Track Settlement Flashcards

1
Q

Where is the majority of settlement from in:
a) old embankments
b) new embankments

A

a) track (ballast)
b) track + embankment + foundation

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2
Q

Give a description of how geomaterials will settle due to the plastic movement of soil grains

A
  • The (granular) soil densifies under repeated loading
  • The void ratio reduces as particles slide past each other into a new physical state
  • Settlement occurs
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3
Q

Why is subgrade settlement really significant in new rail lines?

A

Because the subrade has not experienced cyclic loading at the stress level (axle loads) from the trains

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4
Q

What is the term that defines what can happen if the track is not compacted/not corrected using tampers?

A

Track roughness - high track roughness eventually can compromise track stability

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5
Q

What type of material are modern-day embankments built using?

A

Highly granular material, which are compacted and stiff

NB. both subballast and ballast are both granular soils

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6
Q

Define normally consolidated soils

A

Current stress levels are at the highest ever

This can lead to settlement

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7
Q

Define over consolidated soils

A

Current stress levels are lower than the highest ever

The soil is stronger and stiffer

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8
Q

Give an overview of how tampers can correct track geometry

A
  • The sleepers are lifted to the required level
  • Tamping tines are inserted into the ballast and vibrate
  • At the same time, the tines squeeze to push the ballast under the railhead
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9
Q

How often does a typical (ie. not on switch/crossing) track require tamping?

Why does tamping have to be done regularly?

A

Once a year

As the train loading cycles start again, the ballast densifies and track geometry becomes rough

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10
Q

What can occur following tamping, and what are the consequences?

A
  • Ballast breakage
  • Increases stress on formation
  • Can cause a generation of fines, which migrate to the base of the ballast and block drainage
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11
Q

What can happen at track transition points?

A

Hanging sleepers can occur (sleeper suspended in the air)

Can impact the ballast significantly when trains pass

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12
Q

Define resilient modulus

What is it dependent on?

A
  • A state reached after a high degree of cyclic loading
  • Key parameter when determining track stiffness
  • Depends on the stress/axial load, as well as angularity and compaction
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13
Q

What does this equation represent, and what testing is used?

A

The resilient modulus

Conventional cyclic triaxial test

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14
Q

What factors is the resilient modulus dependent on for granular soils?

A
  • Stress state (magnitude increases with confining pressure)
  • Magnitude of repeated deviator stress
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15
Q

What factors is the resilient modulus dependent on for cohesive soils?

A
  • Stress state (including magnitude of deviator stress)
  • Soil type (structure, compaction)
  • Physical state (e.g. moisture content)
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16
Q

What is a key parameter when determining track stiffness, which can reduce overall track stiffness?

A

Subgrade resilient modulus

17
Q

Define threshold stress

A

The value of deviator stress that separates the two ‘groups’ (stable/unstable)

18
Q

What is the threshold stress for a cohesive soil (ie. up to what point is the soil stable, and when does it quickly go unstable)?

A
  • Stable up to 65 kN/m^2 (deformation rate reduces with increasing load cycles)
  • Quickly becomes unstable at 70 kN/m^2 (deformation rate increases with increasing load cycles)
19
Q

What is threshold stress a function of?

What happens if the induced stress in the subgrade layers is:
a) less than the threshold stress
b) greater than the threshold stress

A
  • Linear function of the effective confining pressure
  • Plastic strain at each layer eventually levels out, and track settlement eventually levels out (NB. may then require tamping)
  • Plastic strain at each layer continues to increase; leading to failure of subgrade and high track settlement
20
Q

Define cumulative plastic deformation

A

The total deformation due to repeated loading

21
Q

Name two test facilities for simulating track settlement/ investigating cumulative plastic deformation

A
  • GRAFT 1 (single axle, no principle stress rotation)
  • GRAFT 2 (multiple phased axles, principle stress rotation)
22
Q

Why is non-uniform track settlement a problem?

A

It can cause differential settlement, which can lead to an increase in track roughness with usage

The majority of track settlement is not uniform

23
Q

What does this equation represent?

A

The fundamental settlement equation for the ballast

For mixed loading conditions, need to use separate versions of the equation (as settlement is non-linear with cycles N for different wheel loads)

Same equation as for the subballast

24
Q

What happens to the ballast with loading?

A

The ballast degrades (attrition), with ballast fines accumulating in the voids

25
What can happen to the ballast if it becomes saturated?
The rate of **plastic deformation** increases markedly
26
What can subballast behaviour be assumed to be?
Linear
27
What type of soil is a typical subgrade made of?
**Fine grained** soils
28
What type of soils (NC or OC) are more resistant to pore water pressure accumulation? Why does pore water pressure accumulation occur?
**Over-consolidated** soils PWP accumulation occurs due to plastic compressive deformation (**compaction**) under repeated loading
29
When does cyclic softening occur? What happens if pore water pressures can dissipate and loading is not too high?
Cyclic softening occurs if **pwp increases significantly**, and leads to rapid and large **plastic strain** accumulation If pwps can **dissipate** and loading is not too high, the soil will become stronger (**compact**) with increasing cycles
30
What does this equation represent?
Plastic accumulation for a fine-grained soil (**subgrade**) NB. in previous questions, A (intercept) and B (gradient) just given in question - A is usually in the range 0.0005% to 6.3%
31
What does this equation represent?
The Li-Selig model for cumulative plastic deformation (for **fine-grained soils***) NB. strength_s is the soil static strength
32
How do you apply the subgrade equation?
- Split the clay depth into a series of **n layers** - Calculate the **deviator stress** at the centre of each layer under a given wheel load - Then, can produce a table for settlement of subgrade
33
What is the equation for the total track settlement?
34
What wavelength track fault (ie. long/short) gives a greater peak-to-peak amplitude, to provide an acceptable passenger ride?
Long wavelength
35
Why are the acceptable values for peak-to-peak amplitude very low on high-speed lines? Where can these acceptable values be found?
As track roughness will make the train and passengers experience unacceptably high **vertical accelerations** These acceptable values can be found in the Track Quality Standards
36
How can track quality be measured? What parameters are measured?
Using a track recording vehicle (**TRV**) Many parameters are measured, including **vertical geometry** and **lateral alignment**
37
What does this equation represent, and what does each value mean?
The roughness standard deviation (SD) settlement, which can be used to calculate track roughness