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ROAD AND RAILWAY CONSTRUCTION

MSC COURSE

2016/2017 AUTUMN SEMESTER

RAILWAY PROTECTION LAYERS

SZÉCHENYI ISTVÁN UNIVERSITY Dr. Ferenc HORVÁT professor

1.1 Functions of protection layer

Functions are listed below, but it isn’t necessary to fulfil all functions simultaneously.

Load distribution function

Reduce the traffic induced stress at the bottom of the ballast layer to a tolerable level for the top of

subgrade.

Separation function

Prevent interpenetration of subgrade and ballast. Prevent subgrade attrition by ballast, which in the

presence of water, leads to slurry formation, and hence prevent this source of pumping.

Protection function

Protect the subgrade against precipitate and frost.

Dewatering

Intercept water coming from the ballast and direct it away from the subgrade to ditches of the track.

Filtration function

Prevent upward migration of fine material emanating from the subgrade

Reinforcement function

Raising of the load bearing capacity of subgrade.

1. SET-UP OF PROTECTION LAYER

Strengthening effect of protection layer


1.2.1 Coarse-grained soil protection layers for track with velocity V 120 km/h

To hinder the drenching of subgrade quasi-watertight material has to be used. It is called coarse-

grained mixture 1 = CGM1). This layer is separated from subgrade by geotextile.

CGM1 has relative higher fine-grained soil content. This mixture is very sensitive to excess of the

optimal construction water content.

Requirements against CGM1:

- mixture has to be produced from crushed stone particles and natural round shaped particles:

o mass percent of crushed stone particles min. 30%,

o mass percent of round shaped particles min. 30%,

o It is allowed to produce the mixture from crushed stone particles in 100%, if the requirements

of compactness and load bearing capacity after construction will be fulfiled according to the

previous tests.

- grain-size distribution curve has to be between the border lines,

- inequality factor has to be Cu 15, it ensure a suitably stable behaviour against dynamic

effects,

- particles’ diameter min. 32 mm max. 63 mm,

- water permeability coefficient k 1x10-6 m/s at Tr = 100% compactness rate, - frost resistance is suitable, if fine particle content d 0,02 mm is not higher than 3 mass

percent at Cu 15 value.

1.2 Protection layers made out of coarse-grained soil


Border lines of material CGM1


Characteristics of mixture has to be ensured and controlled in mine. It is prohibited to modify the

characteristics at place of construction.

Finished track section constructed with CGM1 layer


In case of a subgrade produced with good water permeability feature the protection layer is

constructed from permeable corse-grained material. This material is called coarse-grained mixture 2

= CGM2.

Load bearing capacity of CGM2 layer can be expected equal to CGM2 layer.

Requirements against CGM2:

- grain-size distribution curve has to be between the border lines,

- inequality factor has to be Cu 15, it ensure a suitably stable behaviour against dynamic

effects,

- particles’ diameter min. 45 mm max. 63 mm,

- water permeability coefficient 5x10-5 m/s at Tr = 100% compactness rate,

- mass percent of crushed stone particles max. 30%, fraction 0/16,

- frost resistance is suitable, if fine particle content d 0,063 mm is not higher than 5 mass

percent at Cu 15 value.



Border lines of material CGM2

5

40

7'

0

10

20

30

40

50

60

70

80

90

100

0,0010,010,1110100

Szemcseátmérő D [mm]

Áte

se

tt t

öm

eg

szá

za

lék S

[%

]

Kavics Homok

0,0020,062

AgyagIszap

63

Mk

7' aprózódási próba után

megengedett értékek

Gravel Sand Silt Clay

7’ allowed values

after LA test

Particle diameter D (mm)

Und

ers

ize

in

ma

ss S

(%

)

Use of CGM1 and CGM2 material in tracks with velocity V 120 km/h is possible but not

compulsory.

Characteristics of course-grained material has to be controlled (e.g. grain-size distribution, filtration

function, etc.)

Minimum thickness of protection layer is 20 cm. Due to requirements of frost protection or load

bearing capacity the thickness can be higher than 30 cm.


1.2.2 Coarse-grained soil protection layers for track with velocity V ≤ 120 km/h


Border lines of course-grained material for tracks with velocity V 120 km/h

Border lines of mixtures in Hungarian practice

Border line Soil

replacement

Protection

layer Fill

Barrier layer

of subgrade Gravel pile

Upper E E C B F

Lower A D B E G


In case of secondary tracks on stations the protection layer can be neglected, if

- the load bearing capacity of subgrade is suitably high and equable,

- there is no frost sensitive soil under the surface of subgrade,

- there is no problems between ballast and subgrade in function of filtration and separation,

- there is no water sensitive material in the barrier layer of subgrade,

- it is no necessary to protect the subgrade against the penetrating contaminations.

1.2.3 Needlessness of protection layer


1.2.4 Dimensioning of coarse-grained protection layer

On the surface of subgrade characterized by modulus E2 subgrade should be constructed a course-

grained protection layer with a thickness of „h”. On the top of protection layer the modulus

E2 protection layer should be adequate to values of regulations (see table in 2_Railway substructure

general_Horvat.pptx).

There are two possibilities to determine the values of modulus E2 subgrade:

1.) to execute static plate tests in suitable number according to standard and thereafter planning

values can be calculated with taking in account of geotechnical and hydrological data,

2.) if there are not data in necessary quantity and quality, than the planning values should be

determined with help of table on next foil.


Recommended planning values of modulus E2 subgrade

Hydrological case No. 1:

- subgrade surface is well dewatered,

- there are not periodic drenching to a depth of 1,5 m from the rail head level (consistence index Ic

is above 1,00 permanently).




- dewatering of subgrade surface is inadequate,

- above the depth of 1,5 m from the rail head level periodic drenching can be occurred

(consistence index Ic is between 0,75 and 1,00).


- dewatering of subgrade is not solved, theee is permanent onflow,

- above the depth of 1,5 m from the rail head level permanent drenching is characteristic

(consistence index Ic is lower than 0,75).

Dimensioning diagram for protection layer made out of course-grained material

for increasing of load bearing capacity


1.2.5 Set-up of course-grained protection layer


1.3. Geosynthetics in protection layer

Geosynthetics have to be chosen on the grounds of construction place circumstances and required

functions. In protection layer geosynthetics can be used for following purposes:

- geotextiles

layer separation,

filtration,

drainage in plane of geotextile,

- geomembranes

waterproofing,

layer separation,

- geogrids

strengthening of layer structure,

- geocomposites

to fulfil complex tasks.

1.3.1 About geosynthetics in protection layer in general


General requirements at application of geosynthetics:

- pH-value of the surrounding soil should be between 5 and 9, if geosynthetic is not alkali-proof

(e.g. polyester), it can’t be contacted with soil contains lime or cement and fresh concrete,

- in case of alkali-proof geosynthetic pH-value can be higher then 9,

- geosynthetics has to be put on flat subgrade or protection layer surface which has a side

inclination of 4-5%,

- biaxial geosynthetics should be used,

- geosynthetics should be put on the surface perpendicular to the longitudinal axis of the track,,

- geotextiles should be put on the surface with overlapping of min. 30 cm, in case of geogrids and

geocomposites the overlapping should be min. 50 cm,

- generally design of load bearing connection at overlapping not necessary,

- the geosynthetics laid on surface should be covered with granulate with depth of min. 20 cm,

which is produced according to regulations,

- run of machines or transport vehicles direct on geosynthetic (without filling granulate) is

prohibited,

- direct connection between geosynthetic and machine at covering and compacting work of

granulate should be avoided


1.3.2 Geotextiles

Important parameters from point of view of applicability in railway track:

Physical properties

- mass/unit area,

- nominal thickness.

Mechanical properties

- apparent pore size distribution,

- tensile strength,

- elongation at breaking,

- puncture strength.

Hydraulic properties

- water permeability-

- in-plane transmissivity,

- filtration ability.

Resistance properties

- chemical resistance (e.g. reagents in soil),

- physical resistance (e.g. ultra-violet ray),

- frost resistance (it doesn’t become rigid in cold winter) ,

- other environmental resistance (e.g. microbiological resistance).


Expected functions of geotextile


Position of geotextiles discovered at excavation works in different tracks


Geotextile types Polyfelt Geotextile types Secutex

Secutex 151-GRK 2

Secutex 141


1.3.3. Geogrids

Producing of Tensar type geogrid


Structure of geogrid Tensar and its operational model


Geogrid Tensar SS30


The difference of operational principle between geogrid and geotextile


Increasing effect of load bearing capacity of geogrid


Layer structures with Tensar geogrid


General requirements at application of geogrids:

- a higher strengthening of protection layer can be achieved with crushed stone granulate on

geogrids (granulate with round shaped particles is recommended to avoid),

- grain size of fill material should be correlate to aperture’s size of geogrid on behalf of

suitable confining effect ,

- it is uneconomical to design thicker fill layer, then efficient (about 30 cm),

- if required more layers geogrids can be designed,

- in case of geogrids laid on subgrade with fine-coursed material a geotextile layer should be laid

under the geogrid to ensure the separation effect.


Necessary thickness of crushed stone fill material on Tensar SS30 geogrid,

recommended modulus on top of protection layer E2 rec = 70 MPa

(according to tests in lab of Szechenyi University)


Necessary thickness of crushed stone fill material on Tensar SS30 geogrid,

in function to load bearing capacity on subgrade and requested value on the

top of filling material


Dimensioning diagram calculated by TENSARPave computer program

5.16. ábra

QS 0QS 1

QS 2QS 3

UIC osztályozás

E ( EV 2)

(MN/m2)10 20 30 40 50 100

2 4 6 8 10 15 20 30 40 CBR (%)

SNCF

TGV vonalak

D117

Zúzottkő ágyazatmin. vastagsága 30 cm

Zúzottkő ágyazatmin. vastagsága 25 cm

Javítórétegmin. vastagsága 25 cm

SBB

1 csop. vonalakfővágányai

DB

fővonalak

átmenőfővágányai

100

e, cm

50

0


German dimensioning diagram

Material source: Göbel – Lieberenz: Handbuch Erdbauwerke der Bahnen


German type structures (calculated on load bearing capacity and frost)

Material source: Göbel – Lieberenz: Handbuch Erdbauwerke der Bahnen


Geogrid type Tensar TriAx™

This type of geogrid was introduced in the year 2008.

Pulling rigidity of biaxial geogrids in diagonal direction is much lower then in direction of

ribs. Geogrid type Tensar TriAx™ can hinder the particles’ movement in all directions of

the reinforcing plane with approximately equivalent rigidity.


Rigidity of Tensar geogrids


Cross section of ribs of Tensar TriAx geogrid was reshaped, to ensure a higher grade of

confining between rib and particle.

Tensar TriAx geogrid Normal Tensar geogrid


Layer structure in cut on railway line Zalalövő – Bajánsenye


Load bearing capacity of layer structure built with Tensar SS geogrids

Initial values on subgrade surface before rehabilitation works was under 10 MPa.

Layer structure type No. 1 on line Zalalövő - Bajánsenye

sectioning 404+00 - 415+00


Measured E2 values on the top of layer structure No. 1




sectioning 246+00 - 288+00


Measured E2 values on the top of layer structure No. 1

Reinforcing effect of layer structure No.2 according to the values of control

measurements




sectioning 30+00 - 91+00

Geogrid types Secugrid


Geocomposite (geogrid + geotextile) types Combigrid


Asphalt protection layer can be constructed, if

- the subgrade with weak load bearing capacity should be reinforced and

- a watertight covering layer should be laid on the subgrade..

With asphalt protection layer the further positive effects can be reached as well:

- distribution and damping of static and dynamic actions generated by railway vehicles,

- reduction of deformation on surface of subgrade,

- layer separation (ballast and sublayer),

- reduction of frost danger,

- reduction of vibration,

- resistance against capillarity,

- stabile subgrade surface,

- reduction of track maintenance costs.

General requirements at application of asphalt protection layer:

- minimum thickness 12 cm,

- coarse-grained layer should be constructed between subgrade and asphalt layer (thickness

should be calculated),

- asphalt layer should be constructed on the whole surface of subgrade unto the edge of track

benches,

- slopes should be protected against meteoric water coming from asphalt layer intensively.

1.4 Asphalt protection layer


Cross section on high speed railway line A Rome – Florence

Hungarian example


Double tracked railway line constructed with asphalt protection layer

(Line section Kápolnásnyék – Dinnyés)


Edge of asphalt layer


Breakage of crushed stone ballast and flowing mud


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