BALLAST AND SUB-BALLAST
Economics. Select ballast and sub-ballast with due consideration
of cost and availability.
Specifications and Applications. See AREA Manual, Chapter 1, Part
2, Ballast, for material specifications. The following materials are
acceptable in order of quality as listed:
Pit run gravel.
Cinders or other local materials may be used temporarily or in
Depth. The combined thickness of ballast and sub-ballast required
for a given axle load and subgrade strength should be determined on basis of
engineering calculation and experience. For main running tracks, provide a
minimum of 12 inches of ballast under the ties and 6 inches of sub-ballast.
For all other locations, provide a minimum of 8 inches of ballast under the
ties and 6 inches of sub-ballast. The design strength of the subgrade shall
not be exceeded by the traffic load. See AREA Manual, Chapter 1, paragraph
184.108.40.206, Ballast Thickness. When a geotextile fabric is used between ballast
or sub-ballast and the subgrade, there should be a minimum of 12 inches of
ballast between the bottom of the tie and the geotextile fabric.
See AREA Manual, Chapter 1, Part 2,
Cross Section Dimensions.
Shrinkage Allowance. See AREA Manual, Chapter 1, Part 2.2,
Use 12 to 15 percent Shrinkage Factor.