All tree lawn openings shall be backfilled with premium backfill when the work is to be done in freezing weather. At other times they may be backfilled with earth but the work shall be done under the supervision of the City Engineer. If the trench is back-filled with earth during freezing weather or the work is done at any time without supervision, then the deposit shall not be returned upon completion of the work, but shall be held for a period of six months or for such longer period as may be required, until the City Engineer is satisfied that the settlement is complete.