Ballast & Subgrade Preparation Flashcards

What is the primary function of ballast in railroad track construction?

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.

Which material is the preferred choice for mainline railroad ballast?

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.

What term describes ballast that has become contaminated with fines and degraded particles, reducing its drainage capacity?

Answer: Fouled ballast

Fouled ballast contains excessive fines that fill void spaces, reducing drainage capacity and load-bearing performance below acceptable standards.

What is the subgrade in railroad track construction?

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.

What is the purpose of a subballast layer in railroad track construction?

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.

What is 'pumping' as it relates to railroad track subgrade problems?

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.

What type of soil is generally considered most problematic as a subgrade material for railroad track?

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.