What is a Manhole?
A manhole is defined as, “a vertical access shaft from the ground surface to a sewer or stormwater line, usually at a junction, to allow for cleaning, inspection, connections, and repairs.” Manholes are large enough to accommodate the body of a utility worker and is covered with a top that is flush to the ground. The top of a manhole is called a manhole cover.
What is the Purpose of a Manhole?
There are many functions of manholes when it comes to sewer or stormwater systems. Manholes enable inspections of the sewer or stormwater system as well as maintenance projects such as cleaning or removal of obstructions within the sewer or stormwater line. Additionally, manholes ventilate the sewage system by allowing gases to escape. They allow the municipality to join sewer or stormwater systems, change the direction of the sewer or stormwater system, or align the sewer or stormwater system. In addition, the length of sewer or stormwater lines are based on the placement of manholes.
Where are Manholes Located?
Manholes are along the sewer or stormwater line for easy maintenance access. Although this placement varies depending on the diameter pipe used in the sewer or stormwater system. Due to their primary function’s manholes are typically located where there is a junction of more than one sewer or stormwater lines. Additionally, at locations where the sewer or stormwater line changes size. Also where sewer or stormwater line alignment changes. Finally, where the grade of the sewer or stormwater line changes a manhole will be placed. Manholes are placed 100 to 500 feet apart. This is to correspond with the lengths of blocks within cities and suburbs.
The Different Types of Manholes
Manholes typically fall into one of three categories, which are shallow, normal, and deep. The sewer or stormwater line size as well as the function it serves determines the type of manhole chosen.
- Shallow Manholes
Also known as inspection chambers, shallow manholes are only approximately two to three feet deep. In low traffic areas at the start of a sewer or storm water branch is where shallow manholes can be found.
- Normal Manholes
These types of manholes are typically about five feet deep. They are in the sewer or storm water line and include a heavy manhole cover that is typically square or rectangular in shape.
- Deep Manholes
A deep manhole is always deeper than five feet. Deep manholes incorporate a method for entry, such as a built-in ladder and always have a heavy manhole cover.
There are an estimated 20 million sewer or stormwater manholes across the nation. That works out to about one manhole for every 400 feet of pavement on average; many of those manholes are in serious decay or in need of immediate rehabilitation or replacement. Servicing a sewer line also provides an opportunity to assess and record the condition of a manhole. Inspections are used to document construction material, location, depth, and rate a manhole’s condition. If in the inspection phase it is determined that the manhole needs maintenance or repair those processes can begin. By assessing and rating the manholes and documenting the information, the municipality will know what manholes need immediate attention, which should be tracked for repair in a future year, and which are in good condition.
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Common Manhole Maintenance Issues
To keep a sewer or stormwater system functioning properly regular maintenance on manholes is critical. When maintaining manholes crews should confirm no trash or debris is on the grate opening, in front of the manhole, or inside of the manhole. Check the inside of the structure is clear of any dead animals, vegetation, or sediment. Inspect inlet and outlet pipes for trash or debris. Ensure the manhole cover does not have any cracks or damage. Verify the frame of the manhole is “sitting flush on the riser rings or top slab and firmly attached“. Then confirm that the structure is sound, and all the pipes are securely connected with no cracks. Make sure that the manhole cover is in place, locked securely, and easy to remove. Lastly inspect the ladder within the manhole to confirm it is secure and up to code.
Common Manhole Rehabilitation Issues
When it comes to damaged or aging manholes, rehabilitation is a cost-effective solution for many municipalities. Rather than replacing manholes, the rehabilitation process involves several methods for returning the structural integrity to the existing manhole without the purchase of entirely new manhole. These processes vary based on the material of the manhole (concrete, plastic, or fiberglass). However, some of the main techniques for rehabilitating manholes can include.
- Installing a New Lining System
Options include concrete systems, polyurea-based linings, lining sprays, stretchable systems, two-component systems, rehabilitation liners, life-extension liners, and other materials.
- Applying a Sealant
One common manhole rehabilitation project includes applying a sealant to the manhole chimney. This process reduces the chance of frame-chimney inflow into the manhole.
- Using Epoxies to Repair Manhole Damage
To patch and fill damage, to winterize manholes in cold areas, to patch leaks, and to prevent corrosion. Options for epoxy vary greatly depending on your specific rehabilitation needs.
- Adding a Coating
Another common manhole rehabilitation process is adding a protective coating to the interior of the manhole. This coating is typically a spray-applied polyurethane and will protect the interior from corrosion and abrasion.
- Utilizing Chemical Grout
The application of grout acts as a water barrier that also stabilizes the soil surrounding the manhole. This process strengthens the soil surrounding the manhole and provides the support needed to eliminate issues with water infiltrating manholes, pipe joints, and mainlines.