1 MARCH 2000
(2) Common rock materials suitable for ballast are granites, traprocks, quartzites, dolomites, and hard
limestones. As limestones degrade, they tend to produce fine particles that cement together, and are
thus not the best ballast choice if other hard rock material is economically available. Crushed slag can
also vary greatly in quality and suitability for good ballast.
(1) Table 6-8 gives recommended AREMA ballast gradations. For main running tracks, sizes 4A and
4 will be used. For loading tracks in terminal areas, size 5 may be used to facilitate easier walking along
the cars during loading and unloading operations, but a larger size is preferred for long-term track
maintenance. AREMA ballast gradation 4 is identical to ASTM C33 gradation 4. ASTM C33 gradation 56
is close to AREMA no. 5.
Table 6-8. Recommended Ballast Gradations
Amounts Finer Than Each Sieve (Square Opening)
Percent by Weight
2 to 3/4
1-1/2 to 3/4
1 to 3/8
(2) For smaller projects, where less than 200 tons of ballast is needed, and where the nearest
suppliers do not stock AREMA gradations, the following AASHTO (highway) gradations may be
substituted: CA5 for AREMA 4 or 4A, and CA7 for AREMA 5.
(1) Appropriate ballast depth will be determined by structural analysis using the computer program
specified in paragraph 6. The manual method described in paragraph 7 may also be used, but is not
(2) In all cases, the minimum depth of ballast from the bottom of the tie to the subgrade will be 8 in. In
most cases, however, main running tracks will require more.
e. Cross-Section. Figures 6-1 through 6-7 show standard ballast shoulder widths and side slopes. In
finished or resurfaced track, the top of ballast may be up to 1 in. below the top of the tie, but never above
a. Purpose. Sub-ballast is a layer of material between the top ballast and subgrade with a gradation
finer than the top ballast and coarser than the subgrade. Sub-ballast is often cheaper than top ballast, so
it can be used to reduce total ballast cost or to provide a filter layer between the top ballast and a fine-
grained subgrade. Figures 6-2, 6-4, 6-5, and 6-7 show sub-ballast layer.
b. Application. A sub-ballast layer is recommended for most new construction. In addition to providing
a filter to keep subgrade particles from working up into and fouling the ballast, it provides a good mat to