B-1.1, r. 2 - Construction Code

Full text
8.83. Underground piping must be backfilled with one of the materials referred to in section 8.82 in such manner that
(1)  the piping is bedded on at least 150 mm of backfill;
(2)  there is at least 150 mm of backfill measured horizontally between the piping and the excavation wall;
(3)  the backfill between each pipe is at least twice as thick as the nominal diameter of the largest pipe; and
(4)  the backfill above the piping is at least 450 mm deep including the finishing grade layer.
O.C. 220-2007, s. 1.