403 - Drainage Structures
Drainage structures refer to manholes, catch basins, leaching basins, inlets and drop inlets as detailed on Standard Plan R-1 series or on project plans.
A drainage structure shall not be placed within 5 feet (1.5 m) of the spring point of a street return so that the transverse joint will function properly.
Careful attention to grades and positioning of the footing from offset stakes is required, or the catch basin will not fit curb and gutter alignment. The inspector should observe or check all takeoff measurements made by the workmen. Excavation for this type of structure is included with the structure, regardless of how it is backfilled.
Protection of adjacent sidewalk, curb or gutter, or other items not required to be removed, but endangered by sloping to the angle of repose, is the responsibility of the Contractor under subsection 107.07 of the Standard Specifications for Construction (Protection and Restoration of Property), and the Contractor should be so reminded prior to commencing work.
Excavation for drainage structures is to be performed in accordance with subsection 206.03 of the Standard Specifications for Construction.
The concrete portions of the drainage structures are constructed according to subsection 706.03 of the Standard Specifications for Construction. Concrete is not to be cast when the temperature of the concrete is above 90°F (32°C).
Masonry with mortar is not to be placed when the air temperature is at 36°F (2°C) and falling. Any work damaged by frost is to be removed and replaced.
The first set of bricks or blocks is placed on a full bed of mortar. The bricks or blocks are then laid in courses with mortar joints 0.5 inch (13 mm) thick ± 0.1 inch (3 mm). Mortar joints should be of uniform thickness throughout the structure. Joints are staggered by half the length of the brick or block on adjoining courses. The joints should be struck off and pointed so that the exposed surface is true and smooth.
A 0.5 inch (15 mm) thick plaster coat of mortar shall be applied to the outer surface of all structures and to the inner surface below the outlet flow line on all catch basins with traps and sumps. The joints should be raked, and the brick or block is to be wet prior to placing the plaster coat. The brick or block surface should be allowed to sufficiently dry to provide for proper bonding of the plaster coat.
All brick shall be thoroughly wet. The brick surface should be allowed to dry sufficiently enough to prevent slipping on the mortar. Broken or chipped brick should not be used on the faces of the structure. A course made of headers should be placed a minimum of every seventh course.
Precast reinforced concrete units are to be constructed according to the standard plan or as detailed on the plans. The joints shall be sealed with mortar as described above for Placing Brick and Block Masonry.
The footings shall be either poured-in-place or precast concrete. Precast concrete footings must be supported by placing them on a compacted 6 inch (150 mm) subbase of Granular Material Class II.
Refer to subsection 403.04 of the Standard Specifications for Construction, as well as any special provisions included in the proposal, for information on adjusting drainage structures.
Final adjustment of the drainage structures within the HMA pavement section should be made immediately before the placement of the top course or overlay, if only one course is applied. Adjustments made after the final top course is applied shall be constructed according to the details on the plans. The pay item of Drainage Structure Cover, Adjust, Case 1 (or Case 2) applies when the elevation of the cover is changed up or down 6 inches (150 mm) or less. The cover should be adjusted to the required elevation by supporting it on a metal ring adjustor, on a concrete collar, or on masonry in a full mortar bed. Adjusted covers shall be held firmly in place.
Drainage Structure Cover, Adjust, Case 1
Applies to structures located within existing pavement, curb or curb and gutter.
Drainage Structure Cover, Adjust, Case 2
Applies to structures located outside the existing pavement, curb, or curb and gutter.
The adjacent pavement, curb, or curb and gutter should be removed and replaced to the existing grades or to the required elevations. Sawcutting of the existing pavement, curb, or curb and gutter where required are included in the item Drainage Structure Cover, Adjust, Case 1. Removal and replacement of pavement adjacent to the adjusted structure is also included in this pay item. Removal and replacement of curb and gutter adjacent to the adjusted structure is not included in the pay item and will be paid for separately with the use of other pay items. Placement of new covers on existing structures requires payment for the item Drainage Structure Cover, Adjust, Case 1 (or Case 2) as well as for the item Drainage Structure Cover.