CTB Ballast & Subgrade Preparation 3

Question 1

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

Accepted Answer

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

Answer

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

Question 2

What does density testing of compacted subgrade material measure?

Accepted Answer

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

Answer

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.

Question 3

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

Accepted Answer

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

Answer

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

Question 4

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

Accepted Answer

Properly sized and positioned drainage ditches alongside the track

Answer

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

Question 5

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

Accepted Answer

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

Answer

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

CTB - Certified Track Builder Practice Test

CTB - Certified Track Builder Practice Test

CTB Ballast & Subgrade Preparation 3