water pockets forming: the ballast is soon compressed under the ties. And when a rail line is
either new or recently repaired, water pockets soon form for the same reason, particularly if
traffic over the line is heavy.
(4, In freezing weather. If water pockets are allowed to remain until winter, freezing
temperatures cause ice to form in the pockets. As the ice expands, a heaving roadbed soon
appears. The uneven track can be leveled only by adding shims, bearing pieces used to raise
rails, at the low spots. Shims are placed either between the tie and the tie plate or between the tie
and the rail. Draining the water pockets has to wait until the spring thaw. The amount of labor
and material needed at this stage is much greater than that required earlier to prevent or eliminate
the water pockets.
2.11. CORRECTING WATER POCKETS
Water pockets are simply another drainage problem. And they can be a continual
aggravation where the subsoil's stability or the track's location makes drainage a serious problem.
For example, a wet cut is typical of such locations.
The classic solution to the water-pocket problem is to construct a cross drain from the
location of the pocket to a side ditch that carries the water away from the roadbed. Now that the
water has a way out, the problem is solved. Cross drains are discussed in paragraph 2.15 and
shown in figures 2.8 and 2.9. Side ditches are discussed in paragraph 2.14 and an example of
one is shown in figure 2.5. Grouting, another method used to correct a water pocket, is discussed
in paragraph 2.15d and illustrated in figure 2.10.
2.12. OBTAINING DRAINAGE
Now that you know the great necessity for drainage, how do you go about obtaining it?
There are but two requirements for achieving good drainage: a force to move the water and a
path for it to follow. Gravity provides a universal force to satisfy the first need; it is only
necessary to provide the path required, and gravity will move the water through it.
You read earlier that the roadbed supports the track above the level of surrounding
ground. If nothing on the roadbed obstructs the flow of water, it runs off the tracks and roadbed
to the ground. However, if water is allowed to remain on the ground close to the roadbed, the
stability of the surrounding soil is endangered.