Ballast & Subgrade Preparation Flashcards

What is the primary cause of differential settlement in a railroad track subgrade?

Answer: Inconsistent soil density, moisture content, or composition beneath the track

Differential settlement occurs when areas of varying soil density, moisture, or composition compress at different rates under load, creating an uneven track profile.

What does density testing of compacted subgrade material measure?

Answer: The in-place dry density of compacted soil compared to its laboratory-determined maximum

Density testing (using nuclear gauge or sand cone methods) verifies that compacted fill has achieved the required percentage of the laboratory-determined maximum dry density.

What is 'frost heave' and how does it affect railroad track?

Answer: The upward displacement of track caused by freezing water expanding in frost-susceptible subgrade soils

Frost heave occurs when water in frost-susceptible soils freezes and expands, pushing the track upward unevenly and creating surface and alignment geometry defects.

Which drainage feature is most critical for protecting the integrity of the track subgrade?

Answer: Properly sized and positioned drainage ditches alongside the track

Drainage ditches channel water away from the track structure, preventing saturation of the subgrade that leads to instability, pumping, and frost damage.

What is 'pressure grouting' (mud jacking) used for in track subgrade rehabilitation?

Answer: Filling voids and stabilizing weak or settled subgrade by injecting cementitious material under pressure

Pressure grouting fills voids and strengthens weak subgrade by injecting a cementitious slurry under pressure to consolidate and stabilize the soil beneath the track.

When performing subgrade investigation for new track construction, what is the primary purpose of soil borings?

Answer: To identify soil types, bearing capacity, and groundwater depth at the construction site

Soil borings provide samples and data about subsurface soil types, strength characteristics, and groundwater conditions needed to properly design the track substructure.

What is the recommended approach when railroad track must be constructed over soft, organic, or highly compressible soils?

Answer: Stabilize or replace the weak soil, use geosynthetics, or design a special structural system

Soft or organic soils must be stabilized, replaced with engineered fill, reinforced with geosynthetics, or addressed with special structural designs to achieve acceptable long-term track performance.