If a timber road is being constructed in a 2 ply mat system according to the specs in this diagram, how many board feet of lumber will be required if 6″ x 6″ x 8′ timbers are used?

If a timber road is being constructed in a 2 ply mat system according to the specs in this diagram, how many board feet of lumber will be required if 6″ x 6″ x 8′ timbers are used?