4. GRUPO 2: RASGOS DEL CARÁCTER O COMPORTAMIENTO
4.2 Información semántica
4.2.1 Sentidos y acepciones
C15-1 Introduction
This chapter provides an introduction to rail adjustment and track stability as a basis for adjustment processes described in detail in other manuals. It includes information on:
• Temperature effects in rails leading to compressive and tensile stresses.
• Control of expansion and contraction in rails.
• Maintenance of track stability and the role of track components in track stability.
Since rail adjustment is a principal consideration in the maintenance of track stability in hot weather, information is also provided on the prevention of misalignments including Welded Track Stability, Work in Summer Months and WOLO speed restrictions.
C15-2 What is a misalignment?
A misalignment, also commonly referred to as a `buckle’, is a short, sharp sideways movement of track. It can result in either a sharp radius curve or an `S’ shape. The resulting track geometry cannot usually be travelled over by a train at speed, and in many circumstances is too sharp for any train movement.
The degree of hazard depends on the size of the lateral displacement and the distance over which it occurs, and on the train speed.
Figure 247 – Misalignment on a curve Figure 248 – ‘S’ shaped misalignment on tangent track
C15-3 What causes a misalignment?
Railway track is dynamic. Rails are subject to thermal stresses as temperatures increase or decrease and will go into compression or tension, respectively, as the temperature varies from the neutral temperature at which the rail was installed. When the compressive forces in the rail exceed a certain stress level, the rail will buckle unless it is restrained.
C15-4 Temperature effects in rails
Rails expand and contract (get longer or shorter) as the rail temperature increases or decreases. When rails are free to move they will expand and contract 0.115mm for every 1m in length for every 10°C change in rail temperature. For example a 110m length of rail will get 12.65mm longer when the rail temperature increases 10°C. The rail will get shorter by the same amount if the rail temperature decreases 10°C. An unrestrained 110m rail would grow 19mm if the rail temperature increases from 20°C to 35°C.
When rails are laid in track they cannot expand or contract this amount, because they have been welded to other rails or connected with fishplated joints.
In the case of track with jointed rails, the potential for free movement is only 13mm, which is the gap at each joint. In continuous welded rail there is no potential for free movement because there are no joints.
C15-4.1 Stress
If the rail cannot expand or contract, stress is created in the rail.
There are two types of stress:
Compression As rails expand with heat, any free movement is taken up. When there is no more movement left a force is built up in the rail by expansion. This is called compression.
Tension When the rails cool any free movement is taken up by contraction.
When all movement is taken up, and the rails continue to contract, a force is built up in the rails. This force is called tension.
C15-4.2 Effects of compression
When rail is in compression it tries to relieve the compressive stress by getting longer. It will try to move sideways to get longer.
Compression (stress) is normal and can be contained within the track structure under normal conditions i.e. when the track structure is to standard.
The track structure (the assembly of rails, fastenings, sleepers and ballast) is designed to resist a certain amount of sideways thrust that comes from the compressive stress in the rails. When, however, the amount of compression generated in the rails exceeds the ability of the structure to hold itself in place, track movement occurs. This movement is known as a misalignment (or buckle).
Figure 250 - Misalignment
C15-4.3 Effects of tension
On curved track, tension may have a similar, but less dramatic effect on the track.
When rail is in tension it tries to relieve the compressive stress by getting shorter. It will try to move sideways to get shorter.
Tension (stress) is normal and can be contained within the track structure under normal conditions i.e. when the track structure is to standard.
If the amount of tension generated in the rails is greater than the resistance offered by the track structure, the curve will tend to pull in towards its centre.
Figure 251 – Curve pull-in
This track movement is less dramatic than a misalignment because the track movement occurs over a much longer distance. The movement may not be obvious but it can be extremely dangerous when clearances are affected.
Other indications of excessive tension in rails are broken rails, bent or broken bolts, or breakaways as the rails pull apart.
The adjustment of rails, therefore, is a most necessary and essential part in effective track maintenance. Rails, ballast, ties, fastenings and rail adjustment all interact to provide a stable track structure.
C15-5 Control of expansion and contraction
Rails experience both cold and very hot temperatures in track and will, therefore, experience both expansion and contraction or compressive and tensile stresses.
These stresses are controlled by:
• Establishing and maintaining correct rail adjustment.
• Providing and maintaining lateral resistance to movement.
• Controlling the additional forces that can initiate misalignments or pull-ins.
C15-5.1 Establishing rail adjustment
Rails have to be adjusted so that they don’t generate excessive compression or tension.
The adjustment is carried out by ensuring that each rail will be stress free (no compression or tension) at 35°C rail temperature. This is called the “Neutral temperature”. The temperature has been specially selected for rails in RailCorp infrastructure and is a balance between the extremes of heat and cold experienced in the Railcorp system. If rails anywhere in the system have been correctly adjusted to the Neutral Temperature, the normal range of operating temperatures will not generate excessive compression or tension.
In CWR the rails will always be compression when the rail temperature is over 35°C, if the rails are correctly adjusted. When the rail temperature is under 35°C correctly adjusted CWR will always be tension.
Jointed rails will be in compression if the joints are closed up to zero. What rail temperature this occurs at depends on the length of the rail. For example correctly adjusted 110m rails will be in compression at 40°C rail temperature. Shorter 13.72m rails, however, do not go into compression till the rail temperature reaches 75°C
Likewise, rails will be in tension if the joints are fully open to 13mm. Again, the rail temperature this occurs at depends on the length of the rail. For example correctly adjusted 110m rails will be in tension at 29°C rail temperature. Shorter 13.72m rails however do not go into tension till the rail temperature reaches -11°C.
C15-5.2 Maintaining rail adjustment
C15-5.2.1 Rail creep
Rails in service are subject to longitudinal forces from:
• The braking action of the trains pushing the track in the direction of travel.
• The acceleration of the trains pulling the rail in the opposite direction of travel.
• Wave motion caused in the rails by the passage of the train wheels. This will push the rails in the direction of travel.
These forces can result in the rails moving. If the rails move longitudinally the rail adjustment will be affected irrespective of the type of rail (Jointed Rail or CWR).
To illustrate this point, consider an example where jointed track is on a steep grade. If traffic continually runs downhill, the rails will tend to run downhill as well. This will result in wider rail gaps at the top of the grade and narrower gaps toward the bottom, with the result that there will be too little steel at the top of the hill and too much steel at the bottom.
Continuously welded rails behave in the same way; steel will bunch up in one location and be stretched at another, altering adjustment.
Standard track consisting of formation, ballast, ties, fastenings and rail is designed to interact to resist the longitudinal movement of rail.
C15-5.2.2 Ballast
As well as providing a flexible base for the track, transmitting axle loads to the formation, and providing the vertical and lateral support to the sleepers to maintain track geometry, ballast provides resistance to the longitudinal and lateral movement of sleepers.
The angular shape of the ballast serves to lock the sleepers in place. (Evidence of this can be seen in the marks on the underside of a sleeper that has been removed from the track).
In order to do its job effectively, ballast must have the following qualities:
Grading – Specified shape and size (see TMC 241 - Ballast) to effectively lock together. Poorly rounded or graded ballast will not provide the designed resistance to movement.
Cleanliness – Ballast that contains fine contamination, weeds etc will not provide effective interlocking or resistance to sleeper movement.
Profile – Specified profile (see TMC 241 - Ballast). Ballast deficiencies will significantly reduce resistance to sleeper movement.
C15-5.2.3 Sleepers
In addition to holding the rails to gauge and transmitting the axle loads from the rails to the ballast, sleepers provide resistance to longitudinal movement of the track by holding the rails in position through the fastenings and by engaging the sharp faces of the ballast.
Sleepers that are broken, cracked, split, crushed or rotten will not be effective.
C15-5.2.4 Fastenings
Fastenings are used to connect rails together (fishplates and bolts), tie the rails to the sleepers (spikes, clips and sleeper plates) and to restrict longitudinal movement of rails relative to the sleepers (anchors and resilient fastenings).
Anchors are installed to prevent longitudinal rail movement (creep) and to evenly distribute stresses along the full rail length.
Without rail anchors in jointed rail the majority of stress would occur at the ends of the rails. In CWR the stress will occur at fixed points (e.g. turnouts).
By holding the rail in place with rail anchors these stresses are spread out over the whole length of the track.
To achieve effective anchoring the following points must be remembered:
• Maintain the standard anchor pattern. (i.e. box anchor every second sleepers for 32 ties at mechanical joints and box anchor every fourth tie along the rail).
Standard anchor patterns are detailed in TMC 221 Rail Installation & Repair.
Additional anchors are sometimes installed to control creep on steep grades.
• Make sure the anchors fit hard against the sleeper.
• Make sure the anchors are not overdriven. Overdriving anchors will ‘spring’ them (destroy spring action) and make them ineffective.
• Monitor rail creep by punch marking rails in relation to fixed points.
For effective anchoring with resilient fastenings:
• Do not overdrive clips.
• Replace any “sprung” clips.
C15-6 Providing and maintaining lateral resistance
The standard track structure of formation, ballast, ties, fastenings and rail as previously described is also designed to interact to provide a structure that resists the lateral forces generated by compressive or tensile forces in the rail.
This is achieved by
• Anchors & fastenings or resilient fastenings being properly installed and effective in providing a ladder track structure
• Good sleepers, firmly fastened to the rails and firmly bedded to the ballast
• A full profile of good clean draining, firmly compacted ballast
Overall the contribution of each component in the track to track stability is as follows:
The degree of RESISTANCE is provided by
• Rails approx. 10%
• Fastenings approx. 30%
• Sleepers and Ballast approx. 60%
60% of the RESISTANCE to BUCKLING is given by the SLEEPER in the BALLAST.
• Bottom of tie approx. 25%
• Sides of the tie approx. 25% (Full Crib)
• Shoulder Ballast approx. 10%
C15-7 Maintenance of Track Stability
High temperature is NOT the cause of misalignments. Properly constructed and maintained track will not misalign in the normal range of temperatures experienced in RailCorp.
The most common causes are
1. Track not correctly adjusted to be stress free at 350C.
2. Loss of rail adjustment due to uncorrected rail creep or addition of steel when repairing rail defects or renewing rail.
3. Frozen joints or rail end flow at joints – restricting expansion or contraction.
4. Uneven stresses occurring in track due to fixed or bunching points eg at level crossings, turnouts, welds binding on sleeper/plates, patches of resilient fastenings in dogspiked track, anchor points left in track, twisted sleepers/sleeper plates, angle fishplates with dogspike holes, fishplates
“running” into dogspikes.
Figure 252 – Local bunching point - weld is caught against sleeper - rail cannot
move - sleeper will generally move with rail
Figure 253 – These anchors aren't doing anything
5. Loss of resistance to lateral movement due to inadequate ballast profile or loss of frictional grip between ballast and sleeper. Loss of ballast grip is generally caused by track disturbance, by doing work on the track that breaks the bond.
Figure 254 – Ballast deficiency at a bunching point
6. Trackwork initiators include resurfacing, rerailing, resleepering, ballast cleaning, earthworks and drainage, welding & CWR, trenching or installing cables under or alongside the track, repairing rail defects, damaging ballast shoulder/profile with plant/road vehicles or by repeated walking down the ballast shoulder.
7. Track geometry initiators that create increased lateral force when trains pass over them. In the right circumstances these additional forces will break the bond between sleeper and ballast and trigger a misalignment.
Geometry initiators include loose or flogging joints, pumping track, poor geometry such as poor top, poor line, especially ‘kinks’ or sharp spots in curves, bad weld alignment, both horizontal (line) and vertical (dipped), welding closures not crowed in curves, welding closure not matching rail profile either side eg curve worn rail, insufficient superelevation on curves
Figure 255 – Pumping track Figure 256 – Poor geometry
Figure 257 – Battered joint Figure 258 – Loose joint
8. Rolling stock initiators including trains going too fast, overloaded or defective rolling stock, trains accelerating away from stations, trains braking – coming into stations and down steep grades, train loadings predominantly in one direction.
It is important to note that misalignments will occur at the weakest point. This may only be a short isolated piece of track in an otherwise very stable section.
C15-8 Prevention of misalignments
RailCorp has established a number of systems and practices that if used correctly and in combination prevent misalignments occurring in the hotter months of the year. These are:
• Welded Track Stability Analysis – WTSA.
• Control of rail adjustment.
• Summer work practices.
• WOLO - heat speeds and inspections.
C15-8.1 Welded Track Stability Analysis – WTSA
To provide assurance that the design conditions are maintained during the summer period, track stability examination, analysis and correction are conducted between August and October each year.
This involves inspection of rail adjustment, ballast profile and condition, fastening condition and areas of potential concern, analysis of contribution of these factors to the overall stability, and programmed, prioritised improvement at identified locations. The operation of the WTSA system is detailed in Engineering Manual TMC 203 - Track Inspection.
C15-8.2 Control of adjustment
WTSA relies heavily on control of rail adjustment. In CWR, creep pegs are used to monitor changes. If uncontrolled rail welding occurs, adjustment control is LOST. If unreported, the loss of control is INVISIBLE.
It is essential that all rail welding activities are properly controlled and reported. This is particularly important in colder weather, when uncontrolled rail welding will add steel.
The methods of adjusting track, and of undertaking work on track without losing control of adjustment, are detailed in Engineering Manual TMC 223 Rail Adjustment.
At any time (not just in the hotter weather) the following actions are required to control rail adjustment.
• Extreme care is required whenever rails are cut, to ensure that STEEL IN = STEEL OUT.
• If control is lost in CWR, the 500m section of track BETWEEN CREEP MARKS should be re-adjusted and re-punched.
• Use double creep marks when re-punching.
• Extra care is needed when cutting or laying rails at night, as misalignments can occur the next day if too much steel has been added.
• When tamping curves, ensure that alignment is restored to the survey/offsets agreed with the Civil Maintenance Engineer.
C15-8.3 Work in Summer Months
Restrictions on work affecting the track during the hotter months are provided in Engineering Manual TMC 211 Track Geometry & Stability.
Engineering Manual TMC 211 establishes requirements for pre-season special spot inspections, special inspections throughout the summer and procedures for work in summer months.
It also describes the DO'S and DON'TS of track maintenance in hot weather and provides guidelines for the issue of special instructions for work in Summer months.
Civil Maintenance Engineers may authorize work in Summer by the issue of written instructions which will detail the special actions to be taken prior to, during and after the work to maintain track stability.
In the absence of written authorisation from the Civil Maintenance Engineer, the "safety net" "Work in Summer Months" restrictions apply. These are detailed in Engineering Manual TMC 211. These instructions have also been reproduced on plastic card to be distributed to and carried by all Team Leaders, or staff acting as Team Leaders during the summer season.
It is very important that everyone working on or near the track is aware of and understands special precautions that need to be taken to avoid track buckles
Before you do any work that will affect track stability, you must check Engineering Manual TMC 203.
• What is the current stability loss at the location?
• When you do the work, what will the stability loss be when you've finished?
• What actions are you taking to make sure the track won't buckle after the work is done?
Remember it will take some time for the track to bed down again.
Remember - the ballast shoulder should be at least 400mm wide, level with tops of sleepers.
ALL staff when working on or near the track must beware of:
• Bumping the track e.g. with earthmoving plant.
• Knocking down or removing ballast profile e.g. with trucks or earthmoving equipment running along the ballast shoulder or climbing up the ballast shoulder.
• Undermining the ballast profile by excavation e.g. excavation under or alongside the track for signals cables or services.
C15-8.4 WOLO - heat speeds and inspections
When the AIR temperature on any day reaches or exceeds 38°C OR is predicted to reach or exceed 38°C the speed of trains is reduced in the hottest part of the day by imposing a WOLO speed. Reducing the speed of trains reduces the possibility of misalignments caused by trains and reduces the consequences if a train derails on a misalignment.
The level of speed restriction and the procedure for applying it are detailed in Engineering Manual TMC 211 Track Geometry & Stability.
In addition to WOLO speeds, when the AIR temperature reaches or is forecast to reach 38°C all welded track is inspected in the hottest part of the day for the purpose of detecting signs of misaligned tracks. This requirement is detailed in Engineering Manual TMC 203 Track Inspection.
Engineering Manual TMC 211 Track Geometry & Stability provides further guidance on causes of misalignments and techniques for avoiding misalignments.