21 - Suitability of Shredded Tires for Use in Landfill Leachate Collection Systems

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).

mailto:[email protected]

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.


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.


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.


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


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.


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.


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.


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


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.


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


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.

21 - Suitability of Shredded Tires for Use in Landfill Leachate Collection Systems

Documents