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Effect of cover soil selection and compaction on gas transport and water retention

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In Sri Lanka, engineered landfills are not usually available and the common procedure is to cover the waste with a good cover soil without providing gas venting facilities. Hence, hazardous gaseous and liquids from landfills emit through this cover soil. Therefore, studying the cover soil parameters are of paramount importance in evaluating its future gas diffusion. The Soil gas diffusion coefficient (D p) and Air permeability (k a) govern the transport and emission gases in the unsaturated zone. In this study, soil gas diffusivity (D p/D o, D o is gas diffusion coefficient in free air) and k a were measured for the final cover soil of Maharagama landfill at different soil water matric potentials. k a and D p/D 0 were measured as a function of air filled porosity (s) at different water contents. Finally, it is concluded that final cover soil of Maharagama waste disposal site is a material very low gas exchangeable capacity ie it is a good capping soil.
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1
Effect of Compaction and Moisture Content on Gas
Transport and Water Retention in Landfill Cover Soil;
Maharagama Landfill Site as a Case Study
R.H.K. Ranasinghe 1), U.P.Nawagamuwa2), K. Kawamoto3)
Abstract: In Sri Lanka, engineered landfills are not available and the usual procedure is to cover the
waste with a good cover soil. This can be found at Mahargama dumpsite too. Hence studying the
cover soil parameters are of paramount importance in evaluating its future gas diffusion. The Soil gas
diffusion coefficient (Dp) and Air Permeability (Ka) govern the transport and emission of Green House
gases and volatile organic chemicals in the unsaturated zone. In this study Soil gas diffusivity and Air
Permeability was measured in the soil which was used to construct Maharagama landfill site cover
filling. Measurements were done in repacked samples at soil water metric potentials from pF= 1,2, 3,
4.1 (pF=-Log Ψ) , air dried condition and oven dried condition. Air content was varied from 0 to 0.2
m3 m-3. Gas diffusivity is changed o to 0.05.Air Permeability varies from 0 to 35 µm2. Gas diffusivity
and air permeability was increased with the soil air content. The increase of dry bulk density and
reduction of water content increases the amount of soil gas transport parameters.
Keywords: Cover soil, Gas Transport, Air Permeability, Gas Diffusivity, Soil Air content, compaction.
1. Introduction
Once municipal solid waste is placed in a
landfill, a complex sequence of biologically,
chemically and physically mediated events
occur relating to hazardous gaseous and liquid
landfill emissions. A significant fraction of the
biodegradable portion of the municipal solid
waste is ultimately converted to gaseous end
products during the predominately anaerobic
stabilization of solid waste organic fractions.
In Sri Lanka most of the landfills are
uncontrolled landfills and there are very few
controlled landfills. Therefore there is no gas
collection system in Sri Lankan landfills. Hence
cover soil of the landfill plays a major role in
the emission of landfill gases, because once the
solid waste is covered with the cover soil these
gases are released to the atmosphere with high
pressure, through this cover soil.
This paper discusses the effect of compaction
and moisture content on soil on gas transport
and water retention in landfill cover soil. In this
study the gas diffusivity (Dp/D0), (D0 is the gas
diffusion coefficient of the free air) and air
permeability (Ka) were calculated based on the
measurements in landfill cover soil of
Maharagama landfill site. Currently
Maharagama landfill site is covered with a
cover soil in order to make use as a mini cricket
ground. Radius of the ground is 40m. There
had been a 3-4m height waste layer. When
constructing this mini Cricket ground the waste
layer was compacted and 3-4 layers of soil had
been compacted. The thickness of the soil layer
is 1.5m and there is a 100mm thick different soil
layer on top, which is suitable for the
vegetation. There is a gabion wall around the
ground and parallel to the gabion wall there is a
drain. No gas venting facilities had been
provided when constructing this ground.
Figure 1- Maharagama landfill site
1. Eng.R.H.K. Ranasinghe BSC(Eng),Graduate
Student, De
p
artment o
f
Civil En
g
ineerin
g
,
University of Moratuwa, Sri Lanka.
2. En
g
(Dr) U.P. Nawa
g
amuwa BSc(En
g
),
MEn
g
(AIT), DrEn
g
(YNU), CEn
g
, MIE(SL)
Senior Lecturer, De
p
artment o
f
Civil En
g
ineerin
g
,
University of Moratuwa, Sri Lanka.
3. Dr. Ken Kawamoto,
A
ssociate Pro
f
essor,
De
p
artment o
f
Civil and Environmental
En
g
ineerin
g,
Saitama Universit
y,
Saitama
,
J
a
p
an.
2
Overall plan of the mini cricket ground is
shown in Figure 2. This site is used in the
research as a case study and the materials used
for the cover soil were tested in the laboratory
and also onsite.
Figure2. Overall plan of the mini cricket ground
(Courtesy of Sri Lanka Land Reclamation and
Development Corporation)
Air Permeability (ka) controls air flow due to air
pressure differences in soil. Air permeability
(ka) shows high spatial variability and scale
dependency due to soil heterogeneity. Under
laminar flow conditions, the advective flow of
gas through porous media follows Darcy’s law.
Darcy’s Law was used to calculate air
permeability of the soil samples,
........(1)
ka : Soil-air permeability [ L2 ],
aS : Cross sectional area [ L2 ]
Q : Volumetric flow rate [ L3 T-1 ],
LS : Length of the sample [ L ]
P: Pressure difference across the sample
[M L
-1 T-2]
η : Dynamic gas viscosity [ M L-1 T-1 ]
Fick’s Law is used to calculate the Gas
Diffusivity.
z
c
p
t
cD2
2
δ
δ
δ
δ
= ........ (2)
Dp: Gas diffusion coefficient [ L2T-1]
C : gas concentration (ML-3)
Z : Elevation difference (L)
T : Time (T)
2. Literature review
Accurate description of soil-gas transport
parameters in unsaturated soil profiles is
needed when investigating the emission of
greenhouse gases to the atmosphere (Osozawa
and Hesegawa, 1995[2]). Soil-gas transport
processes in a natural soil profile are largely
governed by variations in soil-water content
(water retention), soil texture, and organic
matter content (Moldrup et al., 2004[2]).
Soil-air permeability is an easily measured
transport parameter both in-situ and in the
laboratory using undisturbed or repacked soil
samples at several measurements scales
(Poulsen et al., 2001(2)]. The Air permeability
(Ka) value provides useful information about
soil structure, and is used for characterizing soil
pore geometry (Ball, 1981(2); Molderup et al.,
2001, 2003[2]).The Ka value also related to
saturated and unsaturated hydraulic properties
(Blackwell et al., 1990; Rasmusssen et al.,
1993[2]). To determine the pore characteristics
of the soil air permeability measurements are
used. Soil air permeability and air filled micro
porosity was used to characterize the soil and
used this relationship to identify changes in soil
structure caused by soil management practices
and biological activity. (Blackwell et al, 1990
[2]). At the same time Ka value is related soil-
gas diffusivity (Dp), and these two transport
parameters have been linked together to
express air-filled pore connectivity and
equivalent pore diameter (Millington and
Quirk, 1964; Ball, 1981; Molderup et al.,
2001[2]). Ka value can be used to determine the
pore characteristics of soils (Blackwell et al,
1990[2]) and air filled micro porosity to
characterize the soil and used this relationship
to identify changes in soil structure caused by
soil management practices and biological
activity. The gas diffusivity and air filled
porosity were used to describe the continuity
and tortuosity of micro pores in the soil (Ball,
1981[2])
The soil water metric potential (Ψ, or pF)
provides a measure of soil moisture status of
unsaturated soil and is directly linked to the
S
Sa
L
aPk
Q×
××
=
η
3
soil water content (θ) and consequently to ε
through the soil water characteristic curve and
soil water characteristic models (Ex Campbell,
1974[2]).
Several studies suggested a direct link of soil
gas diffusivity to pore size distribution through
a SWC function. Freijer (1994) applies the
tortuous jointed-tube model by Ball (1981) to
describe soil gas diffusivity variations with ε,
and also suggested possible links between
Dp(ε) and the parameters in the unimodel (van
Genuchten 1980[2]) SWC function.
3. Materials and Methods
3.1 Material
Measurements on a repacked soil (used in
constructing the Maharagama ground) were
conducted during this study. All measurements
of soil water retention, Ka and Dp were done
using 100cm3 core samples at a wide range of
soil water metric potential values from near
saturation to air dry conditions. The
dimensions of the 100cm3 core cutter were 5.6-
cm in diameter and 4.06cm in height.
Permeameter was used to measure the air
permeability (refer Figure 3) and Gas diffusion
apparatus (refer Figure 4) was used in order to
find the gas diffusion coefficient. Sand box
apparatus (refer Figure 5) and Pressure plate
extractor were used to control the samples to
different moisture conditions.
Figure 3. Air permeability measuring apparatus
3.2 Measurement methods.
First standard Proctor compaction test (ASTM
standard) was carried out to find out the
optimum moisture content and maximum dry
bulk density of the soil. In each compaction test
step, sampling was done. When sampling a 100
cm3 core cutter was inserted carefully by hand
in to the soil mould (Proctor compaction
mould), removed using a hand shovel and
sealed. Field water content was measured when
collecting soil from the site. Soil water retention
was measured using a draining curve, using
either a hanging water column for pF<= 2
(i.e.,Ψ >= 100 cm H2O) or a pressure plate
extractor across the entire pF interval. The soil
samples were first saturated with water and
then drained subsequently to different pF
conditions where Dp and Ka were measured at
each drainage step. Before measuring the Dp
and Ka, the weight of the soil samples were
measured in order to determine the water
content and air content at each pF condition.
The samples were placed inside a convective air
flow oven set at 30 oC for 5 to 7 days to make
the samples air dried. To bring the samples to
oven dry condition, samples were placed inside
an oven set at 105oC for 1 day.
Air Permeability and Gas diffusion coefficient
was measured for each sample at field water
content. Then all the samples were saturated
with water. Then samples were drained to
different soil water metric potential values and
the measurements for Soil gas diffusivity
coefficient (Dp) and air permeability (Ka) were
taken. Weights of the samples were measured
at each pF condition in order to calculate the
soil air content.
Figure 4. Gas diffusion apparatus
Air Compressor
Flow Meter
Manometer
Soil Sample
Fi
g
ure 5. Sand box apparatus
4
4. Results and Discussion
4.1. Results
Table 1: Parameters measured in soil samples
Figure 6- Soil gas Diffusivity changes with the air content of the sample.
Sample No Condition Air content Dp(cm2/s) DP/D0 Ka /(µm2)
1-1 W= 10.38 % 0.057 4.32E-03 2.13E-02 7.007
1-2 W= 10.38 % 0.113 6.63E-03 3.27E-02 27.694
2-1 W= 10.38 % 0.131 9.81E-03 4.83E-02 29.248
2-2 W= 10.38 % 0.083 9.87E-03 4.86E-02 27.433
3-1 W= 10.38 % 0.099 8.21E-03 4.04E-02 31.314
3-2 W= 10.38 % 0.113 1.06E-02 5.21E-02 13.104
B-1-1 W= 16.95 % 0.009 0.00E+00 3.60E-04 6.318
B-1-2 W= 16.95 % 0.008 0.00E+00 1.37E-04 6.021
B-2-1 W= 16.95 % 0.008 2.59E-04 1.27E-03 6.707
B-2-2 W= 16.95 % 0.007 4.59E-03 2.26E-02 8.835
C-1-1 W= 12.16 % 0.061 4.03E-03 1.98E-02 -
C-1-2 W= 12.16 % 0.052 3.36E-03 1.66E-02 -
1-1 @ pF=1 0.079 3.84E-03 1.89E-02 8.333
B- 1-2 @ pF=1 0.037 1.19E-03 5.84E-03 2.025
E-1-1 W=12.55 % 0.080 4.32E-03 2.13E-02 25.343
E-1-2 W=12.55 % 0.086 3.19E-03 1.57E-02 32.355
2-1 @ pF= 1.5 0.174 6.63E-04 3.27E-03 23.346
B-1-1 @ pF= 1.5 0.050 4.97E-04 2.45E-03 1.112
C-1-2 @ pF= 2 0.125 6.56E-03 3.23E-02 62.051
1-2 @ pF= 2 0.181 3.50E-03 1.73E-02 66.287
5
Figure 7- Soil air permeability changes with the air content of the sample
Figure 8- Soil air permeability changes with the dry bulk density of the sample
6
Figure 9 -Soil gas diffusivity with dry bulk density
4.2. Discussion
Figure 6 shows the variation of the gas
diffusivity of the soil sample with the soil air
content. According to the results, air content
varies from 0 to 0.2 m3 m
-3. Gas diffusivity is
changed from 0 to 0.05.When the air content of
the sample is high, the gas diffusivity was also
high.
Figure 7 shows the variation of air permeability
of the soil with air content of the soil. Air
permeability is changing from 0 to 35 µm2 while
air content changes of 0 to 0.2 m3 m
-3. Air
permeability also increases with the soil air
content.
Figure 8 and Figure 9 illustrate the variation of
soil air permeability with soil air content and
soil gas diffusivity with the soil air content
respectively. Bulk density of the soil varies
from 1.75 g/ cm3 to 2.0 g/ cm3.
When the moisture content of the samples was
reduced, air permeability and gas diffusivity of
the samples increased. This was because the
reduction of moisture content of the soil sample
increases the air content of the sample while the
pore volume of the sample remains unchanged.
5. Conclusion
Studies about the gas transport parameters in
solid waste landfill cover soil are very
important to identify the toxic gas emission
variation. Gas Diffusivity (Dp/Do) and air
permeability increase with the soil air content.
When the dry density was increased, the soil air
content decreased. For a soil sample with a
particular dry bulk density, the air content
increased when the water was reduced. So the
increase of dry bulk density and reduction of
water content increased the soil gas transport
parameters. Air content varies from 0 to 0.2 m3
m-3. Gas diffusivity is changed from 0 to 0.05.
Air Permeability varies from 0 to 35 µm2. Gas
diffusivity and air permeability increase with
the soil air content. The increase of dry bulk
density and reduction of water content increase
the soil gas transport parameters. With the
moisture content reduction, the air permeability
and gas diffusivity increase due to the increase
of air content of the sample.
Acknowledgment:
The international collaborative graduate
program of Saitama University is highly
appreciated for giving necessary funds for this
7
research to conduct laboratory and field tests
both in Sri Lanka and Japan.
Mr. P.P. Gnanapala, Additional General
Manager (Research and Design Division) of Sri
Lanka Land Reclamation and Development
Corporation is gratefully acknowledged for
providing necessary information allowing the
authors to use the said Maharagama landfill.
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Methods of Soil Analysis part 4-Physical Methods
  • J H Dane
  • C T Topp
Dane, J. H., Topp, C.T., Methods of Soil Analysis part 4-Physical Methods,Soil Science Society of America, Madison, Wisconsin, USA, 2002, p.1113-1157.
Gas Transport Parameters in the Vadose Zone: Development and Tests of Power Law Models for Air Permeability
  • K Kawamoto
  • P Molderup
  • P Schjφnning
  • T Komatsu
  • D E Rolston
Kawamoto, K., Molderup, P., SchjΦnning, P., Komatsu, T., Rolston, D. E., "Gas Transport Parameters in the Vadose Zone: Development and Tests of Power Law Models for Air Permeability ", J. Vadose Zone Journal 5, Soil Science Society of America, pp. 1205-1215, 2006