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Laying of track
 Before laying the track, it should be verified

LECTURE 9 that the subgrade has been properly

compacted and that the transverse slope (3-
LAYING AND MAINTENANCE OF TRACK
5%) is correctly given
 There are many types of track laying
machines
 A high-speed laying machine (with a workshift
of 6 hours) can achieve an average output of
1.3 km per shift.
 With peak output of 1.5 – 2 km per shift

1 2

Laying of track Laying of track

 Before laying the track, it should be verified
that the subgrade has been properly Sequence of construction of the various track
works
compacted and that the transverse slope (3-
At it’s simplest;
5%) is correctly given
 There are many types of track laying
i. Prepare the subgrade
machines ii. Place and compact all layers up to
 A high-speed laying machine (with a workshift sub-ballast
of 6 hours) can achieve an average output of iii. Place two strips of ballast
1.3 km per shift. iv. Seat sleepers on ballast
 With peak output of 1.5 – 2 km per shift

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Laying of track
Laying of track
v. Place pads on sleepers Note also that;
vi. Place rails in correct alignment Several of these processes can be
vii. Attach fastening devices automated. Clearly it is important to
viii. Weld up/bolt rail sections achieve a good smooth alignment and
ix. Place more ballast around the sleepers machines assist greatly.
x. Tamp (see later) – essentially a
levelling process
xi. Let the trains compact the ballast for
you
5 6

Track maintenance Track maintenance

 After the various railways system components
 Geometrical parameters, the degradation of
start operating, degradation begins and, after
which is usually reversible
a certain time, mtce becomes necessary.
 Mechanical parameters which in most cases
 Track mtce affects both train safety and
cannot be restored without parts replacement
passenger comfort (rails, fasteners, sleepers, welds, etc.)
 With respect to safety, maintenance should be  For lines with 20,000 – 40,000 tons/day,
preventive restoration of geometrical characteristics is done
 Regarding comfort, mtce should be corrective after a traffic load of about 40 – 50 million tons
 The above objectives depend on two  While rails are replaced after about 500 – 600
parameters. million tons.
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Track maintenance Track maintenance

 Deviation between the actual and theoretical Track defects

values of geometrical track characteristics are Longitudinal defect (LD) – (a)
termed track defects  Is the difference between the theoretical and the
 Their restoration is accomplished through track real value of track elevation (Fig. 9.1)
mtce. Transverse defect (TD) – (b)
 Track defects should be distinguished from rail  Is the difference between the theoretical and the
defects real value of cant (Fig. 9.1)
Horizontal defect (HD) – (c)
 Is the horizontal deviation of the real position of
the track from its theoretical position (Fig. 9.1)
9 10

Track maintenance

Vegetation and weed control

Vegetation control
 There are several methods for controlling growth
of vegetation along the track, the following two
are effective
 Chemical means (herbicides – chlorate)
 Installation of an asphalt layer under the
ballast and on the side paths

Fig. 9.1: Track defects

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Track maintenance Track maintenance

Weed control Weed control
 Weeds can cause serious detrimental effects on  Herbicides are extensively used, particularly
the ballast and the subgrade by: hormone selective weedkillers
 Contaminating the ballast with dirt and
vegetation debris, which affect free drainage
 Accelerating the decay of components such as
concrete sleepers, not only by chemical action
but the expansion of roots in cracks and
crevices
 By obscuring the track, and thus defects
normally observed by naked eye would not
have been seen on routine visual inspections
13 14

Railway Maintenance Processes

1st Maintenance requirements
 A railway trackbed is not rigid. Correct track levels
 Ballast, sub-ballast and subgrade are unbound  If settlement occurs, then it is certain that some
materials. differential settlement will be present.
 Long-term movements are expected, most  Perhaps the ballast may be uniform but the
notably settlement. underlying subgrade certainly is not – neither are
the rails.
 The result - some sleepers will settle more quickly
What Maintenance is Needed? than others.

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2nd Maintenance requirements 3rd Maintenance requirements

Clean or replace ballast Restore shape of rail head
Railway ballast is not protected.
Steel rails are put under enormous stress
Dirt gets in from the atmosphere,
trains and from the subgrade soil from train wheels.
underneath. Sometimes this causes changes to the
The ballast can even break down shape of the rail head.
itself. Effects to the ride quality and, eventually,
Ballast slowly changes in nature, the safety of trains.
becoming more like soil. This should
take many years.
17 18

4th Maintenance requirements TAMPING

Replace rails  Tamping takes place by vibrating ballast

Rail head shape can only be corrected around each sleeper.
(by grinding) a certain number of times.
If too much material is lost from the rail  Tamping is carried out every year or so,
head, it can no longer safely fulfil its role. sometimes much more often.
Not only that but cracks will eventually
initiate in the rails.  Tamping induces ballast breakdown.
The rail has to be replaced for safety
reasons.
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TAMPING … TAMPING …

 The tamping machine is the most Advantages:

important track maintenance device.  The automated process is relatively rapid
 It restores the line and level of the track.  It restores the line and level of the track

Disadvantages:
 Leaves the ballast in a loosened state
 Immediate settlement is expected.
 Temporary speed restrictions is required.
 Creates more fine material.

21 22

TAMPING … TAMPING …

23 24

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TAMPING … STONE BLOWING

The Principle:
Small size stones are blown down into any
gap which exists between the bottom of
the sleeper and the ballast.

25 26

STONE BLOWING … BALLAST CLEANING

Stone Blowing Vs Tamping:  Eventually, after several cycles of

 Stone blowing avoids disturbance of the tamping and years of train load
ballast. application, the ballast will need to be
 Therefore, little immediate settlement cleaned and, probably, partially
and no ballast damage. replaced.
BUT  This is carried out using a ballast
cleaning machine.
 Stone blowing is more expensive
 It cannot be used with steel sleepers

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BALLAST CLEANING … BALLAST CLEANING …

The Principle:
 The ballast cleaner is a self-contained
unit.
 It progresses along the track at about 4
kph.
 Being slow, it is therefore expensive.

29 30

BALLAST CLEANING … Causes/sources of Contamination

The Principle:
 Ballast cleaning would normally be
followed by tamping to restore the
correct level to the track.

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Causes of Contamination … Causes of Contamination …

A:Dirt from the atmosphere: B: Due to contact stresses:

 It may be wind-blown or it may fall
directly from passing trains.  Results to a gradual breakdown of the
 Amounts to as much as a millimetre or material, f(stone strength, track
two a year. stiffness and number and weight of
 Washed down to the bottom of the axle loads).
ballast layer to less permeable
material.

33 34

Causes of Contamination … Causes of Contamination …

C:Tamping
 Damages the ballast, causing large
numbers of fractures in critical areas.
D:From the subgrade soil
 If not separated from the ballast (e.g. by
a sand blanket or a geotextile), the two
materials will mix.
 Ballast will tend to sink into the soils as
soil (in the form of slurry) is pumped (see
35 photo – next slide). 36