(a) General.
(1) Manholes shall be made of pre-cast concrete sections in conformance with ASTM C478, N.C. Department of Transportation and the following requirements:
a. Standard manhole diameter shall be 4 feet unless an inside drop structure is installed or the depth exceeds 15 feet. In these cases the minimum diameter shall be 5 feet.
b. Pre-cast concrete manholes shall be as manufactured by Adams Concrete, Carolina Precast Concrete, Inc., D&M Concrete Specialties, Inc., N.C. Products Corp., Stay Right Tank, Tindall Concrete Products, Inc. or approved substitute.
(b) Pre-cast sections.
(1) Minimum wall thickness shall be 5 inches.
(2) Base. Cast monolithically without construction joints or with an approved PVC waterstop in the cold joint between the base slab and the walls. The width of the base extensions on extended base manholes shall be no less than the base slab thickness.
(3) Riser. Minimum lay length of 16 inches.
(4) Eccentric cone. Top inside diameter shall be 24 inches. Width of the top ledge shall be no less than the wall thickness required for the cone section.
(5) Transition cone. Provide an eccentric transition from 60-inch and larger manholes to 48-inch diameter risers, cones, and flat slab top sections. Minimum slope angle for the cone wall shall be 45 degrees.
(6) Flat slab top. Designed for HS-20 traffic loadings as defined in ASTM C890. Items to be cast into special flat slab tops shall be sized to fit within the manhole ID and the top and bottom surfaces. Provide a float finish for exterior slab surface.
(7) Pre-cast or core holes for pipe connections. Diameter of hole shall not exceed outside diameter of pipe by more than three inches.
(8) Grade rings. May be used to adjust frame and cover to finished grade.
(9) Grade rings shall be no less than four inches in height.
(10) Lifting devices. Devices for handling pre-cast components shall be provided by the pre-cast manufacturer and comply with OSHA Standard 1926.704.
(c) Joints.
(1) Manufacturer in accordance with tolerance requirements of ASTM C990 for butyl type joints.
(2) Minimize number of joints. Do not use riser section for manholes up to six feet tall and no more than one riser for each additional four feet in height.
(3) Flexible joint sealants. Preformed butyl rubber based sealant material conforming to Federal Specification SS-S-210A, Type B and ASTM C990.
(4) External seal. Polyethylene backed flat butyl rubber sheet no less than 1/16- inch thick and six inches wide.
(d) Inverts.
(1) Brick and mortar or precast concrete invert.
(2) Form and finish invert channel to provide a consistent slope from inlet(s) to outlet up to four inches.
(3) Channel walls shall be formed to 3/4 of the height of the outlet pipe diameter.
(4) Finish benches with a minimum uniform 1.5:12 slope. Provide a 1/4-inch radius at the edge of bench and trough.
(e) Flexible pipe connectors.
(1) Provide flexible connectors for pipe to manhole that conform to ASTM C923.
(2) Provide stainless steel pipe clamp type band around flexible connection to sewer pipe.
(f) Manhole steps.
(1) Steps shall be in accordance with ASTM C478 and made of 1/2-inch grade 60 steel encapsulated by co-polymer polypropylene and have serrated tread and tall end lugs.
(2) Secure steps to the wall with compression fit in tapered holes or cast-in-place. Align steps along a vertical wall and shall not be located over a pipe opening. First step shall be a maximum of 26 inches from the bottom.
(3) Steps shall be by American Step Co., Inc., Bowco Industries, Inc., M.A. Industries, Inc. or approved substitute.
(Ord. of 11-7-16, No. 10-16)