After the concrete or masonry structure has cured and has been inspected and approved by the engineer, the contractor shall backfill with material removed for the excavation except where sand backfill is specified. No debris, frozen earth, large clods, stones or other unsuitable material shall be allowed as backfill material. The contractor shall simultaneously backfill on all sides of the structure and compact the material in nine inch lifts to a density not less than that specified for the adjacent trench (95% minimum). Backfill shall be terminated at finish grade as shown on the plans and prepared for surface restoration as specified for adjacent trenches. The contractor shall dispose of all excess material as directed by the engineer in an approved disposal area.