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

Excessive rail lubrication applied to curves

Answer

Using too many fasteners per tie during installation

Answer

Selecting an incorrect rail weight for the traffic level

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

The weight of the rail in pounds per yard

Answer

The number of ties installed per track mile

Answer

The concentration of ballast particles per square foot of track bed

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

Ice forming on the rail surface causing wheel slip on locomotives

Answer

Frost damage to wooden ties causing splitting and cracking of the wood

Answer

Ice accumulation in switch points preventing proper operation during winter

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

Rail lubricators installed on curves to reduce wheel wear

Answer

Reflective delineators installed along the right-of-way boundary

Answer

Positive train control (PTC) systems for automated train management

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

Cleaning mud from fouled ballast using high-pressure water injection

Answer

Injecting grout beneath cracked concrete ties to prevent further fracturing

Answer

Applying anti-corrosion compound to switch point mechanisms

Question 6

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

Accepted Answer

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

Answer

To install temporary drainage pipes during the construction phase

Answer

To place explosives for removing rock obstructions from the alignment

Answer

To install anchor bolts for bridge abutment foundations

Question 7

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

Accepted Answer

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

Answer

Install heavier rail to compensate for the poor subgrade bearing conditions

Answer

Increase train operating speed to minimize the duration of load on each weak spot

Answer

Reduce ballast depth to lower the dead weight imposed on the weak subgrade