A manhole looks like one structure from the street, but it is really a stack of separate precast pieces, each doing a job, assembled to fit the depth and the pipes at that spot. Knowing the pieces is how an engineer specifies one and a contractor builds it right. Here are the precast manhole components, from the base at the bottom to the cover at grade, and how they go together.
The precast manhole components are the base, the barrel and riser sections, the cone or top slab, the adjustment rings, and the frame and cover, which stack and seal together to form a watertight structure sized to the run’s depth and pipes. Each piece is a separate precast unit.
The precast manhole components, bottom to top
Read a manhole from the bottom up and the assembly makes sense. The base carries the pipes and the flow, the barrel builds the height, the cone narrows to the opening, and the frame and cover finish it at the road.
A barrel section is a precast ring of a set inside diameter that stacks with others to build the manhole to depth, with a gasketed joint at each connection to keep the structure watertight. The pieces are:
- Base: the bottom unit, with the invert channel shaped to the pipes entering and leaving.
- Barrel and riser sections: the stacked rings that build the structure to the required depth.
- Cone or top slab: the top piece that either narrows to the opening (a cone) or spans flat with an opening (a flat top).
- Adjustment rings: short rings that fine-tune the final height so the cover lands at grade.
- Frame and cover: the cast-iron ring and lid at the road surface.
The base and invert
The base is where the manhole does its real hydraulic work. It carries the pipes entering and leaving, and its invert channel guides the flow smoothly through the structure.
An invert channel is the shaped flow path formed into the base that carries the pipe flow through the manhole, built to match the pipes’ sizes, elevations, and angles. On a sanitary structure, that channel is shaped carefully so solids do not settle and the flow stays clean; on a storm structure, it is often simpler. Because we build the base to the pipes that actually enter it, the flow runs the way the engineer designed it rather than the way a stock base happened to be shaped. Our in-house engineering details the base and invert for each structure.
Barrel and riser sections
The barrel is the height of the manhole, built from stacked precast sections. The number of sections sets the depth, and shorter riser sections handle the in-between measurements so the structure reaches exactly the right elevation.
Each joint between sections is sealed with a gasket so the finished structure is watertight, which matters most on sanitary manholes that must keep groundwater out and wastewater in. Because the depth is built from stacked sections, one diameter of manhole can be assembled to nearly any depth a run requires, which is what makes precast so adaptable on a sewer line that changes depth as it runs.
This is also why precast wins on the site schedule. The sections show up cured and finished, so the crew stacks and seals a structure in a fraction of the time a poured-in-place manhole would take to form and cure. On a sewer line with a manhole every few hundred feet, that speed adds up across the whole job, and it keeps the trench open for less time. The pieces are proven, the joints are engineered, and the assembly is fast, which is exactly what a utility contractor wants underground.
Cones, top slabs, and adjustment rings
Near the top, the structure has to narrow from the working diameter of the barrel down to the roughly two-foot opening of the cover. Two options do that job.
A cone is a tapered precast top that narrows the barrel down to the cover opening, and it is called eccentric when the taper is offset to one side to keep a straight ladder line, or concentric when the taper is centered. The alternative is a flat top slab, used where there is not enough depth for a cone or where a flat structural top is preferred, common on shallow structures and large-diameter junctions. Above the cone or slab, short adjustment rings, sometimes called grade rings, fine-tune the last few inches so the frame and cover sit flush with the finished road.
Frames, covers, and specialty pieces
At the surface, the cast-iron frame and cover carry traffic and provide access. The cover is often marked for its system, “STORM” or “SEWER,” and rated for the load above it, commonly the AASHTO HS-20 highway standard.
Beyond the standard sewer structure, the same component approach builds specialty pieces. We make perforated barrel sections in 36-inch and 48-inch diameters for drainage structures, where the perforations let water pass into the surrounding soil, the same principle behind a dry well. If you are sizing a drainage structure, our dry well sizing calculator helps with that, and our complete guide to precast concrete manholes ties the components back to sizes and the storm-versus-sanitary distinction.
Frequently asked questions
What are the components of a precast manhole?
The precast manhole components are the base, which carries the pipes and the invert channel; the barrel and riser sections, which stack to build the depth; the cone or top slab, which narrows to the opening; the adjustment rings, which fine-tune the final height; and the cast-iron frame and cover at grade. Each is a separate precast piece, and they stack and seal together into one watertight structure.
What is a manhole barrel or riser section?
A manhole barrel section is a precast ring of a set inside diameter that stacks with others to build the manhole to the required depth. Riser sections are shorter rings used to reach an exact height between full sections. Each joint is sealed with a gasket to keep the structure watertight, so the depth can be built to almost any dimension a run needs without changing the diameter.
What is the difference between an eccentric and concentric cone?
Both are tapered precast tops that narrow the barrel down to the cover opening. An eccentric cone offsets the taper to one side so one wall stays vertical, which keeps a straight, climbable ladder line down the structure. A concentric cone centers the taper evenly around the opening. Eccentric cones are common on sanitary manholes for the ladder access; concentric cones and flat tops suit other cases.
What is a manhole adjustment ring?
A manhole adjustment ring, sometimes called a grade ring, is a short precast or manufactured ring placed between the cone or top slab and the frame and cover. It fine-tunes the last few inches of height so the cover sits flush with the finished road surface. Because the barrel sections come in set heights, adjustment rings are how the structure is brought exactly to grade.
What is a perforated barrel section?
A perforated barrel section is a precast manhole ring with holes cast through its wall, used for drainage structures rather than sewer access. The perforations let collected water pass out into the surrounding soil, the same draining principle as a dry well. Glacier Precast makes perforated barrels in 36-inch and 48-inch diameters for stormwater and drainage applications where infiltration is the goal.
One structure, built from proven pieces
The strength of the precast manhole is that it is a kit of standard, engineered pieces: a base shaped to the pipes, barrel sections stacked to depth, a cone or slab to the opening, adjustment rings to grade, and a rated frame and cover on top. Building every piece in one plant means the base matches the pipes, the joints seal, and the structure comes to grade the first time. We make the full set of precast manhole components, from the routine sewer manhole to the perforated drainage barrel, engineered and cast to a certified standard, and we deliver them to the site with our own crane trucks so the structure that leaves the plant is the structure that goes in the ground.
There is one joint detail in how the sections stack that separates a manhole that passes its watertightness test from one that leaks, and we build for it on every structure.
Get manholes built for your project, or start with the complete guide to precast concrete manholes or see precast manhole sizes.