a. For a military main line, a depth of 12 inches of top-quality ballast is usually
maintained. Let us assume that the military standard 6-inch deep ties are being used. They are
set in the top 4 inches of the ballast; the remaining 8 inches extend from the bottom of the tie
to the top of the subballast, if it is used. Subballast, a low-quality ballast, usually consists of a
layer of cinders spread before the top ballast is put down. Using subballast reduces the unit
pressure on the roadbed where a normal ballast section would settle or crush the roadbed.
Subballast helps to keep the track dry; it carries off the water draining down through the ballast
and helps to prevent the mud beneath from working up into and contaminating the ballast itself.
Subballast is not used on old lines because continually adding new ballast through the years
provides a deep enough section of top ballast.
b. On less heavily traveled main lines, subballast is not used and as little as 6 inches of
ballast may support the ties. But a good quality ballast is still desirable. Gravel and slag are
often used on this type of trackage, and even cinders may be good enough. Minor branch lines
are often ballasted with as little as 4 inches of cinders or gravel.
2.8.
SUMMARY
The roadbed provides a means of holding the track above the surrounding land to
prevent water from standing on the tracks. Minor terrain irregularities on the right of way are
compensated for by filling in low spots and cutting through high spots. Slopes on the roadbed
must be protected from erosion by ditches and dikes. A material such as crushed rock, cinders,
slag, gravel, or sand is used as ballast to stabilize ties, distribute the weight of the train to the
roadbed, and aid in track drainage. The amount, depth, and disposition of ballast in the ballast
section are governed by the traffic and the terrain.
Section II. Drainage
2.9.
GENERAL
Adequate, well-maintained track is essential in operating any railroad. Water and the
damage it causes constantly threaten the track, because practically all major track damage can be
attributed directly or indirectly to water. The time and effort spent on achieving and maintaining
adequate drainage are never wasted. However, ideal roadbed drainage is difficult to achieve
because few track
26
Previous Page
