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Abstract
This paper presents results of triaxial testing performed on a ballast material
regarding its permanent deformation and degradation behaviour during cyclic
loading. The tests simulated a large number of passing train wheels. Materials used
in ballast layers are usually comprised of a highly coarse-graded gradation, hence
the implementation of large-scale laboratory tests is difficult to conduct due to the
corresponding large-scale triaxial specimens for railroad ballast material. The main
purpose of this paper was to evaluate the applicability of the parallel gradation
technique in triaxial tests, using small-scale cylindrical equipment with 150mm
(width) x 300mm (height), in which it is easier to manipulate small fractions, as well
as to assess the influence of two different gradations on ballast breakage and
permanent vertical deformation. It was found that granular materials reveal a strong
tendency to settle under higher stress levels, causing a significant increase of their
strength and stiffness. The AREMA No. 24 gradation was found to be the most
resistant to ballast settlement. Results of this study confirm that the confining
pressure should be considered as an important track design parameter. The results
contribute to a better understand of the mechanical behaviour of ballast layers, thus
support ongoing researches on railroad structure.
Keywords: parallel gradation, railway ballast, triaxial tests.
1 Introduction
Rail tracks deform both vertically and laterally under cyclic loads as a result of
varying traffic loads and speeds, causing deviation from its designed geometry. The
geometry of railway tracks requires specific level and alignment in order to have
acceptable ride quality and to meet safety standards. For ballasted railway tracks, the
level and alignment of the track structure strongly rely on the mechanical
characteristics of the granular substructure, especially the ballast layer. Past
Paper 0123456789
Railway Ballast Load Analysis
using Small-Scale Cylindrical Triaxial Test
A. Merheb1, R. Motta1, L. Bernucci1, E. Moura1, R. Costa1, I. Bessa1
T. Vieira1 and F. Sgavioli2
1Polytechnic School, University of São Paulo, São Paulo, Brazil
2Vale S.A., Vitória, Brazil
Civil-Comp Press, 2014
Proceedings of the Second International Conference on
Railway Technology: Research, Development and
Maintenance, J. Pombo, (Editor),
Civil-Comp Press, Stirlingshire, Scotland.
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researches have been giving too much attention to the train and track superstructure
elements, such as rails and sleepers, while the elements composing the substructure,
including ballast, subballast and subgrade were less focused, even though their
importance is well-known. The substructure relevance has been proved to be
remarkable as transported loads increase, traffic speed increases, and time available
for maintenance decreases in order to comply with the increasing modal demand.
The ballast layer has a significant role in dissipating and effectively distributing
the load from the track surface to the underlying bearing subsurface. According to
Selig and Waters [1], ballast degradation is one of the major substructure problems,
which leads to increased track settlement, increased ballast fouling, and reduced
drainage. In this context, the comprehension of the stress-strain characteristics of
those non-cohesive materials under repeated loading is very important for
optimizing maintenance operations, thus ensuring safe and efficient transportation.
Ballast gradation is a key factor for stability, safety and drainage aspects of the rail
tracks. A more uniform gradation implies in larger air voids, thus better drainage.
On the other hand, well-graded ballast gives lower settlement than uniform
gradation, due to higher interlocking among aggregates, and also provides higher
shear strength and better track stability, yet at the expense of ballast drainage
capability. It is also noteworthy that improper ballast stiffness reduces lifetime of
other railway components, including sleepers and rails.
Materials used in ballast layer are usually comprised of highly coarse-graded
gradation, hence the implementation of large-scale laboratory tests are difficult to be
performed due to the corresponding large-scale triaxial specimens required for
railroad ballast material, as well as the small number of facilities that are capable of
testing it. Another procedure that can be done to evaluate these materials in
laboratory is to use the parallel gradation technique. In this case, particle shape,
surface roughness and mineralogy are preserved, so a parallel gradation composed
of smaller aggregates with a maximum particle size can be used in more frequently
available apparatus. Varadarajan et al. [2] reported that there are four techniques that
can be used to reduce the size of the large-sized crushed rock materials, and the
parallel gradation technique was found to be the most suitable. However, according
to Cambio and Ge [3], all the previous work on parallel gradation technique was
done under monotonic loading condition, as there are issues, such as friction and
particle angularity, which have not been addressed under the circumstances of cyclic
loading.
The purpose of this paper is to evaluate the applicability of the parallel gradation
technique in triaxial tests using small-scale cylindrical equipment with 150mm
width and 300mm height, in which handling small fractions is easier, as well as to
assess the influence of two different gradations on ballast breakage and permanent
vertical deformation. The cyclic triaxial tests simulated the behaviour of two
different gradations specified by the American Railroad Engineering and
Maintenance of Way Association (AREMA) – No. 24 and No. 3, which are
currently used on railway mainline tracks around the world under a large number of
passing train wheels.
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2 Experimental Setup
2.1 Triaxial testing apparatus
The study of the mechanical behaviour of railway ballast layers has been done by
several authors, both in laboratory and in the field. In relation to laboratory
researches, the elastic-plastic behaviour of granular materials is usually investigated
through triaxial tests. Shenton [4], Raymond and Williams [5], Alva-Hurtado and
Selig [6], Jeffs and Marich [7], Indraratna [8], Indraratna et al. [9], Indraratna and
Salim [10], Lackenby et al. [11], Anderson and Fair [12] and Nalsund [13] studied
the permanent deformation and the degradation of ballast materials. According to
Indraratna et al. [14], the triaxial test equipment is one of the most versatile and
useful apparatus used to determinate resistance and deformation properties of
geotechnical materials.
In this type of equipment, the vertical load is applied by a load cell, and the
confining pressure is applied to the cylindrical specimens by a vacuum system.
Since the confining pressure is applied on the radial direction, the intermediate
principal stress (σ2) is equal to the minor principal stress (σ3). The major principal
stress (σ1) results from the sum of the confining pressure (σc) and the stress applied
by the loading piston, or deviatoric stress (σd), while the minor principal stress (σ3)
is equal to the confining pressure. The friction at the bearing that supports the
loading shaft is regarded as minimal, and thus neglected in the computation of the
principal stress (σ1). The axisymmetric stress state in a conventional triaxial test is
defined in Figure 1.
Figure 1: The axisymmetric stress conditions applied to the cylindrical specimen.
The apparatus used in the present study was a hydraulic test machine namely
Material Testing Systems 810 (MTS) and its load frame is capable of applying both
monotonic and cyclic loadings to a sample. The maximum applicable force is
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100kN, which is equivalent to a deviatoric stress of 5,659kPa on the designed
sample (150mm diameter). The axial load is applied to the sample by the movement
of the bottom against the stationary load cell at the top of the sample. The
confinement method used was a vacuum system that is sufficient for materials
placed relatively close to the surface, and therefore under relatively low confinement
[15]. The maximum confinement available using vacuum is 95kPa (close to 1atm).
Details of the equipment used in this study are shown in Figure 2.
Figure 2: Small-scale cylindrical triaxial equipment
2.2 Material description
In order to study railway ballast layers, the main difficulty found during laboratory
tests is related to the dimension of the materials, as the maximum size of aggregate
particles can reach up to 63.5mm, thus not allowing the use of conventional triaxial
equipment commonly applied for soil materials. According to Marachi et al., [16],
the largest grain size that can be accurately evaluated in the triaxial apparatus must
be one-sixth the diameter of the testing specimen. Bishop and Green [17] suggested
that the specimen height should be twice the diameter to alleviate end plate
confinement of the specimen during the test. By this mean, the necessary apparatus
would be at least 300mm diameter and 600mm height, so in order to try to solve this
problem, the parallel gradation technique was proposed to test the material using a
small-scaled specimen.
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The granitic ballast aggregate was shipped from the ballast storage of Vale
railway company, located in the city of Cariacica - Brazil, which provides No. 3 and
No. 24 AREMA gradation for ballast aggregates applied in Vitoria-Minas Railway.
Smaller sizes of ballast materials were also available from the site and were used
to manufacture two sets of materials (prototypes of gradations No. 3 and No. 24) in
parallel gradation curves, as shown in Figure 3. A large number of sieves was used
in order to get a smooth and parallel gradation curve as the ballast gradation model,
since this issue is critical in the material preparation.
Figure 3: Grain-size distribution of ballast materials.
2.3 Test procedure and programme
Ballast material was washed, sieved, and blended to compose the two predetermined
gradation curves. Samples were prepared in a cylindrical steel mold on a vibrating
table, where ballast was introduced in four layers of equal height. Each layer was
vibrated for 40 seconds with a 10kg-surcharge on the top of each layer. The sample
mass ranges from 8.4 to 8.5kg, corresponding to a bulk density of 1,590 to
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1,605kg/m3 (based on densities of materials placed in the field), and void ratio
ranging from 0.74 to 0.76.
The sample was covered by two layers of latex membranes. The thicknesses of
the inner and outer membranes were 2 and 1mm, respectively. An o-ring was used to
seal the membranes into the groove at the top and bottom, in order to maintain an
even pressure.
A total of six cyclic triaxial tests were performed. The specimens were
isotropically consolidated to a confining pressure of 65kPa and were then submitted
to a load frequency of 9Hz (a train traveling approximately at 70km/h). Details of
these tests are summarized in Table 1. A typical harmonic cyclic load applied during
the test program is in accordance with Indraratna et al. [18]. The minimum cyclic
stress (qmin) was kept at 45kPa, which represents the unloaded state of the track,
representing weight of sleepers and rails (superstructure) [11]. The amplitudes of
cyclic loading deviatoric stress for each frequency were calculated in accordance
with Esveld [19], for five different axis loads: 15; 22; 27.5; 32.5; and 40 tons.
Test
number Gradation Confining
stress [kPa] σ1/σ3 Number of
cycles (N)
Density
[kg/m³]
1 3 65 3; 4; 5; 6 290,000 1,592
2 24 65 3; 4; 5; 6 290,000 1,605
3 3 65; 50 3; 4; 5; 6; 7 160,000 1,593
4 24 65; 50 3; 4; 5; 6; 7 160,000 1,605
5 3 65 5 250,000
1,592
6 24 65 5 250,000
1,606
Table 1: Summary of the triaxial cyclic tests
Tests number 1, 2, 3 and 4 were conducted under increasing deviatoric stress.
Tests 3 and 4 were conducted under constant confining pressure, however a decrease
on this pressure was applied between cycles 140,000 and 150,000 in order to
evaluate the change on the mechanical behaviour of the material.
3 Experimental results
The six cyclic triaxial tests conducted to simulate a large number of passing train
wheels with two grain size distributions provided information about the ballast
behaviour in terms of resilient modulus, permanent strain (or deformation) and
ballast breakage. The results of permanent axial strain and resilient modulus versus
number of cycles are shown in Figures 4, 5 and 6.
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Figure 4: Results of resilient modulus and permanent strain versus number of loads
for tests 1 and 2.
Figure 5: Results of resilient modulus and permanent strain versus number of loads
for tests 3 and 4.
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Figure 6: Results of resilient modulus and permanent strain for tests 5 and 6 up to
250,000 cycles of load, under one loading condition.
These test results reveal that uniform samples (AREMA 3 gradation) presented
higher axial strain and lower resilient modulus. In contrast, it can be noted that
AREMA 24 gradation yielded the lowest amount of settlement after repeated
loading. According to Tutumluer et al. [20], moderately-graded distributions, such
as AREMA 24, provide denser packing, thus presenting higher shear strength and
lower settlement. When gradation becomes more uniform, the ballast produces more
and more settlement [21].
For tests 1 and 2, the behaviour of each gradation (AREMA 3 and AREMA 24)
was divided into four steps and subjected to a total of 290,000 load applications
(steps I and II with 20,000 cycles each; and steps III and IV with 125,000 cycles
each), as seen in Figure 4. Resilient modulus results of step I for AREMA 24
presented a rearrangement of the material in the initial cycles, and imprecise results
were obtained. The connection between permanent strain and number of load
repetitions is quite linear after a certain number of loads within all four steps, even
after the rearrangement period. According to Selig and Waters [1], permanent
deformation is characterized by a rapid increase during the first cycles, followed by
gradual stabilization. The amount of deformation depends on the characteristics of
the material and the applied load.
Tests 3 and 4 were divided into five steps and subjected to a total of 160,000 load
applications (steps I and II with 20,000 cycles each; and steps III, IV and V with
40,000 cycles each). Figure 5 shows that the permanent strain rate measured within
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each step increases from step I to step V for both AREMA 3 and AREMA 24
gradations. Nevertheless, results for AREMA 3 provided more permanent axial
strain compared to AREMA 24 gradation. This means that a change to a more-dense
grain size distribution curve can lead to great advantages with reduced permanent
strain. Note that in the last step (between load cycles 140,000 and 150,000), there
are significant changes regarding resilient modulus and permanent deformations.
This occurred due to a forced decrease on the confining pressure, from 65 to 50kPa,
imposed to the sample. These experimental results show that permanent deformation
is higher when the deviatoric stress increases and lower when the lateral stress (σ3)
increases. Hence, a higher magnitude of confinement is beneficial for minimizing
permanent and resilient deformation of ballasted tracks. If an appropriate confining
pressure is implemented in local in-situ conditions, rail tracks behaviour can be
improved in terms of degradation, deformation strength and resilient characteristics
[21].
As indicated in Figure 6, results for both elastic and plastic deformations had the
same tendency as presented in the previous tests. Variation on the stress state were
not applied for tests 5 and 6, in order to evaluate if mechanical behaviour of both
gradations had the same pattern found on the previous tests. Note that AREMA 3
always presents a higher rate of vertical deformation and a lower value of resilient
modulus. In addition, it was observed that results obtained for tests 5 and 6 were
similar to the ones obtained on step III for tests 1, 2, 3, and 4, which demonstrate
that permanent vertical deformation was not dependent on the order used for
applying stresses, but is dependent on the maximum applied stress.
Figure 7 shows the particle size distribution of fines after each cyclic test. The
results indicate that ballast breakage decreases as the value of Cu (uniformity
coefficient) increases, as observed by Indraratna et al. [22]. Moreover, in terms of
deformation and resistance to particle breakage, AREMA 24 is quite superior to
AREMA 3 gradation, due to the looser states of the specimens prior to cyclic
loading [22]. Such degradation tends to be higher as the number of loading cycles
increases. In addition, degradation leads to smaller particle sizes and smaller values
of angularity, hence overall shear strength and drainage capability of the ballast
layer decreases [21].
According to Selig and Waters [1], the gradation of ballast materials plays a
significant role in strength, deformation, degradation, stability, and drainage of
tracks. Well-graded ballast gives denser packing, better frictional interlock and
hence, lower settlement. The uniformly-graded ballast gives higher settlement and
also more vulnerability to breakage than well-graded ballast. However, all ballast
specifications demand uniform gradation for free draining. Indraratna et al. [21]
have considered a reasonable balance between the demands for higher strength and
free draining in terms of particle size distribution. The ballast gradation should
provide a stronger and more resilient track without causing any significant delay in
drainage from the substructure.
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Figure 7: Particle size distributions after triaxial cyclic tests
Conclusions
Elastic-plastic behaviour of a ballast material has been explored by means of triaxial
cyclic tests, providing an insight into the deformation characteristics of these
materials under a large number of passing train wheels.
It was found that granular materials reveal a strong tendency to compact under
higher stress levels, causing a significant increase in their strength and stiffness. The
lower the density of the ballast, the higher its resilient modulus is. The results
illustrate that increasing the magnitude of confinement is beneficial for minimising
resilient and permanent deformation of ballast.
Changing the grain size distribution to a denser grading would reduce the
permanent axial deformation. AREMA 24 gradation was found to be the less
resistant to ballast settlement, resulting in more breakage in comparison with
AREMA 3. The cyclic test results when varying the gradation indicate that even a
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small change in the uniformity coefficient (Cu) may affect the elastic-plastic
behaviour of the ballast.
The cyclical test data shown in this study demonstrate that obtaining quality
results is possible when using this apparatus, as it was found that parallel gradation
technique worked well. However, it is necessary to keep particles shape, surface
roughness and mineralogy, in order to create parallel gradation curves that can
precisely represent the materials used in ballast layers. In this context, it is important
to state that a comparative study between large and small-scale samples is essential
to better understand those issues.
It is also necessary to mention that these conclusions were based on a test
program with results of a single repetition, so test values can vary to some extent.
However, the main results obtained in this study are in accordance with relevant
literature on this topic.
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