200703221758歐洲高鐵軌道系統考察報告﹝二﹞

 

歐洲高鐵軌道系統考察報告﹝二﹞

(This is Attachment No.3 of 歐洲高鐵軌道系統考察報告﹝一﹞. The article is extracted from a certain internal report of SIEMENS Company, Germany, 1997)

TRACK SYSTEM

(BALLASTED TRACK vs. FIXED TRACK)

 

1.          Introduction

As an alternative to common ballasted tracks, slab tracks were developed to meet the requirements for combined high-speed and heavy-load traffic.

This construction method allows replacing the old ballast layer by a load-spreading concrete base-plate.

The common ballasted track does not ensure stable track-position under present and expected future conditions with an acceptable amount of servicing.

Numerous tests concerning the comparison between fixed track and ballasted track have been made.

The tests showed that the ballasted track enables a more cost effective construction while the maintenance is costly, the fixed track construction is more expensive but reduces the maintenance costs.

One reason for the cost-exceeding of the fixed track for example is that the development of slab track constructions still on the way.

 

2.          System “Ballasted Track”

The ballasted track is a tried and proven mature technology and is commonly used in major railway systems. The accuracy and permanence of track geometry is sufficient. The track resilience is hardly to exceed. The system will be designed to have a minimum life of 40 years, if the velocity of trains doesn’t exceed 160km/h.

Economical effects on the civil work portion concerning the necessity of an eventually slightly increased tunnel diameter by using the ballasted track should be considered due to greater execution height as well as larger cross sections of the structure beams due to its additional weight.

 

2.1    System Components

2.1.1        Ballast :

The ballast distributes transversal and longitudinal forces, damps rail-borne vibrations and drains occurring surface water.

It’s highly resistant against crushing, grinding and pounding. The material will be weather resistant and of a homogeneous structure.

The minimum height of the ballast is 30 cm between soil and sleeper-bottom.

 

2.1.2        Wooden Sleeper and Fastening :

Wooden sleepers and ballasted track is a common combination in track laying. These sleepers are hardwood-made (e.g. oak) and capable for recreation, when worn-out. The sleepers are impregnated against rottenness and parasitic infestation.

The sleepers will be designed for load characteristics consistent with the design of the passenger and service vehicles and with the performance requirements of the system. For increasing the horizontal track stability, sleeper anchors can be used.

The fastening system ensures permanent tension by means of elastic spring clips. All fastening components are already preassembled onto the sleeper, so the complete sleeper can be laid mechanically.

For quick replacement of the rail, tension clamps are easy to remove in preassembled position.

 

2.1.3        Concrete Sleeper and Fastening :

Concrete prestressed monoblock sleepers form the support for the ballasted track. They are designed to have a minimum life of 50 years.

The sleepers are used on ballasted track and designed for load charactistics consistent with the design of the passenger and service vehicles and with the performance requirements of the system hardwood timber sleepers. In addition, concrete sleepers are also foreseen to be equipped with anchors.

The rail fastening is an elastic system, which is capable of attenuation of rail borne vibrations. This consists of two parts :

l          the elastomeric pad

l          the elastic fastening

The rail rests directly on the concrete sleeper with only a plastic pad in between. The position of the rail is fixed by plastic angled guide plates. Positive permanent tightening between rail and sleeper is achieved by using tension clamps. Sleeper screws are inserted in plastic dowels, which have been preassembled while fabricated into the concrete sleeper.

 

2.2    Construction Operation

Installation :

The ballasted track system is a high-mechanized construction. This ensures low costs and labour expenditure.

Low sensitivity to defects of fabrication and adaptability towards extrinsic impacts are further characteristics of this system. Subsequent execution of cable crossings under the rails are easy to execute.

Moreover, the risk of rail-heating due to the eddy-current-brake and the problem of ballast turbulence require separate assessments.

 

2.3    Maintenance Aspects :

The removal of components in the ballasted track is easier than in the fixed track. In addition the ballasted track allows an easy realignment and adjusting in case of different settlements. Like installation, maintenance is also to a high grade mechanized. This allows immediate execution and cost saving.

However, the wear of ballast requires a great deal of monitoring and control, which expresses shape and quantity as well as distribution of new ballast when necessary. The resistance to lateral displacement in case of lateral acceleration is limited.

 

3.          System “Fixed Track”

Slab tracks are being used more frequently in order to reduce unnecessary maintenance work, such as tamping and lining of tracks, especially in difficult environments such as tunnels and viaducts. Fixed track ensures a stable track, high availability and smooth riding quality at high speed. The approximately service duration runs up to 60 years.

In fixed tracks, the elasticity of the ballast bed has to be simulated. In order to produce the same conditions with regard to vertical elasticity, elastic elements have to be inserted in the rail fastening system, which permit a vertical subsidence. Lateral forces are deflected by means of angular guide plates.

 

3.1    System Components

3.1.1        Slab Track :

The system is principally based on reinforced concrete plinths with linked stand-ups and fixed single rail. These plinths are frictionally connected to the prestressed concrete base plate by means of T-head bolts.

This system allows a lower construction height combined with a saving of weight relating to the design of tunnels and viaducts.

The fixed track shows technical advantages with a view to a higher ride comfort, the unlimited use of the advantageous eddy-current brake as well as the non-existence of ballast turbulence problems.

 

3.1.2        Fastenings :

To reduce the vibration and sound emission, a variable-height fastening system with tension-clamps will be installed.

Liquid grout or epoxy mortar is used to achieve correct vertical alignment of the track of the substructure. Height adjustment shims are used to assist in setting the track to line and level. Injection supported anchor bolts ensure a stable fixation between fastening and concrete plinth. This system allows simple rail transposing, rail changing or distressing in the future.

 

3.2    Construction Operation

Installation :

Because of its less in weight as well as in height, fixed track system enables to create viaduct cross-sections smaller, respectively tunnel diameters more narrow.

Otherwise, because of its little adaptability to setting and to subsidence of the subsoil, the fixed track system shows high demands to soil testing. Another problem is the transition to ballasted track. Due to different elastic modules, the difference in settling amounts to 0.5 mm. This implies damage to the rail joint.

In order to prevent stray currents, full insulation between rail and concrete track bed has to be foreseen.

There is only limited possibility of automisation and regulation, but a considerable potential of mechanization is on the way.

 

3.3    Maintenance :

Fixed track ensures a high operational availability due to long lasting accuracy of track geometry and excellent long-time behavior of component parts.

High resistance to lateral and longitudinal displacement permits a prevention of lateral buckling and rail distortion.

Full track maintenance, as needed by ballasted track, is unnecessary. However, there is only a limited possibility of automization and regulation.

In case of derailment and accidents, there is an easier access by road and saving vehicles.

 

4.          Comparison Table :

Component

Unit

Fixed Track

Ballasted Track

Quantity

Kg/m-Track

Quantity

Kg/m-Track

Rail UIC 60

60 kg/m

2x1 m

120

2x1 m

120

Fastening

50 kg/st

2x1.42 p/m

142

2x1.67 p/m

167

Concrete B35

2500 kg/m3

2x0.2822m3/m

1411

 

 

Sleeper

282 kg/piece

 

 

1.67 p/m

470

Ballast

1650 kg/m3

 

 

2.065 m3/m

3407

Total :

 

 

1673

 

4164

 

5.          Advantages and Disadvantages

5.1    Advantages of Ballasted Track :

l          Good noise and vibration reduction

l          Possibility of later adjusting by different consolidations

l          To a high degree mechanized

l          Greater tolerances in erection are possible

l          Better distribution of the forces

l          Easy execution of cable crossings under the rails

l          Proved, reliable construction

 

5.2    Disadvantages of Ballasted Track :

l          Greater execution height

l          Higher maintenance costs

l          Limited resistance to lateral forces

l          Little long time stability

l          Destruction of ballast when a velocity of 160km/h is exceeded

 

5.3    Advantages of Fixed Track :

l          High availability

l          Stable track bed

l          Quality of smooth riding

l          Low maintenance cost (approx. 0.5% p.a.)

l          High super elevation possible

l          Low construction height

l          Unlimited use of eddy-current brake

l          Can withstand forces by velocities over about 160km/h

 

5.4    Disadvantages of Fixed Track :

l          High investment cost

l          High subsoil stability required

l          Extensive soil testing

 

6.          Conclusions

Fixed track is always the better alternative concerning to installation on viaducts. The reduce of weight, compared to ballasted track, allows to decrease the static demand to the construction and helps so to minimize the visual impression of the building.

All these features may imply, however, a higher investment in track laying but there is also the considerable saving in maintenance.

The decision which system to take is influenced by initial cost and the level of maintenance during operations. If soil conditions are stable (settling<60mm) or if tectonic activity is not expected, fixed track may be an option worth considering. Otherwise, the groundwater table should be at least 1.5m below the upper surface of the rails.

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I, a sincere and diligent man, was born in Nanjing, China and grew up in Kaohsiung and Taipei, Taiwan.
I graduated from a certain university in Taiwan and earned a master degree in civil engineering from an American University. I have ever been a civil engineer for more than thirty-eight years which including 5 year experience of bridge maintenance in the Kingdom of Saudi Arabia. Fortunately I had joined some of very famous bridge construction projects in Taiwan, such as Guan-Du highway bridge in Taipei and Bih-Tan bridge of second Freeway in Hsin-Tien city. In the years from 1997 to 2009 I had worked with Taiwan High Speed Rail, which has ever been the biggest BOT project in the world. Recently since May of 2010 until the end of 2013 I joined in China High Speed Rail project with a Germany consultant company (DB International) in Zhejiang province.
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