10.2 Manholes, Service Taps, Laterals & Sewer Construction Practice
Key Takeaways
- North Carolina requires manholes at the end of each line, at all changes in grade, size, or alignment, at all intersections, and at intervals not greater than 425 feet unless the owner documents the ability to clean at greater distances.
- Minimum manhole diameter is 4 feet with a 22-inch minimum access, and manholes with an inside drop must be at least 5 feet in diameter.
- A drop pipe is required for a sewer entering a manhole more than 2.5 feet above the manhole invert; below that difference the invert is filleted instead, and outside drop connections must be encased in concrete or ferrous pipe.
- Service laterals are commonly connected by wye or tee fittings on new construction and by saddles or tapping machines on existing mains; a protruding or hammer tap is a permanent blockage generator.
- Cleanouts may be used in lieu of manholes on 6-inch private sewer lines at spacings not exceeding 100 feet, and private gravity sewers may not be smaller than 6 inches while public gravity sewers may not be smaller than 8 inches.
10.2 Manholes, Service Taps, Laterals & Sewer Construction Practice
1. Where manholes go
Under North Carolina's Gravity Sewer Minimum Design Criteria, manholes are installed:
- at the end of each line;
- at all changes in grade, size, or alignment;
- at all intersections; and
- at distances not greater than 425 feet for all sewers, unless the owner or authority documents the capability to perform routine cleaning and maintenance at greater distances.
Cleanouts may substitute for manholes on 6-inch private sewer lines, with spacing not exceeding 100 feet. Recall the diameter floor: no public gravity sewer smaller than 8 inches, no private gravity sewer smaller than 6 inches.
2. Manhole construction requirements
| Element | Requirement |
|---|---|
| Minimum diameter | 4 feet (48 inches); larger preferred for large-diameter sewers |
| Access opening | Minimum 22 inches |
| Inside drop manholes | Minimum 5 feet (60 inches) diameter |
| Material | Pre-cast or poured-in-place concrete; lift holes and grade rings sealed with non-shrink mortar or approved material |
| Pipe connections | Gasketed flexible watertight connection, or an arrangement allowing differential settlement between pipe and wall |
| Covers | Watertight where tops may be flooded by street runoff or high water; locked covers where vandalism is a concern |
| Flood protection | Rims 24 inches above the 100-year flood elevation, or watertight and vented 24 inches above it, with vents every 1,000 feet or every other manhole, whichever is greater |
| Buoyancy | Flotation must be prevented where high groundwater is anticipated |
| Channel | Conforms in shape and slope to the connecting sewers; channel walls formed to three-quarters of the height of the crown of the outlet sewer |
| Bench | Provided on each side of the channel when pipe diameter is smaller than manhole diameter; sloped no less than 1/2 inch per foot (4 percent) |
| Corrosion protection | Considered where septicity or other corrosive conditions are anticipated |
3. Drop connections
Incoming sewer > 2.5 ft above the manhole invert
|
+--> DROP PIPE REQUIRED
- Outside drop preferred; encase entire outside drop
connection in concrete or ferrous pipe with blocking
- Inside drop must be secured to the interior wall with
access provided for cleaning (5-ft minimum manhole diameter)
Incoming sewer < 2.5 ft above the manhole invert
|
+--> FILLET the invert to prevent solids deposition
A drop connection prevents the free fall that erodes the bench, aerosolizes wastewater, strips dissolved hydrogen sulfide into the manhole atmosphere, and drenches anyone working below.
4. Service taps and laterals
Connection methods:
| Method | Where used |
|---|---|
| Wye or tee fitting installed with the main | New construction — the best connection |
| Saddle (strap-on or bolted, gasketed) | Existing mains; requires a clean, correctly sized cut |
| Tapping machine / shell cutter | Existing mains, cuts a round hole for a saddle or insert |
| Inserted / expanding tee | Existing mains, no-dig options in some materials |
| Hammer tap | Never acceptable — breaking a hole with a hammer leaves protruding fragments and a permanent snag point |
Installation practice: cut the correct size and shape for the fitting, remove all debris from inside the main, bed and support the lateral so it cannot settle and shear at the connection, and do not let the lateral protrude into the main — a protruding tap catches rags and grease and becomes a recurring blockage that CCTV will find years later.
Grade and alignment: laterals are sloped to drain, typically at a minimum of about 1/4 inch per foot for 4-inch service, with a cleanout at the property line or building. Connections should enter the main above the springline, ideally near the crown, so that flow in the main does not back into the service.
Responsibility. Utilities divide lateral ownership differently — commonly the utility owns to the property line or the cleanout, and the owner owns the rest. That boundary must be written down, because defective private laterals are among the largest infiltration and inflow contributors in most systems and someone has to pay to fix them.
5. Construction and installation requirements
- Cover: minimum three feet over all sewers unless ferrous pipe is specified; ferrous pipe or proper bedding where traffic loads apply.
- Bedding and backfill: ASTM C12 bedding classes for rigid pipe; ASTM D2321 embedment classes I, II, or III for flexible pipe. Ledge rock, boulders, and large stones removed to give at least 4 inches of clearance below and beside the pipe.
- Final backfill: no debris, frozen material, large clods, organic matter, or unstable material within 2 feet of the top of the pipe; stones in backfill no larger than 6 inches on any axis.
- Separation: 10 feet horizontal from water mains (or a separate trench with the water main bottom at least 18 inches above the sewer top); 18 inches vertical at crossings; 24 inches vertical separation from storm sewer lines or ferrous pipe specified; 100 feet from any water supply source, never less than 25 feet from a private well or 50 feet from a public water supply well.
- Stream crossings: ferrous pipe with mechanical joints tested to 150 psi (PVC permitted where three feet of cover is maintained), one foot of cover in rock or three feet in other material, crossings as near perpendicular as possible.
[!WARNING] Sewer manholes are permit-required confined spaces. Entry for bench repair, channel work, or lining requires a full entry permit, atmospheric testing, continuous ventilation, a harness and retrieval system, and an attendant outside. Most manhole inspection work can be done from the surface — and should be.
A new sewer enters a manhole 3.1 feet above the manhole invert. What does North Carolina's gravity sewer design criteria require?
What is the maximum spacing between manholes under North Carolina's gravity sewer criteria, and what allows a longer spacing?
Why is a hammer tap an unacceptable method of connecting a service lateral to an existing main?