Track Settlement

Question 1

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

Answer 1

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

Question 2

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

Answer 2

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

Question 3

Why is subgrade settlement really significant in new rail lines?

Answer 3

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

Question 4

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

Answer 4

Track roughness - high track roughness eventually can compromise track stability

Question 5

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

Answer 5

Highly granular material, which are compacted and stiff

NB. both subballast and ballast are both granular soils

Question 6

Define normally consolidated soils

Answer 6

Current stress levels are at the highest ever

This can lead to settlement

Question 7

Define over consolidated soils

Answer 7

Current stress levels are lower than the highest ever

The soil is stronger and stiffer

Question 8

Give an overview of how tampers can correct track geometry

Answer 8

• 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

Question 9

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

Why does tamping have to be done regularly?

Answer 9

Once a year

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

Question 10

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

Answer 10

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

Question 11

What can happen at track transition points?

Answer 11

Hanging sleepers can occur (sleeper suspended in the air)

Can impact the ballast significantly when trains pass

Question 12

Define resilient modulus

Answer 12

• 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

Question 13

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

Answer 13

The resilient modulus

Conventional cyclic triaxial test

Question 14

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

Answer 14

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

Question 15

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

Answer 15

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

Question 16

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

Answer 16

Subgrade resilient modulus

Question 17

Define threshold stress

Answer 17

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

Question 18

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)?

Answer 18

• 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)

Question 19

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

Answer 19

• 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 increases; leading to failure of subgrade and high track settlement

Question 20

Define cumulative plastic deformation

Answer 20

The total deformation due to repeated loading

Question 21

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

Answer 21

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

Question 22

Why is non-uniform track settlement a problem?

Answer 22

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

The majority of track settlement is not uniform

Question 23

What does this equation represent?

Answer 23

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

Question 24

What happens to the ballast with loading?

Answer 24

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

Question 25

What can happen to the ballast if it becomes saturated?

Answer 25

The rate of plastic deformation increases markedly

Question 26

What can subballast behaviour be assumed to be?

Answer 26

Linear

Question 27

What type of soil is a typical subgrade made of?

Answer 27

Fine grained soils

Question 28

What type of soils (NC or OC) are more resistant to pore water pressure accumulation?

Why does pore water pressure accumulation occur?

Answer 28

Over-consolidated soils

PWP accumulation occurs due to plastic compressive deformation (compaction) under repeated loading

Question 29

Define cyclic softening

Answer 29

Cyclic softening occurs if pwp increases significantly, and leads to rapid and large plastic strain accumulation

NB. if pwps can dissipate and loading is not too high, the soil will become stronger (compact) with increasing cycles

Question 30

What does this equation represent?

Answer 30

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%

Question 31

What does this equation represent?

Answer 31

The Li-Selig model for cumulative plastic deformation (for fine-grained soils*)

NB. strength_s is the soil static strength

Question 32

How do you apply the subgrade equation?

Answer 32

• 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

Question 33

What is the equation for the total track settlement?

Answer 33

Question 34

What wavelength track fault (ie. long/short) gives a greater peak-to-peak amplitude, to provide an acceptable passenger ride?

Answer 34

Long wavelength

Question 35

Why are the acceptable values for peak-to-peak amplitude very low on high-speed lines?

Where can these acceptable values be found?

Answer 35

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

Question 36

How can track quality be measured?

Answer 36

Using a track recording vehicle (TRV)

NB. many parameters are measured, including vertical geometry and lateral alignment

Question 37

What does this equation represent, and what does each value mean?

Answer 37

The roughness standard deviation (SD) settlement, which can be used to calculate track roughness