Sewer Manholes

Sewer maintenance holes, or "sewer manholes",as they are commonly referred to, are built out of concrete and provide a human access point for the maintenance of the sanitary sewer system.  These manholes come in a variety of sizes, but the most common are 48" and 60" diameter manholes.

Manholes are located in the city right-of-way along the path of the sewer main and are installed at approximately every 300 LF to 400 LF of sewer main line and are also built at intersecting main lines and at turns in the main line.  

Sewer manholes are normally precast and can come either with steps or no steps depending on the sewer agency requirements. Some manholes are designed to have a PVC liner pre-installed or may have an applied inner liner installed after the manhole is built.  Standard manholes are made up of a base, shaft, cone, grade rings, and a ring and cover.

The very bottom of a manhole is called the base. Depending on a sewer agency's specifications, the base is either purchased as a precast item with the desired pipe connections and channels already built in to the base, or the contractor can pour the base in place with fresh concrete, using specially designed forms to build the channels. The channels are the troughs in the concrete base that allows the sewer to flow from one pipe to another. The channels are sloped in the base to head to the outlet pipe.

The shaft is the next item of a standard manhole and sets directly on top of the base. Once the base is in place and any wet concrete has set, the manhole shafting is placed on top of the base. The shafting is the body of the manhole and its diameter determines the size of the manhole. A 48" manhole means that the shafting has a 48" diameter. The shafting comes in variable vertical lengths to fit your desired overall manhole height. Careful planning and measuring is required.

The third item of installation of a standard manhole is the cone. The cone reduces the diameter of the manhole from the shaft size to a smaller size opening which is typically 24" or 30" in diameter.

Next, the grade rings, also known as adjustment rings, come in common vertical sizes of 3" and 6" heights and are the diameter of the top of the cone. A cone with a 24" top opening would have 24" diameter grade rings. These grade rings are used to adjust the manhole ring and cover up to the final asphalt paved street section.

The final top part of the manhole is the ring and cover. The cast iron ring is the holder of the cast iron cover. The cast iron cover is the actual part that you may see in the city street and often marked by a letter "S" in the middle of the lid. This is the lid the maintenance worker will open to access the inside of the manhole. The diameter of the ring and cover is the same size as the top of the cone and the grade rings. The ring and cover comes in typical vertical sizes of 4" and 6", depending on the governing agencies specifications.

Manholes are used for all types of maintenance work, but mostly are used to enable access to the existing sewer lines for cleaning and videotaping the sewer pipes to verify their integrity and cleanliness. Manholes are an integral part to a good operating sewer system.

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Published on September 20, 2010 | Comments: 0

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