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Railway track system, is one of the most important infrastructures in the UK. It is said that the railway's contribution to UK economy is acknowledged. According to a survey from Network Rail (2010): over one billion of passengers were carried by railway track system every year, over a third quarter of coal & mines and about a quarter of steel materials were transported by trains. It is estimated that there is about £15 billion pounds benefit for people and business who benefit directly from railway and about £2.9 billion benefit for non-direct users. Railway infrastructures will continue to play a vital role in civil engineering world with future projects such as £ 15.9 billion of Crossrail and £30 billion of High Speed railway 2 (NCE, 2010).
Since railway system had been existed for over 100 years, most of the tracks have poorly managed or no properly maintenance. Nowadays, the number, frequency and weight of trains tracks have increased dramatically, but these old railway tracks were not designed to carry this amount of trains initially. The problem arises with the excess usage. They start deteriorating and riding quality become worse over time. Deteriorated track lead to uneven riding surface, the trains which are passing through these tracks would be forced to travel in a restricted speed, and then, slower riding speed will lead to further deterioration to the track. It is regarded that some of railway accidents such as Bexley derailment in 1997 and Grayrigg derailment in 2007 were caused by high speed trains passing through some failure points over the deteriorated ballast tracks (Railway accident investigation reports, 2007). Further information on the causes of deteriorated tracks will be provided in later section( literature review).
In order to prevent further deterioration of the railway tracks, there are several various of techniques which are used for restoring the quality of ballast track system. The most common technique is tamping. It is used to rearrange the positions of ballast in the track structure, but leaving the ballast in loose state and ballast breakage are the side effects. A further work which is called ballast compaction may be required after tamping; it is used to compact ballast by a special design compactor train. Ballast will then be compacted and become to a more stabilised state. These methods can be carried out to restore the function of ballast until there is too much ballast breakage and physically become more like soil. In this case, more maintenance techniques are going to use such as ballast cleaning, stone blowing or replacement of the whole ballast track structure.
Maintenance is not the only method to extend the life of a railway track; Reinforcement of the ballast is the second option to be considered. Reinforcement of ballast means placing geogrids in the ballast layer in order to improve the speed and the rate of settlement caused by external force such as trains. It can be installed during ballast cleaning. This project will mainly concern the effects of geogrid reinforcement on ballast layer to improve the long term settlement.
This project will continue the work of Mitchell (2009) and Ferguson (2008) by using the large scale of triaxial machine. It was shown that the use of geogrid reinforcement in ballast layer will improve the long term settlement. There were a various range of ballast sample used in these pervious projects. However, there are no works have been done previously approved that the geogrid reinforcement is also effectively improve the settlement on recycle ballast. This project will carry out the investigation of the effects of geogrid reinforcement on recycled ballast with poor quality. This investigation will be carried out simultaneously with Audley (2010) who examines good quality of ballast sample.
An accident happened during a specimen removal in February, the inner cell of the triaxial facility broken and it was impossible to commence any tests afterward. For this reason, pull out test will be carried out to continue the investigation.
The second test of this project will follow the work of Kwan (2006) who develop a pull-out test to investigate the interlocking ability of geogrid. The test is recording the amount of force require to pull out a piece of geogrid from a lump of ballast. Therefore, the aim is finding a geogrid that offers the best interlocking ability for recycle ballast.
1.2 Aims and Objectives
The project will follow the work of Mitchell (2009) and Ferguson (2008) to investigate the effects of geogrid reinforcement on ballast layer by using large scale triaxial machine and pull-out test. Samples in this project are recycled ballast which has smaller size, rounded shape and weaker strength. For the triaxial test, both unreinforced and reinforced specimen are examined for comparsion. For the pull-out test, two different type of geogrid will be examined to find the one with better interlocking ability. The most optimum aperture / particle size ratio will be investigated in both tests.
1.2.1 Aims of the project
Using the large scale triaxial facility to test the behaviour of recycled ballast under repeated loading.
To investigate the effects of the use of geogrid reinforcement on recycled ballast.
To develop a pull-out test to investigate the relationship of the particle size and the interlocking ability of geogrid.
To find the most optimum aperture / particle ratio.
Examine two unreinforced samples for triaxial test.
Perform a triaxial test on a reinforced sample with two layers of small TriaX geogrid.
Perform a triaxial test on a reinforced sample with two layers of large TriaX geogrid.
Perform a triaxial test on a reinforced sample with two layers of small diamond geogrid.
Perform a triaxial test on a reinforced sample with two layers of large diamond geogrid.
Use the best performance of geogrid reinforced sample and perform a triaxial test with one layer in the mid height of sample.
Perform three pull out tests with large triaX geogrid with no surcharge.
Perform three pull out tests with large triAX geogrid with 0.5kN of surcharge.
Perform three pull out tests with large diamond geogrid with no surcharge.
Perform three pull out tests with large diamond geogrid with 0.5kN of surcharge.
Traditional ballast track system is the majority type of existing railway track system in the UK. Ballast track system has been existed in the earliest day of the railway history, so this track system almost covered every km of railway track apart from the recent constructed tracks. These former structures are needed to be replaced or improve their qualities in order to deal with the increasing deterioration rate and frequency of maintenance.
Nowadays, there are numbers of more advance track system appearing such as pre-cast rigid track and flexible track. These new track structures can improve the disadvantages of ballast track but inevitably they are expensive. Besides, these new track structures do not overcome the advantages of ballast track structures such as easy to maintenance and lower maintenance cost. A better option to balance the cost spending and remain the quality of the track is placing reinforcing the ballast by geogrids. This method can extend the life of the track and reduce the frequency of maintanence by improving the settlement of ballast layer.
The literature review of this project will focus on the traditional ballast track system, inspection & maintenance techniques, ballast deformation and improvement of settlement by using geogrids.
Figure â€Ž2â€‘1: physical property of ballast
Ballast is a granular material which sit under railway tracks. Ballasts are usually crushed stones with large size and angular shape. There are no particular definition to limit the size, shape, material and hardness of a ballast. As long as these materials can offer a good performance under the compression, this material can be called ballast.
Ballast is the main source of material used in traditional track system. It has the following functions (Selig and Waters, 1994):
Major structure of the track; Ballast provides a stable, safe and even platform for various types of railway trains.
Resist loads from any directions: vertical loads from trains, lateral and longitudinal forces.
Spread loads from sleepers so that the subgrade will not be overloaded and generate settlement.
Absorb energy such as noise absorption.
Provide good resilient property.
Provide immediate drainage for the structure so that the materials underneath the ballast will not be washed out.
Give friction resistance to the track expansion due to heat.
Provide voids between particles to loose dirt or store any waste materials.
Allows easy maintenance.
2.2.1 Definition of good ballast
Since there is no universal specifications for the properties of ballast, there are various of granular materials are used in the track structure such as mount sorrel, croft, limestone and granite. Different type of ballast are used in different locations which depends on traffic volume and other economic reasons such as usage of the track. In this project, mount sorrel quarry from Lafarge is used for laboratory testing.
According to Bonnett (2005), good quality of ballast should be made by crushed rocks from the ground. It is said that ballast has better performance with angular shape, as the sharp end will enhance the interlocking effects between particles, so that the particles are unlikely to be slipped, so that ballast with angular shape has a lower chance to cause shear failure in the ballast layer.
Ballast under frequency traffic loading will suffer breakage and deformation. Ballast should be strong and hardness in term of internal strength. It is not only able to resist greater vertical forces from railway trains, but also able to resist the damage from tamping machine.
The diameter of ideal ballast should not be smaller than 28mm nor be greater than 50mm. Ballast with smaller size (< 28mm) is stronger in numbers but it's not able to loose dirt and drainage, the condition of the track would become worse if no actions taken for improvement. On the other side, Ballast with greater size (> 50mm) provide immediate drainage and loose dirt but it's weaker because there are less contact points and therefore there will be a higher stress level for each point.
2.3 Forces acting on track structure
Figure â€Ž2â€‘2 forces acting on the trackThere are mainly 3 components of forces acting on the track: Vertical, lateral and longitudinal:
2.3.1 Vertical Forces
Vertical forces are usually the passing railway trains with force acting vertically to the track. This is the main factor that causes the deformation and settlement. According to Selig and Water (1994), there are two types of vertical forces: Vertical wheel forces and uplift forces. For the vertical wheel forces, the weight of railway train is equally divided by the number of wheels. The railway wheels will then transfer the loads to rails, sleepers, ballast and subgrade.
Esveld (2001) states a formula to summary the total vertical force:
Qtotal = Qstatic + Qcentrifugal + Qwind + Qdynamic (Equation 2-1)
where Qtotal is the total vertical loads from trains
Qstatic is the static wheel loads
Qcentrifugal is the centrifugal forces generate at the outer rails
Qwind is the wind loads
Qdynamic is the dynamic wheel loads
The wheel impact forces can be possibly very high due to their high frequency of vibration generates in the track structure. These vibrations contribute further deterioration of the ballast layer and settlement to the subgrade.
Figure â€Ž2â€‘3: The wheel load distribution to the ballast track structure and the deflection of rail under single wheel loading (Selig and Waters, 1994)The figure shows the deflection of rail under a single wheel load, the contact point of the wheel and rail is forced to be pushed downward and causes the maximum downward deflection, but the wheel load will lift up the rail and give upward deflection at non-wheel/rail contact point. This causes rail lift up and it can't be supported by sleepers. If there is no other loads to neutralise this uplift deflection, this action will further deteriorate the ballast underneath the sleepers.
Other vertical forces such as wheels corrugation creating forces and bounding forces from suspension system are depended to the weight of trains.
2.3.2 Lateral Forces
Figure â€Ž2â€‘4: Thermal expansion of a sleeper (Thom, 2009)Lateral forces are usually acting perpendicular to the vertical and parallel to the sleepers. The lateral forces are caused by a train's hunting motion and running into a corner to generate centrifuge forces. Other sources of lateral force is the thermal expansion of sleepers where they expand laterally due to temperature rise.
Esveld (2001) states a formula to summary the total lateral force acting on the outer rail:
Ytotal = Yflange + Ycentrifugal + Ywind + Ydynamic (Equation 2-2)
where Ytotal is the total lateral force acting on the outer rail
Yflange is the force caused by flange against out rail
Ycentrifugal is the centrifugal forces generate at the outer rails
Ywind is the side wind load
Ydynamic is the dynamic force caused by Klingel motion (hunting motion)
2.3.3 Longitudinal Forces
Longitudinal forces are acting perpendicular to vertical & lateral direction and acting parallel to the rails. They are usually caused by thermal expansion of rail due to temperature rises and braking forces of railway trains.
The expansion of rail can cause bending itself, the solution is providing gaps between rails to allow them to expand for further length.
2.4 Ballast deformation
When the good quality ballast under compression for a long period of time until it reaches the serviceability state, the ballast will begin deteriorating to an unacceptable level. Poor quality of ballast has almost opposite properties to the good ballast such as rounded end, smaller size, not able to drain water and not much void to lose the dirt etc.
When the ballast breaks into small pieces, these pieces will have a chance to trap in the gap between other greater size of ballast. Then volumetric of the ballast layer become smaller and make the layer cannot drain the rainwater immediately. Water stores in the ballast layer will weakness the ballast and make them easier to break. Large numbers of ballast breakdown will generate significant deformation in the structure. Therefore, the trackbed will become unstable and the track will response to the deformation, it brings bad quality of riding to the trains passing that section. Trains will be forced to run at a restricted speed, the running cost will be higher for the railway company.
Ballast deformation is occurred because of the following reasons (Thom, 2009):
Ballast is overstressed, it tends to break down into small pieces and get into the voids between ballast (Non-recoverable). Ballast is break down due to high vertical stresses applied to the ballast.
Ballast is slipped to other positions; it may go back to its original positions by an opposite loading or will stay in new position. Ballast is slipped due to high shear stresses applied to the structure.
Figure â€Ž2â€‘5: Deviator stress and axial strain curve from a drained triaxial test under repeated loading (Selig and Waters, 1994)
The figure 2-5 shows stress and strain of granular material behaviour under repeated loading in a triaxial test. The maximum stress for each hysteresis loop is the same. However, there is more ballast breakage or deformation for initial compression. After the initial plastic strain, fewer deformation happening as the granular materials are stronger when they break into smaller pieces. The size of hysteresis loop represents the energy loss during the test, as it shows that the loop is getting smaller for more load cycles.
Figure â€Ž2â€‘6: Hysteresis loop for a load cycle
The figure 2-6 shows only one load cycle of hysteresis loop, as the loop is unlikely going back to its original starting point after unloading. The difference between starting point and ending point is the permanent strain of the sample. Resilient strain is the recoverable strain after unloading. It represents the deformation of the sample but it bounces back to its original position after unloading. Area of the hysteresis loop is the energy loss in this load cycle, this energy is consumed to make permanent strain.
2.5 Ballast Track system
Figure â€Ž2â€‘7: front view of a ballast track structure (Britpave, 2009)
Figure â€Ž2â€‘8: side view of a ballast track structure (Selig and Waters, 1994)
This is the basic railway track system; such structure still exists at the present railway system. The system usually consist the following components: Rails, sleepers, fastening units, ballast layer, sub-ballast layer and sub-grade (Thom, 2009).
Rails are two parallel I-shape beams on the top of the structure which providing even ride to the trains and guide the train wheels safely through the journey. The rails must have low friction in order to reduce the energy loss, so that speed of the railway trains can be maximised with the most efficient of fuel consumption. Rails acts as beams to support railway trains, they must be rigid enough not to have great bending moment under repeat loading and ductile enough not to become brittle. Loads from trains will equally transfer to sleepers via rails.
Fastening units attach rails to sleepers firmly in order to prevent the displacement of rails.
Sleepers are used for spreading loads from the rails to the ballast layer. They also maintain the correct gauge between two rails. Traditional timber sleepers are the major type of sleepers in railway industry; however, concrete, steel and plastic composite made sleepers gradually become common (Railway Track and structures, 2008).
There are two ballast layers in the system: ballast and sub-ballast, ballasts are usually big crushed stones with angular shape located in the top layer. They used to provide a stable platform to the rails, spread loads to subgrade, provide voids to loose dirt, provide immediate drainage and allows maintenance; sub-ballasts are relatively low quality and smaller size because they resist smaller loads, sub-ballast are located in the bottom layer to act as a separator between ballast and subgrade, so that the main ballast layer and soils would not mix together (Thom, 2009).
Subgrade is a layer that absorbs any energy from the top and act as a foundation to provide a stable platform for the entire structure.
2.6 Trackbed investigation
Trackbed investigation is carried out before any maintenance actions taken. It is very important to determine what maintenance actions are going to do for different condition of track. Cost will be more efficiently used if maintenance taken at the right time, right place and right choice. According to Thom (2009), there are several measurement options to be used for investigating how poor the condition of the track would be. Results of the investigation are usually a combination of several different types of data and these data are collected from the following methods:
2.6.1 Longitudinal Track Profile
Civil engineers use New Measurement Train (NMT) to survey the longitudinal profile of a track. These special design trains are using laser beam technology to check the surface of the track structure and using standard deviation to represent the level of evenness. The smaller value of the s.d, the better quality of the track is. If the s.d value is greater than 5, the deformation is large and it is likely that speed restriction will apply to the track and maintenance has to been taken immediately (Thom, 2009).
2.6.2 Visual Inspection
NMTs are installed with digital camera which allows visual inspection to the track structure. Visual inspection can identify the condition of the track immediately so that any problem can be solved rapidly. It is also used to check the contamination level of the track where the slurry is likely to pump out from the track under the passing trains.
2.6.3 Inspection of ballast layer
Internal inspection of trackbed is helpful to determine the condition of ballast in the track structure. It is used to inspect the degree of contamination and breakage of lower ballast layer. Digging trial pits at regular length of a railway track, then samples will be carried back to laboratory for further experiment or investigation.
2.6.4 Radar Survey
Figure â€Ž2â€‘9: Data collected from radar survey showing the thickness of ballast layer (Railway Track Inspection, 2000)Radar survey is used to determine the condition and thickness of ballast layer. It's a more advance technology than ballast sampling. Radar is carried along to the track, it acts as a transmitter and a receiver which transmits and receives radio signal through the track.
2.6.5 Structural Survey
Falling Weight Deflectometer (FWD) is used to measure stiffness related parameters of a pavement structure, however it is still possible to measure stiffness of the trackbed structure directly. It stimulates the behaviour of trains applying load pulse to the trackbed, and then measure the deflection of sleepers or ballast layer by geophones (Thom, 2009).
Deflection measurement offsets are usually 0mm, 200mm, 300mm, 450mm, 600mm, 900mm, 1200mm and 1500mm. It's possible to calculate the stiffness of the trackbed structure by analysing deflection at different offsets.
2.7 Maintenance of railway ballast track
Deformation of ballast layer alters the railway geometry. It's necessary to take maintenance actions for a particular length of track where the deformation or settlement of the lower layer is too great and the ballasts are too dirty. Deformation will influence the riding quality of the track because the riding surface is uneven; it's too dangerous for trains approaching the failure points at high speed, derailment can possibly happen. Therefore, speed restriction will be applied for safety reasons.
When the deformation of the track geometry is too great and beyond the acceptable tolerances, maintenance is required in order to recover the riding quality of that track. There are a few maintenance options for maintaining the riding quality of a railway track. Despite the maintenance actions have been taken, the quality of the track cannot go back to its original level.
2.7.1 1st option: Tamping
Settlement occurs due to the deformation in the trackbed, tamping can recover the track level by lifting the rail up, inserting tines in the trackbed and vibrating. This process rearranges the track geometry (by altering the arrangement of ballast) and correct the track level underneath in order to let the track support the rails at new levels (Thom 2009). The process is automated and can be finished very rapidly. However, the side effect of tamping is the ballast will be leave in a loose state, settlement will possibly occur after tamping and this process can damage the ballast (break into small pieces).
Figure â€Ž2â€‘10: tamping process (Selig and Waters, 1994)
Plate â€Ž2â€‘1: a typical tamping machine
2.7.2 2nd Option: Ballast cleaning or Replacement of ballast
As the track has been used for a long period of time and ballast suffers frequency tamping, the shape of the stones become not angular and more rounded; and due to large amount of ballast breakage, the size of particles become smaller. In this case, tamping is no longer working for this track because it would cause more damage to the ballast layer.
The dirt from the atmosphere or directly deposited from passing trains will get into the track, then the dirt will wash down the bottom layer until it stops at an impermeable layer, the dirt will store in the track structure and gradually build up to the top layer; or the soil from subgrade mix with the ballast, the entire track structure will sink down and soil will be pumped up by passing trains (Thom, 2009). If the contamination of the ballast layer reach a level that tamping is no longer work, ballast cleaning is therefore necessary in this situation.
Only large stones can go back to the track
Fine materials removed
Ballast cleaning is an action of renewing ballast for the track by removing ballast, screening and replacing worn and smaller size of ballast with angular and new ballast. Ballast cleaning is an expensive and slow process, so it may take delay a number of train services.
Figure â€Ž2â€‘11: a typical ballast cleaning machine
2.7.3 Other option: Ballast compaction
Dynamic Track stabiliser compact and vibrate the whole structure of ballast layer giving stability of the track. This process is normally done after the tamping when the ballast will be left in a loose state.
2.7.4 Other option: Stone blowing
Stone blowing is a process that inserting pipe under the sleepers and blow small size of stones to any spaces between the bottom of the sleepers and ballast. The advantages of this method are the ballast won't suffer any damage and no immediate settlement, but this process is expensive and has to be controlled manually.
Figure â€Ž2â€‘12: shows the process of stone blowing (Ernest T. Selig and John M. Waters, 1994)
2.7.5 Final Option: Replacement of the whole structure
When the deterioration of a railway track reaches a level that is not possible to take any maintenance actions or the expected maintenance cost and time is much greater than replacement. Then the final choice is replacement of the whole structure. This case is rarely happen in the UK but it's still possible in some cases.
2.8 Reinforcement of the trackbed
2.8.1 Geogrid Reinforcement
This investment project is mainly about the effect of geogrid made to the ballast in trackbed structure. Geogrids are made of polymeric based material which they have: 1) grids providing interlocking to granular materials and 2) strands stretched to align the long chain polymer molecules providing high tensile durability. It is mainly designed to use in the ground and reinforcing the slope. They are mainly in the form of plastic sheet which is able to cover a large area of sub-structure.
In this project , the geogrids are used for railway industry to give reinforcement to ballast. The function of geogrids is giving interlocks between grids and ballast, so that the ballast can resist higher of shear stresses and prevent them moving laterally which is a main factor of deformation in trackbed structure (Tensar international, 2009).
Figure â€Ž2â€‘13: an example of a typical piece of geogrid (TriAx shape) (Tensar TriAxTM, 2010)
Figure â€Ž2â€‘14: Showing the interlocks between ballast and grids (Tensar International, 2010)
In railway industry, the purpose of using geogrids is reducing settlement in the trackbed structure. With the use of geogrids as figure shows, the grids providing inter 000000locks between ballast so that they will not be deformed or slipped easily.
Figure â€Ž2â€‘15: Geogrid in ballast layer (Tensar International, 2010)
Geogrid located at the ballast layer can significantly reduce the rate of settlement occurred in the trackbed. The interlock at geogrid and ballast in figure 13 acts as a stiff support, so the top ballast layer will become stiffener, stronger and more resistance to deformation. (Tensar international, 2009)
Figure â€Ž2â€‘16: Geogrid in sub-ballast layer (Tensar Internation, 2010)
Geogrid can also be located in the sub-ballast layer. Its interlocking provides support on that position for sub-ballast layer to gain a higher bearing capacity, therefore the required thickness of subgrade can be reduced.
It can also be used in ground condition where the soil is weaker or less bearing capacity; The use of geogrid in this situation can reduce the resources on excavation and fill deposition (Tensar international, 2009). Geogrids can be installed in ballast layer while ballast cleaning is taking place, so that the cost of reinforcement is much less than re-construction of sub-grade.
2.8.2 Effects of geogrid reinforcement on ballast layer
Ferguson (2008) and Mitchell (2009) has examined the performance of non-reinforced and different shape of geogrid reinforced ballast with the use of large scale triaxial facility. They both agreed that the use of geogrid reinforcement can greatly reduce the long term settlement in ballast layer under repeated loading. They also found that there is a linear relationship between the aperture size of geogrid and size of sample; with larger size of ballast, it is found that using approximately the same aperture size of geogrids can effectively improve the performance. However, it is found that the geogrids give minor improvement on poor quality of ballast.
Mcdowell and Stickley (2006) used a box test to examine two samples with different strength. They found that ballast with stronger crushable resistance has been effectively improved its performance with the use of geogrids. On the other side, more crushable ballast only has minor improvement. It is also found that larger aperture size (65mm) of geogrids has better performance with the current ballast specification. They show that the use of geogrids can significantly reduce the long term settlement for the better quality of ballast. It is concluded that track maintenance (tamping) can be halved of its original frequency by using geogrids in ballast layer.
Mitchell (2009) suggests that poor quality of ballast should be continued to examine with triaxial test. It is predicted that the sample may suffer more breakage, and the geogrids possibly offer minor improvement.
Kwan (2006) developed a pull-out test to understand the interaction of ballast and geogrids. He proved that geogrids with thicker ribs with 0.5kN of surcharge give the maximum pull-out force. It is found that the use of geogrids on ballast with surcharge give better grids interlocking.
It is concluded that the use of geogrids reinforcement in railway trackbed can effectively reduce the long term settlement. However, different type/aperture size/tensile strength of geogrids give various of improvement to ballast layer. In this project, it is tried to find out the suitable type of geogrids that give the best improvement to poor quality of ballast. A new type of geogrids will be introduced in triaxial test and pull-out test. These new geogrids have diamond shape aperture shape and have larger aperture size. It is used to compared the performance with TriaX shape geogrids.
Track improvement will be briefly discussed for a particular type of geogrid, also the behaviour of reinforced granular materials under repeated loading.