Ballast & Concrete Slab Track (brief) Flashcards
What is the difference between track alignment and track geometry?
Track alignment describes the line uniformity
- e.g. how long, how much does it rise/fall
Track geometry defines the vertical and horizontal 3D track location
- e.g. does it curve
Describe the plan view of a two-track railway
What does a high-speed track design require?
- A design alignment that gives good vehicle dynamic behaviour (for passenger comfort)
- Limited direction changes to control vertical and lateral accelerations
What type of material is ballast and subballast?
Ballast: highly granular, typically granite in UK
Subballast: granular, typically sand in UK
What are the functions of ballast?
- Supports sleepers in different directions
- Transmits and reduces forces (most stress dissipation is from the ballast)
- Can correct track geometry, because it is unbound
- Drainage
- Damping
- Improves track stiffness
What is used for ballast compaction (name two)?
Vibrating plate
Dynamic track stabilizer
Give a summary of what tampers do
- Correct track geometry
- Tines (vibrating rods) move ballast directly under the sleepers, filling the voids
Give a summary of what stoneblowers do
- Correct track geometry
- Blows new ballast below the track
- Less damaging to ballast than tampers
What are the functions of subballast?
- To further reduce the subgrade bearing pressure
- Prevent mixing of ballast and subgrade; preventing subgrade attrition by the ballast
- Frost protection
- Preventing wetbed formation
What is the top of the subgrade layer called?
The formation
What material does the subgrade consist of?
Either rock or soil
Modern-day embankments consist of granular soil
What are the functions of the subgrade?
- To provide adequate support to the track foundation
- Be capable of supporting induced stresses and not experiencing excessive settlement
- Drainage and must withstand harsh environmental conditions
Why is uniform stiffness required in the subgrade (describe the process)?
- If the subgrade isn’t uniform, the track stiffness also won’t be
- If not uniform, oscillations will set up in the train suspension system; increasing track wear (and reducing ride comfort); inducing settlement and generating track irregularities
Where does most of the stress dissipation come from?
The ballast
Where does most of the track deflection come from?
The subgrade
Name two types of track failure for ballasted track
- Subgrade progressive shear failure
- Ballast pocket formation
Give an overview of subgrade progressive shear failure, and how to prevent it from happening
- Bearing capacity failure due to overstressing of the formation; resulting in shear failure (of formation and subgrade)
- Causes sleeper to tilt
- To prevent, increase subgrade strength and reduce load
Give an overview of ballast pocket formation, and what the solution to the failure is
- Pre-shear failure
- High strains cause the formation of a pocket, after plastic movement of foundations
- ‘Cyclic softening’ happens after pocket fills with water (as failure pocket is reinforced)
- Results in further settlement and pocketing, and weted formation
- Solution is to ‘dig the whole thing out’, as tampers can only temporarily fix
Name two types of slab track
Prestressed:
- improved cracking performance
PACT system (paved concrete track):
- better drainage, used in tunnels
NB. slab track can be preformed or continuous poured in-situ
Name a downside of slab track
If there is lots of cracking, it is very expensive to replace
Describe the different layers of slab track (with sleeper)
Describe how the deformation modulus/stiffness changes through the slab track layers
What does this help prevent?
The stiffness is stepped-down
This controls deformation, prevents cracking and controls the distribution of stiffness onto the subgrade
Name a type of prefabricated slab track, and describe its features
The OBB-Porr system:
- top-down
- no additional devices needed for adjustment of the mutual rail position
- used in HS2 tunnels
- grout layer formed between base slab and top prefabricated slab
- rubber mats underneath absorb vibrations
- in derailments, track slab usually remains intact
What type of slab track would you use for trams?
Embedded rail structure (ERS)
E.g. Balfour Beatty Rail System
When would concrete slab track be used?
For ‘high-fixity’ solutions:
- able to provide an ‘absolute gauge clearance’
- track won’t move out of line/level (e.g. tunnel bore dimensions can be reduced)
- often used in bridges and tunnels
What does slab track offer that ballasted track doesn’t?
A greater degree of trackbed stability
What is a downside of slab track?
Produces more radiated noise, therefore noise absorption features may be needed
