The preliminaries are just about over. Permanent structural repairs are about to begin at Oroville Dam. Five months after an unprecedented emergency forced a mass evacuation, state officials said Wednesday they’re ready to start replacing the now-demolished lower portions of Oroville’s main flood-control spillway. Construction contractor Kiewit Corp. plans to install the first slabs of concrete Thursday in the lower 350 feet of the spillway chute. The so-called “leveling concrete” will create a smooth foundation for structural concrete that will be poured on top, said Jeff Petersen, Kiewit’s Oroville project director.