The construction, maintenance, or existence within the city of any unprotected, unbarricaded, open, or dangerous excavations, holes, pits, or wells, or of any excavations, holes, or pits which constitute or are reasonably likely to constitute a danger or menace to public health, safety, or welfare, is hereby prohibited; provided, however, this section shall not prevent any excavation under a permit issued pursuant to this ordinance or the Building Code of the city, where those excavations are properly protected and warning signs posted in such a manner as may be approved by the Zoning Administrator.