204.05 Rock, Shale, or Coal Subgrade. If an aggregate base is not a part of the
pavement design, undercut the subgrade 2 feet (0.6 m) below the final subgrade elevation
where rock, shale, or coal is encountered. If an aggregate base is part of the pavement
design, reduce the above 2 foot (0.6 m) undercut by the thickness of the aggregate base