11831 AIRPORT RD WHOLE SITE FILE 2020-01-21

corrections shall be made by the Contractor as directed by the Engineer. No additional shafts shall be constructed until the Contractor has modified his rebar cage support in a manner satisfactory to the Engineer. Concrete Placement Concrete placement shall commence within 2 hours after completion of the excavation and shall be placed in one continuous operation to the top of the shaft. Concrete shall be placed through a tremie. The tremie used shall consist of a tube of one-piece construction. Concrete shall be placed through a hopper at the top of the tube so that the concrete is deposited through the center of the reinforcing steel to prevent segregation of the aggregates and splashing of concrete on the reinforcement cage. The Contractor's proposed method for depositing concrete shall have approval of the Engineer prior to concrete placement. The concrete on the top 5 feet of the shaft shall be vibrated. Casing Removal During casing removal, a minimum 5-foot head of concrete must be maintained to balance the soil and water pressure at the bottom of the casing. This casing shall be well coated with form oil prior to concrete placement. Construction Tolerances 1. The centerline of the drilled shaft shall be within 3 inches of Plan position in the horizontal plane, at the Plan elevation for the top of the shaft. 2. The vertical alignment of the shaft excavation shall not vary from the Plan alignment by more than 1/4 inch per foot of depth. 3. After all the concrete is placed, the top of the reinforcing steel cage shall be no more than 1/2 inch above and no more than 1/2 inch below the Plan position. 4. The minimum diameter of the drilled shaft shall be 1 inch less than the specified shaft diameter. 5. The top elevation of the shaft shall have a tolerance of± 1/2 inch from the Plan top of shaft elevation. 6. Excavation equipment and methods shall be designed so that the completed shaft excavation will have a flat bottom. The cutting edges of excavation equipment shall be normal to the vertical axis of the equipment within a tolerance of± 3/8 inch per 12 inches of diameter. Drilled shaft excavations constructed in such a manner that the concrete shaft cannot be completed within the required tolerances are unacceptable. When approved, corrections may be made to an unacceptable drilled shaft excavation by any approved combination of the following methods: 1. Overdrill the shaft excavation to a larger diameter to permit accurate 30 placement of the reinforcing steel cage with the required minimum concrete cover. 2. Increase the number and/or size of the steel reinforcement bars. Swift Green Line—Corridor Stations Special Provisions SP122 May 2017