
Feb-98
Dec-97
Oct-97
Aug-97
Jun-97
Apr-97
Feb-97
0.05
0.30
Dec-96
0.10
0.35
Jun-99
0.15
0.15
Apr-99
Te s t A (S P 1 1 )
Aug-96
(c) S ettlem en t P lates at ap p p r o x. 8m AH D
Feb-99
C o n t ro l B (S P 7 )
Jun-96
0.30
Apr-96
0.00
Dec-98
0.00
Feb-96
0.02
Oct-98
0.04
Cum ula tive Se ttle m e nt (m )
Te s t A (S P 9 )
Aug-98
C o n t ro l A (S P 3 )
Cum ula tive Se ttle m e nt (m )
0.06
Jun-98
Aug-99
Jun-99
Apr-99
Feb-99
Dec-98
Oct-98
Aug-98
Jun-98
Apr-98
Feb-98
C o n t ro l B (S P 5 )
Apr-98
Aug-99
Jun-99
Apr-99
Feb-99
Dec-98
Oct-98
Aug-98
Jun-98
Apr-98
Feb-98
Dec-97
Oct-97
Aug-97
Jun-97
Apr-97
Feb-97
C o n t ro l A (S P 1 )
Feb-98
Dec-97
Oct-97
Aug-97
0.80
Feb-97
1.00
Dec-96
Oct-96
Aug-96
Jun-96
Apr-96
Feb-96
Cum ula tive Se ttle m e nt (m )
Dec-97
Oct-97
Aug-97
Jun-97
Apr-97
0.20
Feb-97
0.25
Dec-96
Oct-96
Aug-96
Jun-96
Apr-96
Feb-96
Cum ula tive Se ttle m e nt (m )
0.08
Dec-96
Oct-96
Aug-96
Jun-96
Apr-96
Feb-96
Cum ula tive Se ttle m e nt (m )
(a) S ettlem en t P lates at ap p p r o x. 4m AH D
(o n to p o f b ase lin er )
(b) S ettlem ent P lates at apppr ox. 6m AH D
0.10
0.20
C o n t ro l A (S P 2 )
C o n t ro l B (S P 6 )
0.10
Te s t A (S P 1 0 )
0.05
0.00
Ss / Hp = Cae log ( t / tp)
(1)
Ca = Cae (1 + ep)
(2)
Ss is the secondary settlement (m),
Hp is the height of waste upon completion of primary settlement (m),
Cae is the slope of the strain versus log-time curve or the secondary compression ratio,
t is the elapsed time (days),
tp is the time for primary compression to complete (days),
Ca is the secondary compression index, and
ep is the void ratio upon completion of primary settlement.
The use of Sowers model has a major limitation: it is sensitive to the value of tp used which is often
difficult to identify as primary and secondary settlement occurs simultaneously (Phillips et al. 1993). To
investigate the sensitivity of tp on Cae, three different tp values (30 days, 60 days and 90 days) were used.
These figures were selected based on the range of primary settlement completion times as suggested in the
literature (Sowers, 1973, Wall and Zeiss, 1995). In this case, the t p of 90 days returned the best fit (Yuen
1999). Thus a 90-day period was taken empirically to be the primary settlement time in the following
analysis.
Based on the monitoring data and equation (1), the secondary compression ratio, C ae, for each individual
layer (i.e. L1, L2, L3 L4 and overall layer as shown in Figure 4) was then determined (Table 1). For
comparison, Table 2 summarises the range of Cae reported in the literature. The values in Table 1 fall within
the published range.
Settlement Plate
Group
Control A
Control B
Test A
Reference
Sowers
(1973)
Burlingame
(1984)
Walker &
Kurzeme
(1984)
Yen &
Scanlon
(1975)
Watts &
Charles
(1990)
Edil et al.
(1990)
Gifford et al.
(1990)
Walls &
Table 1 – Results of Secondary Compression Ratio (Cae)
Layer
Overall
L1
L2
L3
0.028
0.008
0.019
0.041
0.038
0.023
0.030
0.046
0.054
0.035
0.044
0.052
L4
0.030
0.037
0.055
Table 2 - Reported Secondary Compression Ratio, Cae (after Phillips et al., 1993)
Cae
Comment
Type of Landfill
0.075
For ep=3
Highly organic, favourable biodegradation
0.025
0.022
For ep=3
3m thick
Low biodegradation
Upper limit for old landfill, 75 kPa surcharge
0.008
0.08
12m thick
6m surcharge
Typical upper limits for 3-15m thickness of variable age
0.04
0.14
3m surcharge
<12m thick
Upper limits of self-weight creep of recent refuse
0.06
0.10/0.23
12-30m thick
12m thick
Biodegradation component of recent domestic refuse
0.02
0.075
10-30m thick
Corresponding physical creep component
Upper limit, recent refuse under self-weight
0.012
0.020
15m thick
-
Upper limit, old refuse
Upper limit, old landfill
0.033-0.056
0.5m dia. &
Laboratory bioreactor cell
Zeiss (1995)
1.7m thick
0.037-0.049
Laboratory dry cell
Ca
Sowers (1973) suggested that Ca is
L a y e rs
proportional to two factors: initial void
Overall
L1
L2
L3
L4
ratio and favourable decomposition
0 .1 4
conditions.
The
secondary
0 .1 2
compression index (Ca) was then
0 .1 0
determined using Equation 2. This
0 .0 8
would require the estimate of ep (the
void ratio upon completion of primary
0 .0 6
settlement). A value of 1.22 is used
0 .0 4
here for all the settlement plates,
0 .0 2
which was based on the as-capped
0 .0 0
porosity value of 0.55 (above). While
C o ntro l A
C o ntro l B
Te s t A
this as-capped void ratio is not exactly
G ro u p s o f S e ttle m e n t P la te s
the same void ratio at the completion
of primary settlement, any error
Figure 6- Secondary Compression Index (Ca)
would be small considering the
relatively small magnitude of
primary settlement. The results of Ca
for all layers at the three different settlement plate groups are plotted in Figure 6.
CONCLUSIONS
Because of its simplicity, the Sowers model was used in the above analysis. It also offers the advantage
that the Cae values obtained can be compared with other similar published data. In this case they are within
the published range. With knowledge of other associated waste characteristics, the parameters obtained can
be used in predicting settlement of similar landfills with a higher degree of confidence.
Table 1 and Figure 7 suggests that while similar results were obtained from the two settlement plate
groups (Control A and B) in the control section, the Test A group of plates in the recirculation section
returned the highest Cae and Ca. Monitoring of landfill gas, leachate quality, and in-situ temperature
generally indicated a better biodegradation occurring in the leachate recirculation section (Yuen 1999). The
higher settlement rate is likely to be associated with enhanced biodegradation as reported by Wall and Zeiss
(1995). It can also be observed from Table 1 and Figure 7 that within each settlement plate group, there is a
large variation in Cae and Ca among individual layers at various depths. This implies that there is a presence
of heterogeneity along the vertical profile.
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