On public works projects in which trench excavation will exceed a depth of four feet, any contract therefor shall require adequate safety systems for the trench excavation that meet the requirements of the Washington industrial safety and health act, chapter 49.17 RCW. This requirement shall be included in the cost estimates and bidding forms as a separate item. The costs of trench safety systems shall not be considered as incidental to any other contract item and any attempt to include the trench safety systems as an incidental cost is prohibited.