CTB Ballast & Subgrade Preparation 1 — Questions and Answers
Question 1: What is the primary function of ballast in railroad track construction?
- To provide a decorative appearance for the track right-of-way
- To prevent rail from expanding due to thermal changes
- To distribute loads, provide drainage, and maintain track position (Correct answer)
- To lubricate the interface between rails and wheel flanges
Correct answer: To distribute loads, provide drainage, and maintain track position
Ballast distributes vertical loads from ties to the subgrade, provides drainage, and anchors the track against lateral and longitudinal movement.
Question 2: Which material is the preferred choice for mainline railroad ballast?
- River sand
- Gravel and river cobbles
- Recycled concrete aggregate
- Granite or trap rock (crushed stone) (Correct answer)
Correct answer: Granite or trap rock (crushed stone)
Crushed granite or trap rock provides angular particles that interlock well, resist degradation under repeated loading, and allow excellent drainage.
Question 3: What term describes ballast that has become contaminated with fines and degraded particles, reducing its drainage capacity?
- Fouled ballast (Correct answer)
- Spent ballast
- Depleted ballast
- Soft ballast
Correct answer: Fouled ballast
Fouled ballast contains excessive fines that fill void spaces, reducing drainage capacity and load-bearing performance below acceptable standards.
Question 4: What is the subgrade in railroad track construction?
- The layer of ballast directly under the ties
- The prepared native soil or engineered fill on which the track structure is built (Correct answer)
- The layer of asphalt placed between ballast and native soil
- The concrete base used under slab track systems
Correct answer: The prepared native soil or engineered fill on which the track structure is built
The subgrade is the prepared natural ground or compacted fill that forms the foundation of the track structure below the ballast and subballast layers.
Question 5: What is the purpose of a subballast layer in railroad track construction?
- To anchor rail spikes firmly into the track bed
- To support overhead electrical wiring systems
- To provide a walking surface for track maintenance workers
- To separate ballast from subgrade and improve load distribution and drainage (Correct answer)
Correct answer: To separate ballast from subgrade and improve load distribution and drainage
Subballast acts as a filter and separator between ballast and subgrade, distributes loads more uniformly, and prevents subgrade material from migrating upward into the ballast.
Question 6: What is 'pumping' as it relates to railroad track subgrade problems?
- Using water jets to clean fouled ballast in place
- Installing drainage pumps along the track right-of-way
- The upward ejection of wet, fine-grained subgrade material through ballast when trains pass (Correct answer)
- Spraying water on ballast to aid compaction during installation
Correct answer: The upward ejection of wet, fine-grained subgrade material through ballast when trains pass
Pumping occurs when train loads force saturated fine-grained subgrade material upward through ballast voids, causing progressive track degradation and instability.
Question 7: What type of soil is generally considered most problematic as a subgrade material for railroad track?
- Well-graded gravel (GW)
- Expansive high-plasticity clay (CH) (Correct answer)
- Clean poorly-graded sand (SP)
- Dense limestone bedrock
Correct answer: Expansive high-plasticity clay (CH)
Expansive high-plasticity clay swells when wet and shrinks when dry, causing seasonal heave and settlement that destabilizes the track structure.
CTB Ballast & Subgrade Preparation 1
What is the primary function of ballast in railroad track construction?