What is a Precast Concrete Manhole?
Manholes are a key element in a sewer system and provide a junction point for converging sewer pipes. They are also key avenues for accessing these junction points for maintaining, observing and servicing the sewer system.
A precast concrete manhole is a simple, round- or box-shaped structure, with an opening in the top that is covered with a slotted or perforated metal grate, rounded beehive grate, or a flat lid. These structures are often installed near a sewer branch or line, or near important utility features.
Manholes are essentially a column consisting of a base, multiple sections, and one or more cones which affect the depth and circumference of the underground cavity and the surface opening. They are topped with one of several surface lid or grate options, depending on whether they are to collect and channel rainwater.
Manhole Design Considerations
- Specific site use requirements and special constraints
- Size and entry angle for converging sewer pipes
- Debris volume and density
- Anticipated surcharge loading on the structure
- Soil bearing capacity below the structure
- Ease of access for future maintenance and/or cleaning
Precast Concrete Advantages – For Designers
- Unmatched durability and strength
- High quality concrete mix designs
- Documented QA/QC programs
- Easily customizable to meet project needs
Proper sizing is critical to the long-term use and structural integrity of the precast manhole. One of the first aspects in sizing is the intended use and accessibility of the manhole. For utility manhole structures this is the most important consideration. Utility manholes must be large enough to accommodate all service panels and connections that are required as well as provide enough space for entry and work area. For sanitary and storm sewer manholes sizing is more often driven by the size of pipes the structure must accommodate. The diameter of the manhole should be such that it can accept the largest pipe and maintain minimum structural leg thickness (approximately 6-inches) between pipe openings. Proper manhole sizing is ultimately the responsibility of the engineer who must follow State and local guidelines but your local precast concrete supplier can assist with design requirements and product specifications.
State, City and Municipality Requirements
ASTM C913-21 Standard Specification for Precast Concrete Water and Wastewater Structures
This specification covers the recommended design requirements and manufacturing practices for monolithic or sectional precast concrete water and wastewater structures with the exception of concrete pipe, box culverts, utility structures, septic tanks, grease interceptor tanks, and items included under the scope of Specification C478-12.
ASTM C478 Standard Specification for Circular Precast Reinforced Concrete Manhole Sections
This specification covers the manufacture and purchase requirements of products used for the assembly and construction of circular vertical precast reinforced concrete manholes and structures used in sewer and water works.
Glossary of Terms
A tapered connection that allows for a wide manhole with a smaller surface hole.
The rate of rise or fall along the length of the road with respect to the horizontal.
A component at the top of a manhole that allows storm water to run into the cavern while preventing larger debris from falling in.
Casting concrete in a reusable mold or “form” which is then cured in a controlled environment, transported to the construction site and maneuvered into place.
Soil Bearing Capacity
The capacity of soil to support the loads that are applied to the ground above.
The result of objects on the surface that add loading to the protective system.