All proposals for any such improvement shall be signed by the person bidding and shall be delivered to the City Clerk enclosed in a sealed envelope upon the outside of which shall be stated the nature of the enclosure. Each bid shall be accompanied by a bond or certified check in such amount as may be named in such advertisement, and in case of bond with sureties to be approved by the City Council, conditioned that, in case that the contract be awarded to such bidder, he will enter into contract and bond, as required, to perform such work according to such specifications and plans.
('72 Code, § 3.20(e))