Whenever there is a period of excessive unemployment in Illinois, if a person or entity is charged with the duty, either by law or contract, of (1) constructing or building any public works, as defined in this Act, or (2) the clean-up and on-site disposal of hazardous waste for the State of Illinois or any political subdivision of the State, and that clean-up or on-site disposal is funded or financed in whole or in part with State funds or funds administered by the State of Illinois, then that person or entity shall employ at least 90% Illinois laborers on such project. Any public works project financed in whole or in part by federal funds administered by the State of Illinois is covered under the provisions of this Act, to the extent permitted by any applicable federal law or regulation. Every public works contract let by any such person shall contain a provision requiring that such labor be used: Provided, that other laborers may be used when Illinois laborers as defined in this Act are not available, or are incapable of performing the particular type of work involved, if so certified by the contractor and approved by the contracting officer.