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

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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%

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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

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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)

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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.

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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

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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

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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

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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)