Skokie, Ill.-based U.S. Robotics Corp. said it will lay off as many as 600 workers, or about 10% of its work force, to cut costs. . . . Richmond, Va.-based Circuit City Stores Inc. said it plans to create a second series of common stock to track the separate performance of its CarMax used- and new-car retail operations. . . . San Francisco-based BankAmerica Corp. plans to cut 80 jobs, or 10% of the work force, in its brokerage unit, BA Investment, mainly by automating administrative duties. . . . Lockheed Martin Corp. received a $567.3-million Navy contract to continue production of the Trident II nuclear missile through 2000. . . . Beverly Hills-based Giant Group Ltd. reported third-quarter net income of $1.5 million, or 33 cents a share, contrasted with a net loss of $8.3 million, or $1.64 a share, last year.