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Challenge H: For an even safer and more secure railway
1
Development of the Countermeasure against Roadbed Degradation
under Ballastless Tracks for Existing Lines
Katsumi MURAMOTO, Takahisa NAKAMURA
Track Structures and Geotechnology Laboratory, Railway Technical Research Institute, Tokyo, Japan Recently, ballastless tracks with cement grouted into ballast have been constructed on a full-scale construction operation in Japan Metropolitan Area. If the ballastless tracks are laid on clayey roadbeds, the soils are likely to be fluidized and flow out by trainloads. Consequently, track degradations typified by a large track depression sometimes occur. In this study, the authors carried out the test with a full-scale ballastless track model laid on a saturated clayey roadbed. From the results of the test, it was confirmed that degradation of the ballastless track on a clayey roadbed is caused by local progressive failure; specifically, the outflow of the fine-grain fraction in roadbed surface. In addition, the authors confirmed that the BLITS (Bentonite Liner for Track-bed Surface) method is one of the reasonable countermeasures for the roadbed degradation under the ballastless tracks.
Keywords: roadbed, ballastless track, roadbed degradation, mud pumping, bentonite
1. Introduction When ballastless tracks (Fig. 1) are constructed on sound roadbeds, the maintenance work and the
maintenance costs become generally less than conventional ballast tracks. However, if the ballastless
tracks are laid on clayey roadbeds, the soils are likely to be fluidized and flow out by the trainload. Consequently, track degradations typified by a large track depression sometimes occur. The authors,
therefore, have carried out moving-load tests with small-scale ballastless track models on clayey
roadbeds and reappearance tests of the roadbed degradation using small-scale cylindrical molds, so that
the basic defect mechanism of clayey roadbeds has been clarified (Fig. 2).1)
Accordingly, three fundamental policies of countermeasures against clayey roadbed defects were
clarified as follows2):
1) Decrease a dynamic water pressure on the roadbed surface, which is excited by trainload
2) Decrease roadbed pore water level
3) Increase roadbed soil cohesion
The results of small-scale model tests, however, might not be applicable to actual conditions because
the phenomenon involving pore water movement differs significantly with the effect of scale. Furthermore,
the progressive failures involving roadbed soil outflow are hardly predictable using numerical simulation at
2
this time. In this study, we performed tests with full-scale track models to clarify the details of roadbed
degradation and countermeasure effectiveness.
Rail
PC-Sleeper
Ballast
Cement Grouted Layer
Ballast Penetration
Side DitchGround
Fig. 1 Ballastless Track for Existing Lines
Water Channel
Outflow of mud
Cave FormationInflow of rain water
Train Load
Fig. 2 Basic Defect Mechanism of Clayey Roadbeds
2. Simulation of the roadbed degradation process by full-scale model tests
2.1 Setup of full-scale clayey roadbed model Where a full-scale roadbed model is made with clayey soil, the roadbed is, generally, constructed by two
methods; compaction with the soil, which is controlled by optimal water content; consolidation with slurry,
which is controlled by higher water content than the liquid limit. Because roadbed saturation and loading
history were more important in these tests, the models were made by consolidation.
Fig. 3 shows an outline of the construction process of the full-scale roadbed model. The clay slurry was
pumped into an earth tank (Fig. 4), which was then depressurized after the roadbed surface had been
covered with a polyethylene film. The differential stress between the internal pressure and atmospheric
pressure acts on the roadbed surface and squeezes pore water from the slurry. As a result, a saturated
clayey roadbed with a controlled stress history is constructed, namely by the vacuum consolidation
method.
3
Clay Slurry
Supernatant Water
Filter (Nonwoven Cloth)
Polyethylene Film Air-Water Separation Layer(Plastic Pallet)
SandLayer
1) Cast in slurry and settle out 2) Set plastic pallet and polyethylene film
Drain Pipe
to Vacuum Pump
Clay Slurry
Atmosphere Pressure
Vacuuming
WaterTrap
Atmosphere PressureVacuuming
Ballastless Track Model
Clayey Roadbed
3) Vacuum and consolidate 4) Set track model
Fig. 3 Making Procedures of Roadbed Table 1 Properties of Arakida clay
Fig. 4 Slurry Casting
2.2 Model outline The properties of Arakida–clay, which was used in this experiment, were as shown in Table 1. This clay
is derived from volcanic cohesive soil and distributed around the Arakawa (a river in Japan’s Kanto
region). Although Arakida–clay is well known as a good compaction soil and used for playing fields, we
had already clarified that this clay easily causes degradation under trainloads3).
Fig. 5 and Fig. 6 show the outlines of the model. The slurry was formed into a clayey roadbed
approximately 700 mm thick by the vacuum consolidation method. Ballast penetration and concrete plates
that simulate irregularities at the bottom of the grouted layer were buried in the roadbed surface. In
addition, pore pressure meters were set near these buried items.
In substitution for the ballastless track for existing lines, an A-type Shinkansen concrete slab was used
Particle Density 2.712g/cm3
Liquid Limit 49.4%
Plastic Limit 27.7%
Plasticity Index 21.7
Rate of Sand Content 2.7%
Rate of Silt Content 50.6% Rate of Clay Content 46.7%
4
for the model in this experiment because the slab width is practically equal to that of a cement grouted
layer of the ballastless track. Besides, it had been confirmed by FE analysis that bending rigidity of the
slab is also almost equal to the ballastless track (Fig.7)4).
A-Type Concrete Slab for Shinkansen
BallastPenetrationLayer
Concrete Plate
Dep
th=7
00m
m
Pore WaterPressureMeter
3000
mm
(5 fa
sten
ings
)
Pore WaterPressureMeter
2340mm
BallastPenetration
ConcretePlate
Fig. 5 Cross Section of the Model Fig. 6 Roadbed Surface
0 500 1000 15001.5
1.0
0.5
0.0 Track Structure Model
Wheel Load = 50kN
Rail Displacement
Roadbed Displacement
Verti
cal D
ispl
acem
ent (
mm
)
Distance from Center of the Model (mm)
Ballastless Track for Exisiting Lines A-Type Slab Track for Shinkansen
Fig. 7 Result of FE Analysis Fig. 8 Situation of the Cyclic Loading
2.3 Test specifications Fig. 8 shows a situation of the cyclic loading. The loading points were the center of the rails with five rail
fasteners on the slab. The cyclic load, which was a 0 to 100kN sine wave, was acted on the loading
points with a 1-Hz frequency. This 1-Hz frequency was determined from the predominant frequency of the
dynamic water pressure that was measured under an actual ballastless track on a conventional
(non-Shinkansen) line (Fig.9). This frequency depends on car length; therefore, eighty thousand times
correspond to the monthly operation of a busy line in Japan.
The roadbed settlements under ballastless tracks on operation lines must be approximately finished because sufficient loading history was applicable under the operation with ballast tracks. Therefore, the
maximum consolidation pressure was set at 80kPa because the model roadbed has to be over
Loading Actuator
Slab
5
consolidation condition. In addition, static preloading, in which a drainage layer is put on the roadbed, was
imposed with a 300kN axle load for 24 hours before a cyclic loading test with a 100kN axle load. Fig. 10
shows an outline of the loading history.
0 2 4 6 8 10 12 14 16 18 20-4
-3
-2
-1
0
1
2
-1.5-1.0-0.50.00.51.0
Car Length
Bogie to Bogie
Roadbed Displacement
Water Pressure of the Roadbed Surface
Wat
er P
ress
ure
(kPa
)
Time (sec)
Dis
plac
emen
t (m
m)
M
ean
Roa
dbed
Pre
ssur
e (k
Pa) 80
46
18
Cyc lic Loading Test(Axle Load = 100kN)
Preloading(Axle Load = 300kN)
Consol idationPressure(80kPa)
4
Self Weight of The SlabApproximately 3 ton
Fig. 9 Water Pressure under Ballastless Track Fig. 10 Outline of Loading History
2.4 Observation of the degradation Fig. 11 shows the primitive phase of the degradation, when muddy water with a high water content
moves in and out through water channels that have developed around the ballast penetration layers or
the concrete plates. Then, as shown in Fig. 12, silty soil, which has lower water content than the primitive
phase muddy water, is pushed out from the water channels and accumulates on the roadbed at the terminal phase of degradation. Fig. 13 shows the roadbed surface when the concrete slab was removed
after the loading test. Emanating from the ballast penetration layers, ramal channels filled with soft mud
arose on the roadbed surface. The entire roadbed surface was thinly covered with soft mud.
The relationship between number of times under load and track settlement, in other words rail settlement,
is shown in Fig. 14. The track settlement increased by 4 mm to 5 mm at a stretch until primal thousands
times under loading. It was assumed that this primitive settlement occurred because the roadbed surface
became softer and weakened by the cyclic water pressure. The primitive settlement corresponded to the
primitive outflow of muddy water shown in Fig. 11. The settlement rate of the track was reduced
immediately after the primitive settlement, however that eventually accelerated by degrees. It was
assumed that this acceleration occurred due to spreading of the water channels over the roadbed surface
(Fig. 13) and flowing out of silty soil which has less viscosity with little clay content (Fig. 12).
Fig. 15 shows the relationship between number of times under load and amplitude of the track
displacement. The amplitude increases at a stretch until primal thousands times under loading and then
increases at a constant rate. It is conceivable that a primitive rising of the amplitude occurred in a
softening process of the roadbed surface, and that the latter constant-rate rising of the amplitude occurred
while a spreading of the roadbed degradation decreases a bearing rigidity of the slab.
6
Fig. 16 shows the relationship between number of times under load and water pressure of the roadbed
surface around the buried items. The water pressure increases at a stretch until primal thousands times
under loading and then decreases by degrees. Because the clay content does not flow out at the primary
phase, the roadbed surface maintains a low permeability and thus the large water pressure occurs. Then,
after the clay content flows out, the permeability of the roadbed surface increases slightly; besides, water
channels on the roadbed grow to water pressure meter. Consequently, the water pressure seems to
abate.
Fig. 11 Primitive Phase of the Degradation Fig. 12 Terminal Phase of the Degradation
0 10 20 30 40 50 60 70 80 9014
12
10
8
6
4
2
0
80,000 times is correspond to a monthly train operation
The Number of Loading Times (1,000 times)
Trac
k S
ettle
men
t (m
m)
Fig. 13 Roadbed Surface after Loading Fig. 14 Track Settlement
Ballast Penetration
Water Channels
Installation Location of The Slab Concrete Plate
7
0 10 20 30 40 50 60 70 80 902.0
2.1
2.2
2.3
2.4
2.5
The Number of Loading Times (1,000 times)
Am
plitu
de o
f Tra
ck D
ispl
acem
ent
0 10 20 30 40 50 60 70 80 90
-5
0
5
10
15
20 Max MinConcrete Plate Ballast Penetration
The Number of Loading Times (1,000 times)
Wat
er P
ress
ure
of R
oadb
ed S
urfa
ce
Fig. 15 Amplitude of Track Displacement Fig. 16 Water Pressure of Roadbed Surface
2.5 Roadbed degradation process According to the above results, roadbed degradation under ballastless tracks is assumed to be due to
the following process;
1) Dynamic water pressure between the grouted layer or a slab and the roadbed occurs due to trainloads;
therefore the roadbed surface, on which effective stress hardly acts, is fluidized.
2) The clay content becomes mud water and flows out at the primary phase; therefore, density of the
roadbed surface decreases. Consequently, the water in the boundary layer becomes free to move;
therefore, water channels arise on the roadbed.
3) The roadbed surface, from which clay content has flown out, has reduced cohesion; therefore, the
surface soil becomes prone to move with water. The surface soil consequently flows out through the
water channels.
4) The water channels extend all over the roadbed; therefore roadbed degradation is accelerated. In
addition, bearing rigidity of the track is reduced; as a result, the grouted layer finally collapses.
To conclude, degradation of a clayey roadbed under ballastless tracks is not caused by shear
deformation or by consolidation, which are due to a lack of roadbed strength, but rather by a local
progressive failure, which is due to the outflow of small-particle content from the roadbed surface.
3. Countermeasures against roadbed degradation
3.1 Basic countermeasure policies From the reappearance tests it was evident that the clayey roadbed degradation under ballastless tracks
is a local progressive failure. In consequence, countermeasures to increase the strength of the entire
roadbed can be dispensed with. In fact, it is assumable that the degradation is preventable by some
appropriate treatments only for the roadbed surface. However, even if the surface is simply reinforced
with likes of cement, a new border is formed between the cemented layer and the uncemented layer;
therefore, the result must be similar to an untreated case. The improvement method should satisfy either
8
of the condition to show below at least:
(1) Prevention of decrease in effective stress
(2) Ejection or confinement of free water between the roadbed and the grouted layer
3.2 BLITS method To prevent a decrease in effective confining stress on the roadbed surface, a permeable layer set
between the grouted layer and the roadbed is assumed to be effective, because that layer diffuses the
water pressure that acts directly on the roadbed. But actually, this method might not be able to continue
the effect for a long term, because the permeable layer has to use a filter which will be clogged by small
particles sooner or later. For the practical application, therefore, the confinement of free water between
the roadbed and the grouted layer has been adopted.
As one of the method to confine free water movement, we regarded Bentonite-clay that has become
often used as the material of impermeable layer at waste disposal sites. As a result, the Bentonite liner for
track-bed surface (BLITS) method, in which the Bentonite liner is used as a protection layer of the
roadbed surface, has been developed. Fig. 17 shows an example of the BLITS method. If water is
present between the roadbed and the grouted layer, the Bentonite liner hydrates and swells; as a result, it
forms an impermeable layer to protect against an inrush of the free water.
In the BLITS method, a special granular Bentonite (Fig. 18) which is colored with red food coloring for visibility against roadbed soil is used. The thickness of the Bentonite liner is basically from 5 mm to 10
mm. Fig. 19 shows a concept of the Bentonite liner. Because the Bentonite has little bearing strength,
train roads from the grouted layer are directly borne by the penetrated ballast on the roadbed.
Bentonite Liner(approximately 5-10mm)
Fig. 17 Example of BLITS Method
Bentonite
Ballast Penetration
Grouted Layer
Fig. 18 Granular Bentonite Fig. 19 Concept of the Bentonite Liner for BLITS Method
9
3.3 Confirmation of the improvement effect The full-scale model test with improved roadbeds using BLITS methods, as referred to above, was
performed under the same condition as the unimproved case. Fig. 20 shows the relationship between
number of times under load and track settlement. It is clear that the BLITS method shows sufficient
improvement because the track settlement was much less than the unimproved case.
Fig. 21 shows the relationship between number of times under load and amplitude of the track
displacement. However the BLITS method showed larger displacements than the unimproved case at the
primary phase, those displacements became small according to number of times under load. This
phenomenon shows that because the Bentonite liner is compacted by cyclic load, the bearing strength of
the track increases.
With regard to the BLITS method, more term loading tests were performed with other frequencies, as
shown in Fig. 22. The frequency dependence of the settlement is hardly shown; in the end, the final track
settlement was estimated in 2 mm up to one million loadings. Because there was not enough
consolidation time in these tests, the roadbed consolidation has not been finished yet. Therefore, almost
all of the settlement is thought to have been caused by roadbed consolidation. Generally, the existing
roadbed was already consolidated enough; hence, if only the degradation of the roadbed surface can be
prevented, the settlement of the ballastless track on an existing roadbed can be kept to a very small level.
0 20 40 60 80 100 120 140 160 18014
12
10
8
6
4
2
0
Unimproved
BLITS Method
80,000 times is correspond to a monthlytrain operation
The Number of Loading Times (1,000 times)
Trac
k S
ettle
men
t (m
m)
0 20 40 60 80 100 120 140 160 180
2.0
2.1
2.2
2.3
2.4
2.5
unimproved
BLITS Method
The Number of Loading Times (1,000 times)
Am
plitu
de o
f The
Tra
ck D
ispl
acem
ent
Fig. 20 Track Settlement Fig. 21 Amplitude of Track Displacement
10
0 100 200 300 400 500 600 700 800 900 100014
12
10
8
6
4
2
0
Unimproved
BLITS Method
1Hz2Hz 5Hz
The Number of Loading Times (1000 times)
Trac
k S
ettle
men
t (m
m)
Fig. 22 Track Settlement (Long-Term Tests)
4. Conclusions From the results of the full-scale model tests, it can be confirmed that degradation of the ballastless
tracks on a clayey roadbed is caused by local progressive failure; specifically, the outflow of the fine-grain
fraction in the roadbed surface. Even if only the roadbed surface is saturated with water, the roadbed is
likely to be degraded. The degradation, therefore, is caused not only by groundwater, but also by
temporal surface water from rainfall. In addition, the degradation mechanism of the ballastless track is
assumed to be different from the mud pumping of the ballast track; therefore, the ballastless track has the
potential to degrade after replacement, though mud pumping had not occurred when the ballast track was
used in service. The ultimate countermeasure is improvement of whole roadbed, for example,
replacement of the poor roadbed soil or cement stabilization.
However, if consolidation of the existing roadbed has been sufficiently finished, additionally, if the ballast
track has been used without any train running problems, it is assumed that the reasonable treatment of the roadbed surface as discussed in this report can prevent roadbed degradation after replacement of
ballast track by ballastless track.
References [1] Muramoto, K., Sekine, E. et al., “Dominant Factors of Degradation of Cohesive-Soil Roadbed under Ballastless Tracks” RTRI Report, Vol. 18, No. 3, pp. 23-28, 2004 (in Japanese)
[2] Muramoto, K. and Sekine, E., “Fundamental Tests of Anti-degradation Methods of Soft Roadbed under Train Load,” JGS, 39th Japan National Conference of Geotechnical Engineering, pp. 1067–1068, 2004 (in
Japanese) [3] Muramoto, K. and Sekine, E., “A Study on Degradation under Ballastless Tracks,” JSCE, 59th JSCE
Annual Meeting, Vol. 3, pp. 41–42, 2004 (in Japanese)
[4]Muramoto, K. Nakamura, T. and Sekine, E., "An Effective Repairing Method of Ballastless-track for
Existing Lines", JSCE Journal F, Vol. 63, No.3, pp. 335-348, 2007 (in Japanese)
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