Track Settlement Flashcards
Where is the majority of settlement from in:
a) old embankments
b) new embankments
a) track (ballast)
b) track + embankment + foundation
Give a description of how geomaterials will settle due to the plastic movement of soil grains
- The (granular) soil densifies under repeated loading
- The void ratio reduces as particles slide past each other into a new physical state
- Settlement occurs
Why is subgrade settlement really significant in new rail lines?
Because the subrade has not experienced cyclic loading at the stress level (axle loads) from the trains
What is the term that defines what can happen if the track is not compacted/not corrected using tampers?
Track roughness - high track roughness eventually can compromise track stability
What type of material are modern-day embankments built using?
Highly granular material, which are compacted and stiff
NB. both subballast and ballast are both granular soils
Define normally consolidated soils
Current stress levels are at the highest ever
This can lead to settlement
Define over consolidated soils
Current stress levels are lower than the highest ever
The soil is stronger and stiffer
Give an overview of how tampers can correct track geometry
- 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
How often does a typical (ie. not on switch/crossing) track require tamping?
Why does tamping have to be done regularly?
Once a year
As the train loading cycles start again, the ballast densifies and track geometry becomes rough
What can occur following tamping, and what are the consequences?
- Ballast breakage
- Increases stress on formation
- Can cause a generation of fines, which migrate to the base of the ballast and block drainage
What can happen at track transition points?
Hanging sleepers can occur (sleeper suspended in the air)
Can impact the ballast significantly when trains pass
Define resilient modulus
What is it dependent on?
- 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
What does this equation represent, and what testing is used?
The resilient modulus
Conventional cyclic triaxial test
What factors is the resilient modulus dependent on for granular soils?
- Stress state (magnitude increases with confining pressure)
- Magnitude of repeated deviator stress
What factors is the resilient modulus dependent on for cohesive soils?
- Stress state (including magnitude of deviator stress)
- Soil type (structure, compaction)
- Physical state (e.g. moisture content)
What is a key parameter when determining track stiffness, which can reduce overall track stiffness?
Subgrade resilient modulus
Define threshold stress
The value of deviator stress that separates the two ‘groups’ (stable/unstable)
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)?
- 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)
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
- 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
Define cumulative plastic deformation
The total deformation due to repeated loading
Name two test facilities for simulating track settlement/ investigating cumulative plastic deformation
- GRAFT 1 (single axle, no principle stress rotation)
- GRAFT 2 (multiple phased axles, principle stress rotation)
Why is non-uniform track settlement a problem?
It can cause differential settlement, which can lead to an increase in track roughness with usage
The majority of track settlement is not uniform
What does this equation represent?
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
What happens to the ballast with loading?
The ballast degrades (attrition), with ballast fines accumulating in the voids
What can happen to the ballast if it becomes saturated?
The rate of plastic deformation increases markedly
What can subballast behaviour be assumed to be?
Linear
What type of soil is a typical subgrade made of?
Fine grained soils
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
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
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%
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
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
What is the equation for the total track settlement?
What wavelength track fault (ie. long/short) gives a greater peak-to-peak amplitude, to provide an acceptable passenger ride?
Long wavelength
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
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
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
