www.elsevier.com/locate/wasman
Waste Management 24 (2004) 967–979
Suitability of shredded tires for use in landfill leachatecollection systems
M.A. Warith a,*, E. Evgin b, P.A.S. Benson c
a Department of Civil Engineering, Ryerson University, 350 Victoria Street, Toronto, Ont., Canada M5B 2K3b Department of Civil Engineering, University of Ottawa, Ottawa, Ont., Canada
c Golder Associates Ltd., Ottawa, Ont., Canada
Accepted 11 August 2004
Abstract
The suitability of shredded tires or ‘‘tire chips’’ for use in the leachate collection drainage layer of a municipal solid waste landfill
was investigated in terms of the: (1) compressibility of the tire chips and resulting changes in hydraulic conductivity under varying
applied loads, and (2) effect of leachate pH on the shredded tries compressibility and hydraulic conductivity behavior. A constant
head hydraulic conductivity apparatus was fabricated to measure the hydraulic conductivity of the tire shred sample under different
axial strains. Further, the fabricated assembly was capable of measuring hydraulic conductivity of the sample at various sample
locations at a given strain level. One aim of this study was to provide supporting information for permission to use tire chips as
an alternative to crushed stone in the leachate collection system of a landfill. Shredded tires from two different sources were used
in this study to investigate any differences in the sensitivity of the shredding process to compressibility and hydraulic conductivity
responses under varied applied loads. Under applied vertical loads resulting in average vertical stresses of up to 440 kPa, equivalent
to over 50 m of waste, the maximum normal strain recorded in each type of tire chip was observed to plateau at a strain level near or
slightly greater than 0.5. The results of the permeability testing indicated average hydraulic conductivity values ranging between 0.67
and 13.4 cm/s under average applied normal stresses ranging from approximately 60 to 335 kPa and strain increments between 0.3
and 0.5. These results are one to three orders of magnitude higher than the hydraulic conductivity typically specified for drainage
layers in leachate collection systems of 0.01 cm/s. Additional tests were also carried out to identify how landfill leachate and varied
pH levels may affect the compressibility and hydraulic conductivity of the shredded tires. Care should be exercised in extending these
results to field conditions, as the results presented are based on limited experimental testing data and a limited time frame.
� 2004 Elsevier Ltd. All rights reserved.
1. Introduction
1.1. Background
Leachate collection and removal systems are a key
component of modern-day engineered landfill sites.
The purpose of a leachate collection and removal system
is to remove contaminated water from the base of a
0956-053X/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.wasman.2004.08.004
* Corresponding author. Tel.: +1 416 979 5000x6459; fax: +1 416
979 5122.
E-mail address: [email protected] (M.A. Warith).
landfill waste containment cell for the purpose of mini-
mizing the hydraulic head on the liner system or sub-
grade of the landfill cell. Typical components of
leachate collection and removal systems include ashaped subgrade, leachate collection piping, depressed
sumps (or low areas in a waste containment cell for leac-
hate collection) with leachate evacuation pumps and a
drainage medium to convey leachate to the collection
piping and sumps. The drainage medium is typically
comprised of poorly graded, uniform crushed stone,
overlain by a filter medium of either geotextiles or sand
(Evans, 1997).
968 M.A. Warith et al. / Waste Management 24 (2004) 967–979
The crushed stone component of leachate collection
system drainage layers can be a very costly component
in the construction of an engineered landfill facility. This
is due, in part, to the extensive processing involved in
producing quantities of crushed stone with the required
gradation and haulage costs for this relativelyheavy material. Furthermore, crushed stone is a non-
renewable resource (Humphrey and Manion, 1992).
This paper presents the results of a laboratory study,
which was carried out to investigate the suitability of
shredded rubber tires or ‘‘tire chips’’ as an alternative
to crushed stone in the leachate collection drainage layer
of a municipal solid waste landfill site.
Recommendations for drainage layer thickness andgradations are set forth in Ontario, Canada by the On-
tario Ministry of the Environment (MOE, 1998), and
the regulatory and approval requirements for new or
expanding municipal landfill sites in Ontario, Canada,
are described under Ontario Regulation 232/98
(O.Reg. 232/98) made under the Environmental Protec-
tion Act. Similar regulations and guidance documents
exist for other governing bodies and regulatory agen-cies in North America. These recommendations typi-
cally pertain to the required service life of the
engineered components of the landfill site. Demonstra-
tion that the granular drainage material in the leachate
collection system will provide adequate hydraulic con-
ductivity or hydraulic conductivity during the service
life is an essential requirement for landfill design
approval.Review of the use of tire chips in the design of high-
way embankments (Edil and Bosscher, 1994; Bosscher
et al., 1997) revealed that the tire chip-soil matrix could
exhibit a significant initial plastic compression under
vertical loads. This could be as high as 40% of the initial
placement thickness for pure tire chips. Once the mate-
rial is subjected to this level of vertical stresses and com-
pression and the associated reduction in porosity, itbehaves like an elastic material.
1.2. Study objectives
The main objective of this study was to investigate
the performance of tire shreds as an alternative to
crushed stone in landfill leachate collection systems.
The specific objectives of this study were to:
� Evaluate the suitability of tire chips for use in the
leachate collection drainage layer of a municipal solid
waste landfill site in terms of the compressibility of
the tire chips and resulting changes in hydraulic con-
ductivity under varying applied loads for site specific
and generic landfill design conditions.
� Investigate differences in compressibility and hydrau-lic conductivity testing results on tire chips from dif-
ferent sources and shredding methods.
� Investigate the effect of leachate acidity (low pH val-
ues in the range of 4–5) during early stages of the
landfill operation and organic waste biodegradation,
and leachate alkalinity (high pH values in the range
of 8–9) during later stages of organic solid waste bio-
degradation on the shredded tires drainage layercompressibility and in turn drainage layer hydraulic
conductivity properties under varying conditions of
compressibility and pH values.
Information of shredded tires thickness and hydraulic
conductivity during its application as a drainage layer in
landfills is essential and is often required by the regula-
tory standards. The compressibility of the material iscritical to the evaluation of the use of shredded tires be-
cause as the material compresses, the thickness and
hydraulic conductivity of the drainage layer will de-
crease, negatively affecting its performance. Knowledge
of the interrelationship between compressibility, thick-
ness, and hydraulic conductivity for various types and
treatments of shredded tires would increase our ability
to design appropriately for their use.Shredded tires from two different sources were used
in this study to investigate how different shredding proc-
esses may affect the compressibility and hydraulic con-
ductivity response under varied applied loads.
2. Literature review
Municipal solid waste (MSW) landfills are con-
structed with a leachate drainage layer overlying single
or composite multi-layer liner materials. The purpose
of a leachate drainage layer is to provide positive control
and discharge of landfill leachate to the leachate collec-
tion system. The design criteria as stipulated in Ontario,
Canada Regulation 232/1998 and by Subtitle D (USE-
PA, Code of Federal Regulations 40 CFR258 of 1992)for the drainage stone layer are as follows:
� drainage layer should be designed to prevent the leac-
hate head from exceeding 300 mm over the liner;
� drainage layer should be at least 300 mm thick; and
� the drainage material should possess a hydraulic con-
ductivity equal to or greater than 1 · 10�3 cm/s.
The requirements set out in the Ontario, Canada
Landfill Standards for stone in a primary leachate col-
lection system for a 100, 75 and 60 year service life state
that stone must have D85 > 37 mm, D10 > 19 mm, a
coefficient of uniformity <2, and <1% passing the
#200 (0.075 mm) sieve (MOE, 1998).
The drainage layer consists of a natural material with
a high hydraulic conductivity such as gravel or crushedstones. The stone drainage media provide a uniform and
continuous connection between the solid waste and the
M.A. Warith et al. / Waste Management 24 (2004) 967–979 969
leachate collection system, in which the liquid collected
on the base liner is transmitted to the collector drainage
pipes. Conventional materials used in the leachate col-
lection layer include granular soils and geosynthetics
(geotextiles and geonets). In addition to allowing free-
flow of leachate, the drainage layer must also serve asa protective layer to guard the liner system, particularly
the geomembrane liner, from damage during construc-
tion and after subsequent placement of waste. Finally,
the drainage layer must not damage the liner when great
depths of waste and soil could lead to stress concentra-
tions in the angular materials in a drainage layer push-
ing into the liner they are meant to protect. Thus, this
layer plays a complex role of protective cover anddrainage.
Tire shreds were proposed and approved as an alter-
native to drainage stone in the construction of a new
landfill cell at the Ryley Regional Landfill Site in the
County of Beaver, Alberta. A new landfill cell was con-
structed at this site during 1995 and 1996 using tire
shreds as a complete replacement for drainage stone in
the leachate collection system with the exception of thecrushed stone that was placed surrounding the leachate
collection pipes. It was noted during this project that the
tire shreds could not provide sufficient lateral support to
the pipe due to the compressibility of the tire shreds and
the uncertainty of the quality of the shredded tires closer
to the main drainage pipes (Evans, 1997). In total,
approximately 6000 ton of tire shred was incorporated
into the leachate collection system of the new landfillcell, which is equivalent to around 600,000 passenger
car tires (Evans, 1997).
The design criteria for the landfill cell at the Ryley site
were a minimum liner grade of 2%, a maximum allowa-
ble leachate head of 1 m over a compacted clay liner,
and a minimum leachate collection layer hydraulic con-
ductivity of 0.01 cm/s. Leachate collection pipes were
spaced at approximately 40 m, with a granular (tireshred) leachate collection layer thickness of 300 mm
(Evans, 1997). The tire chips used had a nominal size
of 50 mm (in length). Laboratory hydraulic conductivity
tests on the tire chips under a variety of confining pres-
sures were reportedly consistently two or more orders of
magnitude greater than the design performance criterion
of 0.01 cm/s.
Evans (1997) reported that ‘‘the preliminary find-ings show [hydraulic conductivity] performance to be
as expected or better and there is no marked difference
in leachate quality from that collected via a conven-
tional gravel system’’. Some practical considerations
for the use of tire shreds in a leachate collection sys-
tem, however, include: health and safety aspects of
handling shredded tires, as the steel belt fragments
and steel rims in the tires results in many sharp finesteel pieces being present; availability of tire shreds
in sufficient quantities can be a challenge; and compac-
tion of waste placed initially over the tire shreds (i.e.,
the initial waste lifts) may be up to 50% lower than
the compaction achievable over gravel-based systems
due to the initial high compressibility of the shredded
tire layers.
The City of Calgary, Alberta has also been incorpo-rating the use of tire shreds in the drainage layers at
three of the City�s landfill sites, including the use of
approximately 12,500 ton of tire shreds in their 1998
program alone (Reddy and Saichek, 1998).
Landfilling of whole tires is generally an unaccept-
able practice in the United States as well as other
developed countries and, consequently, the incorpora-
tion of tire chips into the engineered components ofnew landfill site construction has been gaining popu-
larity and acceptance in several states. For example,
Modern Landfill Inc. received the New York State
Department Environmental Conservation�s (NYS-
DEC) approval to permit the substitution of tire chips
for drainage aggregate in its primary leachate collec-
tion system in October 1991, becoming the first land-
fill site in New York to utilize tires for this purpose(Goehrig, 1996).
3. Apparatus and experimental approach
3.1. Tire chip samples
Tire chips from two suppliers were obtained for usein the laboratory study. These tire chips are represent-
ative of what is presently readily available in Ontario,
Canada, and in North America from existing facilities
outfitted with rubber shredding equipment. Tire shreds
are basically flat, irregularly shaped tire chunks with
jagged edges that may or may not contain protruding,
sharp pieces of metal, which are parts of steel belts or
beads. As previously noted, the size of tire shreds mayrange from as large as 460 mm to as small as 25 mm,
with most particles within the 100–200 mm range. The
average loose density of tire shreds varies according to
the size of the shreds, but can be expected to be be-
tween 390 and 535 kg/m3. The average compacted den-
sity ranges from 650 to 840 kg/m3.Two types of tire
chip samples are referred to as ‘‘higher quality’’ (HQ)
tire chips and ‘‘lower quality’’ (LQ) tire chips. Thehigher quality tire chips are characterized by sharper,
more regular edges with the appearance of being
‘‘cut’’ as opposed to torn or shredded. The shapes of
the individual tire chips in the higher quality samples
are somewhat more regular with average length-
to-width aspect ratios of about 1:1 to 2:1. The lower
quality tire chips have torn appearances and the indi-
vidual pieces are more elongated.Particle size analyses of shredded tires selected for this
experimental study were conducted on representative
0
10
20
30
40
50
60
70
80
90
100
0.01 0.1 1 10 100 1000
Grain Size (mm)
Per
cen
t fin
er
Trial 1 Trial 2
Fig. 1. Grain size analysis for tire shreds.
Fig. 2. The high quality shredded tires (HQ) used in the experimental study.
970 M.A. Warith et al. / Waste Management 24 (2004) 967–979
HQ and LQ tire chip samples. The resulting grain size
distribution curves are presented in Fig. 1, while Fig. 2
shows the HQ shredded tires used in this experimental
study.
Tire chips and tire shreds are non-reactive under nor-
mal environmental conditions. The principal chemical
component of tires is a blend of natural and synthetic
rubber, but additional components include carbonblack, sulfur, polymers, oil, paraffins, pigments, fabrics,
and bead or belt materials.
3.2. Compressibility test
A testing program was carried out in the laboratory
to investigate the compressibility of the tire shreds in
terms of the normal strain resulting from varied applied
vertical loads. A schematic of the compressibility testing
apparatus is provided in Fig. 3. A PVC cylinder was
used to house the tire chip samples. The dimensions ofthe cylinder measured approximately 1000 mm long
with an average outside diameter of 317 mm and an
CONCRETE CYLINDER
PLYWOOD ENDCAP(WITH ALUMINUM FACING)
TIRE CHIP SAMPLE
PVC CYLINDER
PLYWOOD ENDCAP(WITH ALUMINUM FACING)
LOAD CELL
STEEL SPACER PLATES
LOAD CELLREADOUT BOX
TINIUS OLSEN UNIVERSAL
TESTING MACHINE
Fig. 3. Schematic of compressibility test apparatus.
M.A. Warith et al. / Waste Management 24 (2004) 967–979 971
average inside diameter of 296 mm. The dimensions of
the PVC cylinder were selected to permit testing on sam-
ples with diameters exceeding the width of several indi-
vidual tire chips (i.e., at least four tire chips at a
maximum size of 75 mm) and thickness representative
of the thickness of the tire shred layer that would typi-cally be placed initially in the leachate collection system
drainage layer of a landfill site (i.e., greater than 500
mm). The end-caps used to provide the bearing surfaces
consisted of a plywood laminate with 3-mm thick alum-
inum faceplates.
A hydraulic Tinius Olsen Universal Testing Machine
was used to compress the tire chips. A load cell was
placed under the bottom end-cap to record verticalloads at the bottom of the tire chip sample, in compar-
ison to the vertical applied load measured by the univer-
sal testing machine, for the purpose of determining how
much load was being lost to friction between the tire
chips and the walls of the PVC cylinder.
Compression testing involved slowly loading the tire
chip samples while recording sample deformations at
various load increments. The reference point for meas-uring deflections was the overall distance between the
top and bottom bearing plates of the universal testing
machine. Tire chip sample mass was measured and the
corresponding bulk density was calculated for various
compression tests.
A silicon-based lubricant spray was used on the in-
side walls of the PVC loading cylinder. The purpose of
using the lubricant was an attempt to decrease loss inapplied load due to the friction between the tire chips
and the cylinder walls as the tire chip sample was com-
pressed. Significant friction losses were observed based
on the difference in applied load at the top of the sam-
ple (as measured by the universal testing machine) and
the load measured by the load cell at the bottom of the
sample.
The average maximum vertical stresses (top stress
and base stress divided by 2) computed were in the range
of approximately 250–440 kPa. Assuming an average
density of 750 kg/m3, which is representative of moder-ate to good waste compaction for municipal solid waste
in modern landfills, the maximum loading employed in
the compressibility tests were equivalent to between 30
and 50 m of solid waste.
3.3. Hydraulic conductivity testing
Following completion of the compressibility testing,the PVC cylinder used in the compressibility testing
was used as part of a constant head hydraulic conductiv-
ity apparatus for laboratory testing on the hydraulic
conductivity of tire chips under varying applied loads.
Incorporation of the ability to compress the tire chips
to a predetermined strain level and then to lock in the
required applied load such that hydraulic conductivity
testing could be carried out at that particular strain levelwas required. A schematic of the ‘‘tire chip permeame-
ter’’ is illustrated in Fig. 4.
Four 12.7-mm diameter aluminum-locking pins were
used to support aluminum porous end-caps at the top
and bottom of the tire chip samples. The locking pins
were inserted through holes that were drilled through
the wall of the PVC cylinder. The pair of locking pins
supporting the porous cap at the bottom of the tire chipsample remained in a fixed position during the hydraulic
conductivity testing. The pair of locking pins supporting
the porous cap at the top of the tire chip sample were in-
serted through holes which were drilled at locations rep-
resenting strain levels of 0.3, 0.35, 0.4, 0.45 and 0.5. The
PIEZOMETER PANEL
POROUS CAP
COMPRESSED TIRE CHIPS
SINK(With known
cross-sectionalarea)
LOCKING PIN
DRAIN PLUG
LEGS
WATER HOSES
VENT HOLE
OUTLET TOSINK
OVERFLOW RESERVOIR
PVC CYLINDER
LOCKING PIN
METRE STICK
PIEZOMETERTUBING
POROUS CAP
Fig. 4. Schematic of permeameter used for tire chip testing.
972 M.A. Warith et al. / Waste Management 24 (2004) 967–979
locations of these holes were calculated based on an ini-
tial tire chip sample height of 84 cm (i.e., the height ofthe PVC cylinder above the top of the bottom porous
cap). For each of the strain increments, the tire chip
samples were compressed using the hydraulic Tinius Ol-
sen Universal Testing Machine. A concrete cylinder and
steel plates were used as spacers to push the top porous
cap down to the desired stain level, such that the locking
pins could be inserted to lock the porous cap in place,
keeping the tire chip sample in a compressed state forthe duration of the hydraulic conductivity testing at that
strain increment. Window sealant putty was used to pro-
vide a near watertight seal around the perimeter of the
locking pins. Duct tape was used to prevent leakage
from the locking holes not being used during each
hydraulic conductivity test and around the base of the
PVC cylinder.
Fifteen (15) taps were drilled into the side of the PVCcylinder at 2-in. (50.8 mm) spacings to allow flexibility in
the connection location of each of the eight (8) piezom-
eters making up the piezometer panel. Taps not in use
during hydraulic conductivity testing were sealed with
screw caps. Water used during the hydraulic conductiv-
ity testing was provided from high-pressure faucets in
the laboratory with removed aerators. The hydraulic
conductivity testing was carried out on representativesamples of the HQ and LQ tire chips, compressed to
each of the five strain levels indicated above. Testing
was repeated at selected strain intervals in order to dem-onstrate result consistency and reproducibility.
Hydraulic head levels were measured at eight loca-
tions in the testing cylinder during each permeability test
run. These locations were selected from 15 possible ports
installed at regular intervals along the testing cylinder
(the seven unused ports were sealed shut). The specific
taps selected varied between tests due to the different
sample thicknesses, which were dependent on the strainincrement being tested. The intervals over which head
losses and resulting permeability calculations were made
were selected in order to provide a permeability profile
through the sample. Adjacent ports were not considered
(e.g., 1 and 2, 3 and 4, etc.) because the minimal loss in
hydraulic head between ports spaced closely together
introduced the potential for significant experimental er-
ror (i.e., minor fluctuations in piezometric levels repre-sented significant proportions of total head loss
between ports). Instead, intervals were selected which al-
lowed for permeability measurements at different loca-
tions in the sample, while being spaced far enough
apart that the head loss across the interval was relatively
large. The y-axis coordinates for points plotted in Figs. 8
and 9 represent the midpoint of the interval over which
permeability (x-axis) values were calculated (e.g., halfway between ports 1 and 4, 2 and 5, etc.). The average
M.A. Warith et al. / Waste Management 24 (2004) 967–979 973
of the permeability values calculated between each of the
five piezometer pairs was compared to the average bulk
permeability of the entire tire shred sample, based on the
total head loss between the upper most and lower most
ports (i.e., ports 1 and 8). This was done as a check to
address potential errors in interpretation.
3.4. Effect of pH on shredded tires’ compressibility and
hydraulic conductivity
An experimental program was designed to investigate
the compressibility and the hydraulic conductivity prop-
erties of shredded tires in drainage layers in a landfill
environment, where leachate pH varies during the land-fill life span. In this experimental study, pH conditions
ranging from 4 to 9 were used to simulate landfill leac-
hate pH (Warith and Sharma, 1998).
The HQ shredded tires were exposed to three pH
solutions:
� Low pH solution (pH 4) to simulate landfill leachate
conditions at an early stage of organic solid wastebiodegradation and generation of fatty acids.
� Neutral pH solution (pH 7) to simulate intermediate
stages of waste degradation.
� High pH (pH 9) to simulate a later stage of solid
waste biodegradation and conversion of acetic acids
and hydrogen gas to methane and carbon dioxide.
The pH values, due to these methanogenic reactions,
rise to pH values in the range of 6.8–8.5.
The solution pH values were adjusted using nitric
acid (HNO3) and sodium hydroxide (NaOH) solutions.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0 50 100 150 20
Average St
Str
ain
(cm
/cm
)
Fig. 5. Stress/strain beha
The shredded tire samples were exposed to these three
pH solutions for 2, 4 and 6 week time intervals. The
shredded tire samples were examined at each time inter-
val for their compressibility and hydraulic conductivity
properties. These tests provide insight on the effect of
various landfill leachate samples on the compressibilityand hydraulic conductivity of tire chips.
4. Experimental results and discussion
4.1. Tire chip compressibility
A graphical representation of the compressibility re-sults is shown in Fig. 5 with the average normal stress
plotted against the corresponding observed normal
strain. As described in Section 3.2, the average normal
stress at each load increment was calculated, assuming
a linear distribution of stress through the tire chip sam-
ple, by averaging the applied loads measured at the top
and bottom of the sample. Average normal strain was
calculated by dividing the observed normal deformationat each load increment by the initial sample thickness.
The compressibility testing showed consistent results
with no significant differences between the LQ and HQ
tire chips. Additionally, the use of a silicon-based lubri-
cant for the intended purpose of decreasing the fric-
tional losses between the tire chips and the walls of the
PVC testing cylinder did not appear to have any signif-
icant impact on the compressibility.The average applied normal stress was calculated
from the compressibility results at strain increments of
0.3, 0.35, 0.4, 0.45 and 0.5. These average values were
0 250 300 350 400
ress (kPa)
HQ Average
LQ Average
vior of tire shreds.
974 M.A. Warith et al. / Waste Management 24 (2004) 967–979
calculated through linear interpolation of the applied
stress at each of these strain levels in each of the com-
pressibility tests. The interpolated stress values at each
strain level were averaged separately for all of the tests
on the HQ chips and the LQ chips. The average applied
stress values at each of the above strain increments forthe HQ and LQ chips are shown in Figs. 5 and 6. The
average stress values computed for the HQ chips and
the LQ chips at each strain increment were very similar,
particularly at higher strain levels (e.g., 0.45 and 0.5). At
strain levels below 0.45, slightly less applied stress was
required to compress the HQ tire chips to the given
strain increments than was required for the LQ chips.
This could be due to a slightly higher angle of internalfriction in the LQ chips than in the HQ chips.
Similar to the results reported by Humphrey and
Manion (1992), the tire chip samples were observed to
be highly compressible at relatively low stress levels
(i.e., up to 100 kPa), with decreasing compressibility at
stress levels above 100 kPa. In each case, the maximum
normal strain recorded was observed to reach a constant
limit at a strain level near or slightly greater than 0.5(i.e., when samples were compressed to approximately
50% of the initial thickness). This is also consistent with
Donovan et al. (1996). The observed normal strain was
plotted against the average normal stress on a logarith-
mic scale. An approximately linear relationship between
the strain and log stress was observed, as shown in
Fig. 6.
The unloading results showed a combination of elas-tic and plastic deformation in the tire chip samples. Fol-
lowing compression of the tire chips to strain levels
above 0.5, an apparent total plastic deformation on
0.0
0.1
0.2
0.3
0.4
0.5
0.6
1 10
Average Str
Str
ain
(c
m/c
m)
Fig. 6. Stress/strain behavior of tir
the order of about 40% of the initial sample thickness
was observed. It is considered that some of this defor-
mation may not have been permanent (i.e., plastic)
and that, given sufficient time, the samples may have
decompressed to a higher degree. The high amount of
apparent plastic deformation observed may be due tothe loose sample placement methodology used with no
compaction effort.
Significant frictional losses were observed between
the tire chips and the walls of the PVC testing cylinder
during the compressibility testing. This was apparent
by the significantly lower applied loads measured by
the load cell at the bottom of the tire chip sample, com-
pared to the applied load measured at the top of thesample by the universal testing machine. The maximum
applied loads recorded at the bottom of the sample were
about 40% of the maximum applied loads measured at
the top of the sample.
4.2. Tire chip hydraulic conductivity
Fig. 7 provides a graphical presentation of the aver-age hydraulic conductivity measured at strain incre-
ments of 0.3, 0.35, 0.4, 0.45 and 0.5, found from each
of the compressibility tests described above. As indi-
cated in the preceding section, average stress values at
each of the strain increments in which hydraulic conduc-
tivity testing was carried out were calculated based on
the compressibility test results through linear interpola-
tion, as the applied load recorded to compress the tirechip samples to the desired strain level for hydraulic
conductivity testing was not considered to be represent-
ative of the ‘‘locked in’’ applied load. The average
100 1000
ess (kPa)
HQ Average
LQ Average
e shreds on a semi-log scale.
0.00E+00
5.00E+00
1.00E+01
1.50E+01
2.00E+01
2.50E+01
0.25 0.30 0.35 0.40 0.45 0.50 0.55
Strain (cm/cm)
Ave
rag
e H
ydra
ulic
Co
nd
uct
ivit
y (c
m/s
)HQ Average
LQ Average
Fig. 7. Average hydraulic conductivity versus strain for tire shreds.
M.A. Warith et al. / Waste Management 24 (2004) 967–979 975
hydraulic conductivity values plotted in Fig. 7 were
computed as the average of the calculated hydraulic
conductivity values based on five piezometer pairs from
each constant head hydraulic conductivity test. A total
of eight piezometers were connected to the tire chip per-
meameter during each test. The piezometer pairs used inthe average hydraulic conductivity calculation represent
the hydraulic conductivity of the tire chips at various
positions within the sample. This was desired due to
the significant stress gradient through the tire chip sam-
ple. The piezometer taps were numbered from the top of
the tire chip permeameter to the bottom. The hydraulic
conductivity values used to calculate the average
hydraulic conductivity were calculated based on the to-tal head loss between the first (upper most) and fourth
tap, the second and fifth tap, the third and sixth tap,
the fourth and seventh tap, and the fifth and eighth
(lower most) piezometer tap connected in each hydraulic
conductivity test. Using this method in the hydraulic
conductivity calculations allows for discrepancy due in
compaction along the depth of the shredded tire layer.
The average hydraulic conductivity based on the fivepiezometer pairs, as described above, for each hydraulic
conductivity test was compared to the hydraulic conduc-
tivity calculated based on the total head loss between the
upper most and lower most piezometer taps connected
during the test. The hydraulic conductivity based on
the piezometers attached near the top and bottom of
the tire chip sample should provide an ‘‘average hydrau-
lic conductivity’’ across the entire tire chip sample. Ineach case, the average hydraulic conductivity calculated
based on the five piezometer pairs was similar to the
average hydraulic conductivity measured between the
upper most and lower most connected piezometer taps.
Figs. 8 and 9 illustrate the differences in hydraulic
conductivity calculated at different positions within the
tire chips for the tests conducted on the higher qualitytire chips and the lower quality chips, respectively. The
locations included in Figs. 8 and 9 represent the position
in the sample at the midpoint between each of the five
piezometer pairs. As expected, the hydraulic conductiv-
ity of the tire chip samples decreased with increasing
height above the bottom of the sample. This decrease
was more predominant at lower strain increments under
lower applied stress. The decrease in hydraulic conduc-tivity also appeared to be slightly more predominant
in the HQ samples. This may be an indication that the
upper portions of the HQ tire chips compressed more
easily, compared to the upper portions of the LQ tire
chips, again supporting the hypothesis that the LQ chips
may have a higher angle of internal friction, thus provid-
ing more interconnection between individual tire chips.
Some hydraulic conductivity test results indicatedslightly higher hydraulic conductivity values in the
upper most portion of the tire chip samples, compared
to portions directly below (refer to Figs. 8 and 9). This
could reflect an interaction between the tire chips and
the upper porous end-cap.
The above results are in agreement with Edil and
Bosscher (1994) who concluded that tire chips have high
hydraulic conductivity (more than 1 cm/s) when uncon-fined. Additionally, Edil and Bosscher (1994), Gonzales
0
5
10
15
20
25
30
35
40
45
50
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0 28.0 30.0 32.0
Hydraulic Conductivity (cm/s)
Hei
gh
tab
ove
bo
tto
m o
f sa
mp
le (
cm)
Strain = 0.35 Strain = 0.3 6 Strain =0.4 Strain =0.45 Strain = 0.5
Fig. 8. Hydraulic conductivity of high quality (HQ) shredded tires versus depth of the compressibility cell.
0
5
10
15
20
25
30
35
40
45
50
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0 28.0 30.0 32.0
Hydraulic Conductivity (cm/s)
Hei
gh
t ab
ove
bo
tto
m o
f sa
mp
le (
cm)
Strain = 0.3 Strain = 0.35 Strain =0.45 Strain = 0.5 Strain =0.4
Fig. 9. Hydraulic conductivity of low quality (LQ) shredded tires versus depth of the compressibility cell.
976 M.A. Warith et al. / Waste Management 24 (2004) 967–979
and Williams (1995) and Bosscher et al. (1997) indicatedthat overburden pressure reduces hydraulic conductiv-
ity; however, a relatively high hydraulic conductivity
on the order of 0.1 cm/s or more can be expected under
typical drainage conditions in structure fill.
The hydraulic conductivity test results obtained on
tests conducted on the HQ chips were very similar to
the test results obtained on tests conducted at the corre-
sponding strain increments on the LQ chips. The HQ
tire chips exhibited slightly higher average hydraulicconductivity values than the LQ tire chips at the lower
strain levels of 0.3 and 0.35. It is considered that this
may be a result of the more uniform individual tire chip
dimensions of the HQ chips, which could result in a
slightly higher void ratio.
The results of the hydraulic conductivity testing indi-
cated average hydraulic conductivity values ranging be-
tween 13.4 and 0.67 cm/s under average applied normal
M.A. Warith et al. / Waste Management 24 (2004) 967–979 977
stresses ranging from approximately 60 to 335 kPa and
strain increments between 0.3 and 0.5. These results are
similar to the results reported by Donovan et al. (1996)
and Evans (1997) and higher than the hydraulic conduc-
tivity values reported by Duffy (1995). The constant
head hydraulic conductivity testing carried out in thestudy presented by Duffy (1995) was conducted on tire
chip samples ranging in thickness from 23 to 33 cm.
The thickness of the tire chip samples in this study
was significantly higher than this (i.e., 42–84 cm). It is
considered that different sized tire chips could result in
varied hydraulic conductivity results. The nominal size
of tire chips used in this study was 75 mm.
The initial bulk density of the tire chips was observedduring this study to result in a noticeable difference in
hydraulic conductivity under load, as evidenced by the
results of the hydraulic conductivity of initial testing
with lower initial bulk density tire shreds, compared to
the other hydraulic conductivity tests shown in this
study. The initial bulk density of the tire chips employed
in the field is directly related to the construction vehicle
traffic permitted over the tire chips once placed. Con-struction traffic could compact the tire chip layer or
align the individual tire chips in a less random fashion.
If tire shreds are used as an alternative to drainage stone
in the leachate collection system of a landfill site, it is
therefore considered that limiting construction vehicle
traffic over the tire shreds would help to ensure adequate
0.280.3
0.320.34
0.360.380.4
0.420.440.460.48
0 100 200 300 400 500 600 700
Stress (kPa)
Str
ain
(cm
/cm
)
Control pH = 4, 2 weeks exposure
pH = 4, 4 weeks exposure pH = 4, 6 weeks exposure
0 100 200 300 400
Stress
0.280.3
0.320.34
0.360.380.4
0.420.440.460.48
Str
ain
(cm
/cm
)
Control
pH = 9, 4 weeks exposur
Fig. 10. Stress/strain relationship for vario
hydraulic conductivity of the tire shred layer. This can
be carried out by applying a layer of selected waste over
the drainage layer as soon as practical.
In any case, the results of this study have indicated
that, under the applied load imposed by the anticipated
range in waste height at an engineered landfill facility,the vertical hydraulic conductivity of tire chips is on
the order of one to three orders of magnitude higher
than the hydraulic conductivity typically specified of
0.01 cm/s. These test results were obtained on samples
with a thickness similar to the thickness of tire shreds
that would be used as the drainage layer of a municipal
solid waste landfill site leachate collection system. It is
noted that individual tire chips are relatively flat and,once placed, may have a tendency to lie flat in a more
horizontal position than vertical. The direction of leac-
hate flow in a leachate collection system is primarily hor-
izontal, as leachate follows the contours of the shaped
subgrade to the leachate collection piping. It is consid-
ered that the vertical hydraulic conductivity measured
in this study is a conservative estimate of the hydraulic
conductivity in the horizontal direction.It is worthwhile to note that the overall hydraulic
conductivity of the shredded tire drainage layer may
be reduced in a landfill environment due to the biologi-
cal clogging and migration of fines (Reinhart and Town-
send, 1998). These aspects were not examined in this
study.
0 100 200 300 400 500 600 700 800
Stress (kPa)
0.280.3
0.320.34
0.360.380.4
0.420.440.460.48
Str
ain
(cm
/cm
)
Control pH = 7, 2 weeks exposure
pH = 7, 4 weeks exposure pH = 7, 6 weeks exposure
500 600 700 800 900
(kPa)
pH = 9, 2 weeks exposure
e pH = 9, 6 weeks exposure
us pH values for HQ shredded tires.
0
2
4
6
8
10
12
14
16
0.25 0.3 0.35 0.4 0.45 0.5
Strain (cm/cm)
K (c
m/s
)
control pH = 9, 2 weeks exposure
pH = 9,4 weeks exposure pH = 9, 6 weeks exposure
0
2
4
6
8
10
12
14
16
18
0. 25 0 .3 0 .35 0 .4 0. 45 0 .5
Strain (cm/cm)
K (c
m/s
)
control pH = 7,2 weeks exposurepH = 7, 4 weeks exposure Ph = 7, 6 weeks exposure
0
2
4
6
8
10
12
14
16
18
0. 25 0.3 0.35 0.4 0.45 0. 5
Strain (cm/cm)
K (c
m/s
)
control pH = 4,2 weeks exposure
pH = 4, 4 weeks exposure pH = 4,6 weeks exposure
Fig. 11. Shredded tires (HQ) hydraulic conductivity and strain relationship for various pH values.
978 M.A. Warith et al. / Waste Management 24 (2004) 967–979
4.3. Effect of pH on HQ tire chip compressibility and
hydraulic conductivity
Compressibility and hydraulic conductivity testing
were carried out on HQ shredded tire samples havingsimilar grain-size distributions (nominal size of 75
mm). The stress and strain relationships are provided
in Figs. 5 and 6. Under applied vertical loads resulting
in average vertical stresses of up to 500 kPa, the maxi-
mum normal strain recorded was observed to reach a
strain level near or slightly greater than 0.5.
The applied load range used in the compressibility
testing would be equivalent to a range from 30 to 50m of solid waste in a landfill environment. The effect
of various solutions pH in the range from 4 to 9 on
the shredded tire compressibility was noted to be min-
imal. After two weeks, the shredded tire samples were
slightly softer than at initial conditions. As noted in
Fig. 10 the duration time has a slight effect, on the or-
der of less than 5%, on the stress and strain
relationships.Hydraulic conductivity values (k), calculated by the
entire HQ shredded tire samples, decrease as the stress
increases as shown in Fig. 11. For example: when sub-
jected to a load of 650 kPa (for pH 4, 6 week exposure),
the tire chips were compressed approximately 47% and
the hydraulic conductivity was measured to be 0.64
cm/s.
Under different pH exposures ranging from pH 4 to
9, average hydraulic conductivity values range between
0.47 and 16.90 cm/s under average applied normal stres-
ses ranging from approximately 100 to 680 kPa withstrain increments between 0.28 and 0.47.
In summary, tests at various pH conditions and for
various exposure periods indicated that pH and expo-
sure duration did not significantly influence the com-
pressibility and hydraulic conductivity characteristics
of these shredded tire samples. It should be emphasized
that these results apply to specific experimental condi-
tions and their applicability to full-scale landfills meritsfurther examination.
5. Conclusions
Under applied vertical loads resulting in average ver-
tical stresses of up to 440 kPa, the maximum normal
strain recorded for each type of tire chip was observedto plateau at a strain level near or slightly greater than
0.5. A linear relationship between the strain and log
stress was observed. At lower strain levels, slightly less
applied stress was required to compress the ‘‘higher
quality’’ (HQ) tire chips than was required for the ‘‘lower
quality’’ (LQ) chips. This could be due to a slightly high-
er angle of internal friction in the LQ chips than the HQ
chips due to longer protrusions of steel belt fragments
M.A. Warith et al. / Waste Management 24 (2004) 967–979 979
from the LQ tire chips, possibly providing more inter-
connection between individual tire chips. The applied
load range used in the compressibility testing would be
equivalent to over 50 m of solid waste.
The results of the hydraulic conductivity testing
indicated average hydraulic conductivity values rang-ing between 13.4 and 0.67 cm/s under average applied
normal stresses ranging from approximately 60 to 335
kPa and strain increments between 0.3 and 0.5. These
results are one to three orders of magnitude higher
than the hydraulic conductivity typically specified
for drainage layers in leachate collection systems of
0.01 cm/s.
Based on the results of this study, it is concluded thatthe use of uniformly graded 75-mm shredded tire chips
is worthy of further consideration as an alternative to
crushed stone in the leachate collection drainage layer
of a municipal solid waste landfill site. The use of shred-
ded tires in the construction of engineered landfill facil-
ities may offer economic advantages, in comparison to
the cost of crushed stone, and provides the opportunity
for a second use of tires, sparing the use of a non-renewable resource.
Some practical considerations for the use of tire
shreds in leachate collection systems include health
and safety aspects of handling tire shreds containing
many fine, sharp, steel pieces and the inability to
achieve high waste compaction rates in initial waste
lifts overlying shredded tires. The high compressibility
observed for tire shreds may also require the ‘‘over-building’’ of leachate collection drainage layers with
a greater layer thickness, such that the ‘‘effective’’
thickness after compression of the tire shred layer will
meet the design criterion. For example, assuming an
initial shredded tire layer thickness of 600 mm, using
the stress vs. strain relationship presented in this pa-
per, and assuming an applied MSW load of 100
kPa, it is estimated that the shredded tires would com-press by approximately 28%, reducing the layer thick-
ness to 432 mm (effective thickness), which exceeds the
typical design criteria of 300 mm for leachate drainage
layers.
This experimental investigation also highlights the
need for further investigation of the use of tire shreds
in leachate collection systems. Further research is
needed into the potential for bio-clogging and long-termexposure of tire shreds to leachate to reduce hydraulic
conductivity in field applications.
Acknowledgements
The authors thank K. Sze and R. Moore. R. Moore is
thanked in particular for his assistance in apparatus de-
sign and set-up and his technical support in the labora-
tory. The authors also thank Lafleche EnvironmentalInc. for supplying the tire shreds used in the laboratory
experiments.
References
Bosscher, P.J., Edil, T.B., Kuraoka, S., 1997. Design of highway
embankments using tire chips. J. Geotech. Geoenviron. Eng. 123
(4), 295–304.
Code of Federal Regulations, 40 CFR, 1992. Protection of Environ-
ment. The Office of the Federal Register National Archives and
Records Administration, US Printing Office, Washington, DC, 40
CFR 258, pp. 355–392 and 40 CFR 264, pp. 154–330.
Donovan, R., Dempsey, J., Owen, S., 1996. Scrap tire utilization in
landfill applicationsProceedings from Wastecon 1996, Portland,
Oregon, September 23–26. Solid Waste Association of North
America, Publication #GR-G 0034.
Duffy, D.P., 1995. Using tire chips as a leachate drainage layer. Waste
Age.
Edil, T.B., Bosscher, P.J., 1994. Engineering properties of tire chips
and soil mixtures. Geotechnical Testing J., GTJODJ 17 (4), 453–
464.
Evans, P.A., 1997. Use of tire shred in landfill construction. In:
Proceedings from The Geotechnical Society of Edmonton Third
Annual Symposium, Environmentally Friendly Technologies in
Geotechnical Engineering, Edmonton, Alta., April 4.
Goehrig, J.P., 1996. Using shredded tires in landfills, The New York
PerspectiveProceedings from the Conference on Using Tire Shreds
in Landfill Design, Texas, March 12. Natural Resource Conserva-
tion Commission, Arlington, TX.
Gonzales, L., Williams, J., 1995. Use of shredded tires as lightweight
backfill material for retaining structures. Research Project by
University of Illinois, Chicago, pp. 433–451.
Humphrey, D.N., Manion, W.P., 1992. Properties of tire chips for
lightweight fill: grouting, soil improvement and geosynthetics. In:
Proceedings of the Conference Sponsored by the Geotechnical
Engineering Division of the ASCE, New Orleans, LA, February
25–28, vol. 2.
MOE, 1998. Landfill Standards – A Guideline on the Regulatory and
Approval Requirements for New or Expanding Landfilling Sites.
Ontario Ministry of the Environment, 127p.
Reinhart, D.R., Townsend, T.G., 1998. Landfill Bioreactor Design and
Operation. CRC Press LLC.
Reddy, K.R., Saichek, R.E., 1998. Characterization and performance
assessment of shredded scrap tires as drainage materials in landfills.
In: Proceedings of the Fourteenth International Conference on
Solid Waste Technology and Management, Philadelphia, PA, 1998.
Warith, M.A., Sharma, R., 1998. Review of methods to enhance
biological degradation in sanitary landfills. Water Quality Res. J.
Canada 33 (3), 417–437.