The trench width should be no wider than what is represented in the attached table, except as necessary to construct the trench or install the pipe. According to ASTM D 2321, the trench width should be no wider than what is required to safely and conveniently compact backfill material on either side of the pipe. As I understand it, in clay soils you do not want to make wider trenches, rather, you will need additional drain line laterals. More info
HERE.