Modeling the Behavior of Recycled Materials in Embankment Construction
Ayşe Edinçlilera* and Gökhan Baykalb
a
Assist. Prof., Department of Earthquake Engineering,
Kandilli Observatory and Earthquake Research Institute, Boğaziçi University,
Istanbul, Turkey
Prof. Dr., Department of Civil Engineering, Boğaziçi University, Istanbul, Turkey
b
Abstract
The use of recycled materials and wastes in embankment construction is becoming more popular due to lack of
natural mineral resources and increasing waste disposal costs. Suitable waste materials such as tires, fly-ash,
plastics, wood, glass, etc. may be used in embankment construction. The waste materials can be either used
alone or can be mixed with local soil available. The main objective of using wastes in embankment construction
is to facilitate large volume disposal of waste materials. The successful design of a highway embankment
requires information about material properties and foundation soil. Shear strength is a fundamental mechanical
property that governs fill stability design. The behavior of highway embankments under static loading may be
predicted and modeled by using shear strength and deformation parameters obtained from large-scale laboratory
tests. In this study two processed wastes such as tire buffings and fly ash pellet aggregates are evaluated in terms
of engineering performance for embankment construction. A large size direct shear device was used to determine
the shear strength properties and deformation behavior of the wastes considered. A typical embankment cross
section was modeled using finite elements technique and the experimental values obtained were input. The
vertical displacement at the middle bottom part of the embankment, the differential settlement at the top section
of the embankment, and the factor of safety against failure values were obtained. The analysis results
demonstrate that the waste materials perform as good as natural materials when used in embankment
construction. The differential settlements at the highway platform level were lower than those for natural soil
investigated.
Keywords: embankment, modelling, recycled materials, fly-ash, shear strength
1. Introduction
Industrial wastes or by products are produced in large quantities throughout the world and their disposal
problems increase each year. With need for disposing larger amounts of wastes, the cost for disposal increases.
Recycling and reuse of these waste materials, especially in highway construction, is increasing nationwide.
Proper use of these recycled materials may lead not only to quality roads at considerable savings, but also to
solutions to environmental problems. Construction of highways requires large volumes of construction material,
so highway agencies are frequent participants in efforts to recycle and reuse waste materials.
Waste tires and fly ash are two materials widely used in the world for highway embankment construction.
Millions of waste tires are being discarded each year and their disposal creates big problems like fires,
mosquitoes etc. New areas for utilization are being investigated to utilize tires in large volumes. Waste tires as a
whole are used to construct retaining walls. In other applications waste tires are processed to form tire shreds,
tire crumbs and tire chips. These products are being utilized as lightweight aggregates. Another form of waste
tire is tire buffings. In contrast to tire chips, crumbs or shreds, tire buffings are the by-product of tire retread
industry. The contact surface of worn tires are stripped of and resurfaced with new rubber. The fiber shaped tire
buffings are produced during stripping process. Their production do not require extra energy.
*
Correspondence author: Tel: +90 216 3080511/225; fax : + 90 216 3084639; e-mail: aedinc@boun.edu.tr
Fly ash is the waste of coal burning thermal power plants. The annual production of fly ash in Turkey reaches 17
million tons ranking tenth in the world, but only a small amount is utilized. Proper disposal of fly ash in landfills
is costly and this cost directly affects energy costs which is very important for economical development. Most of
the disposal areas are nearly full and there is an urgent need for utilization in large volumes.
Shear strength is a fundamental mechanical property that governs fill stability design. The behavior of highway
embankments under static loadings may be predicted and modelled by using shear strength and deformation
parameters obtained from large-scale laboratory tests. In the limit equilibrium analysis the ultimate load leading
to failure is determined from the strength parameters. In the elastic analysis the stress strain behavior measured
from the laboratory tests are used to predict the response of embankment under static loading.
In this study, two different potential embankment applications developed at Boğaziçi University will be
presented: the first application is with tire buffings added to sand, and the second application is related to
artificial lightweight aggregates manufactured using fly ash. A disc pelletizer was constructed and fly ash pellets
were produced using cold binding technology. A large-scale direct shear testing device was developed and used
to determine the shear strength parameters and deformation behavior of tire buffings, tire buffings-sand mixture
and fly ash pellets. By using the laboratory test results, the several embankment analysis by using the Finite
Element Program, Plaxis 7.2, were performed. A typical embankment cross section, over a typical clay
foundation is used for comparison. The differential settlement at the top part of the embankment, the total
vertical settlement at the center of the embankment and the factor of safeties are presented and compared.
2. Background
2.1 Static properties of the waste materials
Several researchers conducted large scale direct shear tests to determine the shear strength parameters of the
recycled materials to use them as lightweight fill materials. In a study conducted by Humphrey et al. (1993), tire
chips obtained from three different suppliers were used for the large-scale direct shear tests. They have reported
friction angles ranging between 19 and 25o and cohesion of 7.7 - 8.6 kPa. They stated that tire chips are useful
in constructing lightweight embankments over soft soils. In addition, tire chips can be used to replace natural
aggregate, to improve drainage, end to provide thermal insulation.
Edil and Bosscher (1994), and Foose et al. (1996) have reported that sand can be reinforced using tire chips.
These reported studies have shown that adding tire chips increases the shear strength of sand, with friction angles
as large as 65o being obtained for mixtures of dense sand containing 30% tire chips by volume.
Foose et al. (1996) investigated the feasibility of using shredded waste tires to reinforce sand. They have
investigated the effect of five factors affecting shear strength such as normal stress, sand matrix unit weight,
shred content, shred length, and shred orientation. They found that shred content and sand matrix unit weight
were the most significant characteristics affecting the shear strength of the mixture. They reported that sand
containing shredded tires had higher shear strength than that of sand alone.
Tatlisoz et al. (1998) conducted the large-scale direct shear tests with tire chips, sand, sandy silt, sand-tire chips
and sandy silt-tire chip mixtures. They reported that the shear strength of the sand-tire mixtures increases with
increasing tire chip contents of up to 30 % by volume. In contrast, the friction angle of the sandy silt-chip
mixtures is nearly independent of tire chip mixtures. However the shear strength of the sandy silt-tire chip
mixtures increases with tire chip content primarily due to an increase in apparent cohesion.
Humphrey et al. (1993), Foose et al.(1996) and Tatlisoz et al. (1998) have reported that sand can be reinforced
using tire chips and tire shreds. These studies have shown that adding tire chips or tire shreds increases the shear
strength of sand with friction angles as large as 54 o being obtained for mixtures of dense sand containing 30%
tire chips by weight. Comparatively, the corresponding friction angle of sand was only 34 o.
Large-scale direct shear tests were performed by Edinçliler et al. (2004). Sand, tire buffings, sand-tire buffings
mixture, and manufactured fly ash pellet aggregates were tested in the dry condition. The sand used in the tests
was uniformly graded, medium dense, with a dry unit weight of 15.3 kN/m 3. Tire buffings having maximum
lengths of 40 mm were used in the tests. The unit weight of the tire buffings was 5.1 kN/m3. The tire buffings
did not have any metal pieces in them. Ten percent by weight tire buffings was added to sand. The unit weight
of the tire buffings-sand mixture was 13 kN/m3. The internal friction angle of 10 percent tire buffings added sand
was 29 degrees. Pellet aggregates having maximum diameter of 10 mm were used in the tests. The unit weight
of the fly ash pellet aggregates was 10.5 kN/m3. The internal friction angle of fly ash pellet aggregates was found
as 46 degrees.
3. Methodology
3.1 Waste materials
Tire buffings used in this study are the by-product of the tire retread process and were obtained from tire retread
companies in Istanbul. In order to eliminate the uncontrolled effects of the particle-size distribution, tire
buffings, which varied in size, were graded and then mixed according to a desired gradation curve. The tire
buffings tested in the large-scale direct shear test device had diameters ranging between 1 to 4 mm and lengths
ranging from 2 to 40 mm. In contrast to tire shreds or tire chips, the small diameter and fiber shape of the
buffings make it an ideal material to form a composite with soil.
Use of tire buffings only for embankment construction will not be feasible due to comparatively lower shear
strength of the material. Due to its fiber shape and smaller size, it can be used to modify the properties of soil. A
large-scale direct shear testing device was used to determine the shear strength parameters and deformation
behavior of tire buffings, tire buffings-sand mixture having 5%, 10%, 20%, and 30% tire buffings by weight.
The purpose of using tire buffings was to modify the properties of soil as an additive, rather than using it as a fill
material for highway embankment construction. Due to lightweight of rubber the volume percentages correspond
to approximately two times the weight percentages given. For example 10 percent tire buffing addition by weight
corresponds to 20 percent by volume.
Sand, tire buffings, and sand-tire buffing mixture having 5%, 10%, 20%, and 30 by weight were tested in the
dry condition. The sand used in the tests was uniformly graded, medium dense, with a dry unit weight of 15.3
kN/m3. Tire buffings having maximum lengths of 4 cm were used in the tests. The unit weight of the tire chips
was 5.1 kN/m3. The tire buffings did not have any metal pieces in them. The unit weight of the tire buffings –
was 15.19 kN/m3, 14.89 kN/m3, 14.22 kN/m3, and 13.56 kN/m3, respectively.
Fly ash from Soma thermal power plant in Turkey was used for the production of pellet aggregates. Annual fly
ash production of Soma Thermal Power plant is 4 million tons. Soma fly ash is a self cementitous fly ash with
high calcium oxide content (33.5 percent). Ten percent by weight Portland cement was added to the fly ash. The
increased durability of the pellets by adding cement fulfilled the durability criteria as determined by soundness to
sodium sulfate test for concrete production. This type of harsh durability criteria is not needed for embankment
construction. Using only fly ash will definitely be more economical for embankment construction. A disc
pelletizer developed at Boğaziçi University was used to pelletize the mixture (Baykal and Doven, 2000). Water
was used as the binder. Pellet aggregates having maximum diameter of 10 mm were used in the tests. The unit
weight of the fly ash pellet aggregates was 10.5 kN/m3.
3.2 Experimental procedure
The shear strength properties of the waste materials were determined by using a 300 mm by 300 mm large scale
direct shear test device. The equipment was specially developed for determining shear strength parameters of
waste materials. Up to 50 mm diameter wastes can be tested in this device. The application of vertical pressure
was provided by means of an air compressor, pressure regulator and an air bag. A range of vertical pressures
from 20 kPa to 80 kPa were applied. The load readings were taken with a moment compensated load cell and the
axial deflections were measured by means of displacement transducers. The rate of shearing was 1 mm per
minute.
3.3 Numerical modelling
Plaxis 7.2 were used for the analysis. Plaxis 7.2, finite element software package specially intended for analysis
of deformation and stability in geotechnical engineering projects. The program uses advanced constitutive
models for the simulation of non-linear and time dependent behavior of soils. Mohr-Coulomb (MC) constitutive
model is used in the model. Staged construction option was used for loading input. The embankment height was
taken as 5 meters and the side slopes were width as 1:3. The embankment width at the top was taken as 20 m.
Considering the symmetry about the vertical axis, half of the embankment is analyzed. The appropriate fixity
codes are specified for all the element sides along the boundaries. The lateral and vertical displacements are
restricted along the bottom boundary and on the sides the horizontal displacements are restricted. The FE mesh is
presented in Figure 1. The foundation of the embankment is assumed to be clay. The material properties for the
FE element modeling are given in Table 1.
Based on the previous study (Edincliler et al., 2004) and this study, the results of the large-scale direct shear test
results for the tire buffings added to the medium dense sand at various compositions and fly ash pellet aggregates
are used as input for the Finite Element Modeling. The material parameters used in the models are directly taken
from the laboratory tests conducted. The material properties used are presented in Table 1. A typical clay
material has been taken as the foundation layer.
.
Figure 1. Finite Element Modeling of Embankment Construction on Soft Soils
Table 1. Material properties of road embankment and subsoil
Parameter
Name
Unit
Clay
Sand
Tire
Buffings
MC
%10Tire
Buffings +
Sand
MC
Fly-ash
pellet
aggregates
MC
Material
model
Type
of
behaviour
Dry
soil
weight
Wet
soil
weight
Horizontal
permeability
Vertical
permeability
Young’s
modulus
Poisson’s
ratio
Cohesion
Name
-
MC
MC
Type
-
Undrained
Undrained
Undrained
Undrained
Undrained
dry
kN/m3
15
15
5
13
10.5
wet
kN/m3
18
17
7
15
12.0
kx
m/day
1.10E-4
1.000
1.000
1.000
2.000
ky
m/day
1.10E-4
1.000
1.000
1.000
2.000
Eref
kN/m2
2000
3000
500
2500
2000
-
0.33
0.30
0.30
0.30
0.30
kN/m2
50
7
3.00
8.70
1.00
Friction
Angle
Dilatancy
Angle

o
1.00
33
22.00
29.00
46.00

o
0.00
2
2.00
2.00
2.00

cref
4. Results
4.1 Experimental results
Shear stress vs displacement curves obtained from large scale direct shear tests are presented in Figures 2 and 3
for tire buffings mixtures and fly ash pellet aggregates respectively. Maximum shear stress values were used to
calculate the shear strength parameters. Five percent tire buffings addition to sand showed similar behavior as
that of sand only up to a displacement of 12 mm (Figure 2). At displacements smaller than 7 mm, the 10 percent
tire buffings added mixture showed a stiffer behavior that of sand only. This initial stiffening is important
especially for highway load applications where displacements are small. Tire buffings only had low shear
strength. The test results of ten percent tire buffings addition to sand were selected for numerical analysis.
Summary of the shear strength data is given in Table 2.
Sand, Tire Buffings, Sand-Tire Buffings Mixture Direct Shear Test at 80 kPa
80
Sand
Sand-Tire Buffings
Sand-Tire Buffings
Sand-Tire Buffings
Sand-Tire Buffings
Tire Buffings
70
Shear Stress (kPa)
60
(5%)
(10%)
(20%)
(30%)
50
40
30
20
10
0
0
5
10
15
20
25
30
35
40
45
Horizontal Displacem ent (m m )
Figure 2. Shear stress vs. horizontal displacement curves for sand, tire buffings and sand-tire buffing mixtures at vertical
stress 80 kPa.
The shear stress versus displacement values for fly ash pellet aggregates for 20, 40 and 80 kPa vertical stress are
presented in Figure 3 (Edincliler et al, 2004). Although the fly ash pellet aggregates were compacted at a loose
state, the internal friction angle was 46 degrees which makes it an ideal material for embankment construction.
With a 10.5 kN/m3 unit weight, a free draining nature due to high porosity and a high internal friction angle, fly
ash pellet aggregates have many properties that are superior to many naturally available fill materials.
4.2 Numerical results
One embankment model with four different embankment materials was analyzed. Sand fill was studied as the
reference material and the other three waste materials’ behaviors were compared. The stability of embankments
is generally studied by limit equilibrium method which only takes into account the failure condition. With the
application of finite element method the stresses and displacements leading to failure can be observed throughout
the model geometry. The vertical displacements for sand, tire buffings, ten percent tire buffings added sand and
fly ash pellet aggregates are presented in Figure 4 and Figure 5. The factor of safety against failure for all
studied cases were high and adequate for design. However when the vertical displacements obtained from the
FEM study for different embankment materials are considered some interesting findings can be revealed.
The vertical displacements at the center bottom of the embankment are mainly affected from the unit weight of
the embankment material. The lowest values are obtained for tire buffings only which has the lowest unit weight.
The values obtained are for undrained conditions and do not include consolidation settlement in the foundation
clay layer. When the differential settlement at the highway platform level is considered some interesting findings
are observed. The differential settlement at this level is important because the stiffer wearing course will be
affected directly. When sand fill is considered the differential settlement between the center axis and the edges of
the platform is 80 mm for 10 meters. For tire buffings and sand-tire buffings mixture the differential settlement is
reduced to 40 mm for 10 meters. For fly ash pellet aggregates which has the highest shear strength among the
studied materials the differential settlement further decreased to 20 mm for 10 meters.
70
Shear Stress (kPa)
60
50
40
30
20
80 kPa
10
40 kPa
20 kPa
0
0
5
10
15
20
25
30
Horizontal Displacement (mm)
Figure 3. Shear stress vs. horizontal displacement curves for fly ash pellet aggregates at different vertical stresses.
Table 2. Summary of shear strength data
Material
Tire buffings
Unit Weight
(kN/m3)
5.1
Shear Strength Parameters
c = 3.1 kPa ;  = 22o
Sand
15.3
c = 6.91kPa ;  = 33o
5% Tire Buffings +
95% Sand
10%Tire buffings +
90%Sand
20% Tire Buffings + 80%
Sand
15.19
c = 10.4 kPa ;  = 28o
13.0
c = 8.7 kPa ;  = 29o
14.22
c = 15.45 kPa ;  = 5 o
30% Tire Buffings + 70%
Sand
Fly ash pellet aggregates
13.56
c = 10.7 kPa ;  = 8o
10.5
c = 0 kPa ;  = 46o
Figure 4. Vertical settlements for the embankment models (sand, tire buffings, %10 tire buffings addition to sand)
Figure 5. Vertical settlements for the embankment model (Fly-ash pellet aggregates)
5. Conclusions
The conlusions presented below are valid for the materials tested under given conditions:
1. The test results obtained for manufactured fly ash pellet aggregates demonstrate the fact that higher
engineering performance may be obtained from recycled or waste materials when processed.
2. Understanding the material properties and engineering behavior of waste materials will increase
confidence in their use which will lead to increased volume utilization.
3. Special laboratory techniques for determining waste properties and appropriate engineering behavior
modelling are needed.
4. Fly ash pellet aggregates as embankment material showed excellent performance with high internal
friction angle and low unit weight. The differential settlements at the highway platform level were the
lowest among the studied cases.
5. Ten percent by weight tire buffings addition to sand decreased the differential settlement at the platform
level by half when compared to the case for sand only.
6. The numerical modelling results demonstrate the high performance of processed waste materials for
highway embankment construction. By this way large volume utilization of wastes will be easily
achieved. The replacement of natural mineral aggregates by these processed wastes will help in
preservation of the ecology. Large volume utilization of wastes will decrease huge disposal costs and
decrease potential environmental and aesthetics pollution.
7. The experimental results and numerical findings must be confirmed with field tests.
References
Baykal G, and Doven GA, 2000, Utilization of Fly-ash by Pelletization Process: Theory, Application Areas and Research
Results. Resources, Conservation, and Recycling, 30: 59-77, Elsevier, Amsterdam.
Edincliler, A., Baykal, G. and Dengili, K., 2004, Determination of Static and Dynamic Behavior of Recycled Materials for
Highways, Resources, Conservation, and Recycling 42; 223-237, Elsevier, Amsterdam.
Edil T, and Bosscher P, 1994. Engineering Properties of Tire Chips and Soil Mixtures. Geotechnical Testing Journal, 14(4):
453-464.
Foose GJ, Benson CH, and Bosscher PJ, 1996. Sand reinforced with shredded waste tires. Journal of Geotechnical
Engineering, 122 (9): 760-767.
Humphrey D, Sandford T, Cribbs M, and Manison W, 1993. Shear strength and compressibility of tire chips for use as
retaining wall backfill. Transportation Research Record, No.1422, Transportation Research Board, Washington, D.C., pp.
29-35.
Tatlisoz N, Edil TB, and Benson C, 1998. Interaction between reinforcing geosynthetics and soil-tire chip mixtures. Journal
of Geotechnical and Geoenvironmental Engineering, 124(11): 1109-1119.