CTB Ballast & Subgrade Preparation 2 — Questions and Answers
Question 1: What is the primary purpose of undercutting ballast during track maintenance?
- To lower the track profile below its original elevation
- To remove fouled ballast from beneath the ties and replace it with clean ballast (Correct answer)
- To install new ties without disturbing the existing rail
- To compact the subgrade through vibration from the undercutter machine
Correct answer: To remove fouled ballast from beneath the ties and replace it with clean ballast
Undercutting removes contaminated fouled ballast from under and around ties, restoring proper drainage and structural load support to the track section.
Question 2: What does the term 'surfacing' mean in track maintenance and construction?
- Painting the top of the rail with a protective coating
- Installing a paved surface between rails at road crossings
- Lifting and tamping the track to restore correct grade, cross level, and surface geometry (Correct answer)
- Cleaning the ballast surface with power washing equipment
Correct answer: Lifting and tamping the track to restore correct grade, cross level, and surface geometry
Surfacing refers to the process of raising the track to proper grade and eliminating surface irregularities using ballast tamping and track lifting equipment.
Question 3: Which property of ballast is most important for ensuring good drainage performance?
- Color and mineral composition
- Particle angularity and interconnected void space (porosity) (Correct answer)
- Weight per cubic foot of the ballast material
- Chemical reactivity with rail steel components
Correct answer: Particle angularity and interconnected void space (porosity)
Angular ballast particles create large, interconnected void spaces that allow water to drain rapidly away from the track structure.
Question 4: What is the purpose of geotextile fabric when installed in a railroad track structure?
- To provide electrical insulation between the rails and subgrade
- To replace the need for ballast in light-rail applications
- To act as a separator and filter between ballast and subgrade, preventing particle migration (Correct answer)
- To waterproof the top surface of the subgrade against all moisture
Correct answer: To act as a separator and filter between ballast and subgrade, preventing particle migration
Geotextile fabric separates the ballast from the subgrade while allowing water to pass through, preventing fine particles from migrating upward into the ballast layer.
Question 5: What equipment is typically used to compact newly placed subgrade fill material in track construction?
- Ballast tamping machine (tamper)
- Rail anchor applicator machine
- Vibratory roller or plate compactor (Correct answer)
- Rail saw and magnetic drill press
Correct answer: Vibratory roller or plate compactor
Vibratory rollers and plate compactors apply dynamic compaction energy to achieve the required density in subgrade fill materials.
Question 6: What is a 'ballast section' as used in track engineering drawings?
- The cross-sectional shape and dimensions of the ballast layer, including depth and shoulder width (Correct answer)
- The portion of the project budget allocated to ballast material procurement
- A section of track where ballast has been removed for subgrade inspection
- The specification sheet listing approved ballast material suppliers
Correct answer: The cross-sectional shape and dimensions of the ballast layer, including depth and shoulder width
The ballast section defines the required cross-sectional geometry including ballast depth under ties, crib width between ties, and ballast shoulder width.
Question 7: What is the minimum FRA-required ballast depth under the base of ties for Class 4 track operations per 49 CFR Part 213?
- 4 inches
- 6 inches
- 8 inches
- 12 inches (Correct answer)
Correct answer: 12 inches
FRA Track Safety Standards (49 CFR Part 213) require a minimum ballast depth of 12 inches under the base of ties for Class 4 and Class 5 track.
CTB Ballast & Subgrade Preparation 2
What is the primary purpose of undercutting ballast during track maintenance?