Crews from Western Builders began work today on the foun dation for a 100,000-square-foot addition to the Levi Strauss io -Prest plant at 4720 NE 24. A $50,000 building permit for the foundation work was issued Tuesday afternoon. The plant expansion is scheduled for completion by June 1971. Levi Strauss officials estimate tthat the expansion of facilities will increase plant employment by more than 550 persons. The increase would make Levi Strauss the largest employer in Amarillo with about 1,800 persons on the payroll. The addition will be on a landscaped tract adjacent to the existing distribution center and Sta-Prest plant. The structure will be of masonry construction.