Manholes: The Ultimate Resource

Covers, accessories, engineering, installation, maintenance and more. Everything you’ve ever wanted to know about the underground world of manholes.

What is a Manhole?

According to Dictionary of Construction, a manhole is, “a vertical access shaft from the ground surface to a sewer or stormwater line…, usually at a junction, to allow cleaning, inspection, connections, and repairs.”

What is the Purpose of a Manhole?

As manhole manufacturers, we often get asked about the purpose of manholes and the uses of manholes. And the answer is, that there are many functions of manholes when it comes to sewer or stormwater systems. The four main purposes of a manhole are:

To facilitate 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
To assist in ventilation of the sewage system by allowing gases to escape
To 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
To assist in ensuring the sewer or stormwater line is laid in convenient lengths

QUICK NAVIGATION

Common Manhole Locations

Manhole locations vary, however, due to their primary functions they are typically located in the below areas:

Where there is a junction of two sewer or stormwater lines (or more)
At locations where the sewer or stormwater line changes sizes
In situations where the sewer or stormwater line alignment changes
When the grade of the sewer or stormwater line changes

Manholes are also periodically placed along the sewer or stormwater line for easy maintenance access. This placement varies depending on the diameter of the sewer or stormwater system in question.

The Different Types of Manholes

Manholes typically fall into one of three categories, which are shallow, normal, and deep. The type of manhole chosen for a specific area depends on both the size of the sewer or stormwater line as well as the function that the manhole is supposed to serve.

Shallow Manholes

Also known as inspection chambers, shallow manholes are only approximately two to three feet deep. They are typically located at the start of a sewer or storm water branch and are placed in areas that are not heavily trafficked.

Normal Manholes

These types of manholes are typically about 150 centimeters or five feet deep. They are located in the sewer or storm water line and include a heavy manhole cover that is typically square or rectangular in shape.

Deep Manholes

Any manhole deeper than 150 centimeters is considered a deep manhole. These manholes incorporate a method for entry, such as a built-in ladder, always incorporate a heavy manhole cover.

How are manholes engineered and what materials are they made from?

Manholes can be manufactured using a variety of materials including precast concrete, plastic, and fiberglass. The chosen manhole material can vary based on a variety of factors including the shape of the manhole and the intended manhole function. Depending on the material that is chosen, the manufacturing process will vary. Due to technological advances in recent years, fiberglass and plastic polyethylene models have increased in popularity, as they have many advantages over traditional precast concrete manholes. In this section, we discuss the construction of manholes, including plastic manholes, precast manholes, and fiberglass manholes.

Plastic Manholes

Plastic manholes are typically manufactured from polyethylene with a durable one-piece construction with no seams or seals that can cause maintenance issues. Similarly to fiberglass manholes, polyethylene manholes are environmentally-friendly and do not pose a risk of contaminating the ground that they are placed in.

Polyethylene also has the advantage of being extremely resistant to corrosion, unlike precast manholes that degrade with time and require frequent rehabilitation and maintenance. Another similar feature plastic manholes have to fiberglass manholes is that when they are engineered, additional features, such as ladders, can be built directly in, which eliminates the need to add on additional accessories after the manhole and manhole cover installation.

Precast Concrete Manholes

Traditionally, that construction of manholes has used precast concrete. These precast manhole frames are engineered in segments in an off-site factory (for more details on this process, watch the above video) to ensure quality and to facilitate easy and quick installation. After being manufactured, precast manholes [1] are then assembled on site. One reason for the continued popularity of this manhole material includes its durability, with studies showing it can last for up to 100 years.

Fiberglass Manholes

Fiberglass manholes [2] are typically engineered to include a manhole barrel and cover. Within this basic structure many additional features and accessories can be easily integrated, such as separation units for stormwater, grinder channels, flumes, weirs, and more. Although there may be multiple components, the construction of manholes from fiberglass seals all pieces together to form a monolithic shape with no seams or seals that can be damaged by tree roots and other obstructions.

Unlike their precast cast iron counterparts, this manhole material is easy to handle, weighing in at one-tenth the weight of a concrete manhole. This lightweight design also makes them easier to install. They are also popular due to their environmentally-friendly nature and their durability.

You can learn more about the installation process of a fiberglass manhole in the video provided.

Manhole Covers: Functions, Materials, and Engineering

What is a Manhole Cover

Chances are you regularly walk over manhole covers on a daily basis, especially if you are a city dweller. Manhole covers are simply the removable cover that serves as a lid to a manhole. This cover serves several purposes, the most obvious of which, is preventing pedestrians from falling into these oftentimes quite deep access points.

Manhole covers also serve the purpose of keeping unauthorized personnel from accessing manholes and, in turn, keep sewer or stormwater systems safe. Manhole covers also function as a source of ventilation to prevent sewer or stormwater gases from entering residences and as an easy entry point for maintenance workers to access sewage and stormwater lines.

Manhole Cover Materials

Traditionally, manhole covers have been constructed of either cast iron, concrete, or even some combination of the two. This is because both of these materials are inexpensive, durable, and heavy enough to stay in place. However, similarly to manholes, technology has led to a variety of alternative manhole cover materials, including composite, fiberglass, and plastic. In this section, we discuss the many different types of manhole covers including cast iron manhole covers, plastic manhole covers, and composite manhole covers.

Cast Iron Manhole Covers

Historically manhole covers have been manufactured from cast iron [3], which is iron that is melted down and poured into a mold. When it comes to manhole cover material, gray cast iron and ductile cast iron are preferred as they are both economical and extremely durable. Gray cast iron is made up of iron alongside alloys including carbon and silicon. It is the carbon that results in the strength associated with cast iron. Ductile cast iron differs from gray cast iron because manganese is added to the molten iron during the casting process, which influences the formation of the carbon, resulting in an even stronger finished product.

Common Issues with Cast Iron Manhole Covers

In more recent years a variety of alternative materials have increased in popularity when it comes to manufacturing manhole covers. This is due to many issues associated with cast iron manhole covers. Some of the top drawbacks of cast iron manhole covers include scrap metal theft, injury to workers associated with the weight, and corrosion.

How Manholes Are Made

Cast iron manhole covers are manufactured in factories known as foundries, with a casting process made up of five simple steps:

Making the pattern
Preparing the mold
Melting and pouring
Cooling the metal
Finishing

You can learn more about the construction of manholes from cast iron in the video provided.

Alternative Materials for Manufacturing Manhole Covers

Due to the engineering and maintenance issues associated with cast iron manholes, a variety of alternative manhole cover types have increased in popularity in recent years including composite materials, plastics, and fiberglass. Some of the major benefits [4] of composite manhole covers, fiberglass manhole covers, and plastic manhole covers include:

Lightweight – One of the biggest problems with cast iron manhole covers is their weight, which not only makes them cumbersome and difficult to work with, but has led to concerns regarding worker safety. Composite manholes can be up to one-third lighter than their cast iron counterparts, which can weigh anywhere from 100 to 200 pounds. The lightweight design of these types of manhole covers makes them easier to maneuver and also drastically cuts down on worker injuries and workers compensation payouts.
Corrosion Resistant – Another reason for the popularity of composites, fiberglass, and plastic manhole covers is that they do not corrode like cast iron models. Since sewer or stormwater systems are notorious for being highly corrosive due to the presence of hydrogen sulfide [5] gases within them, this is a critical feature in manufacturing a long-lasting solution that will not need frequent maintenance or replacement. Plastic, fiberglass, and composite manholes can also be engineered to resist corrosion [6] caused by saltwater in coastal environments as well as salt used to treat roads in colder regions of the country.

Do Not Contain Scrap Metal – With manhole theft increasing [7] due to the scrap metal value of traditional cast iron models, many municipalities are seeking out alternative materials that do not have scrap value. Alternative materials such as composites, fiberglass, and plastic manholes save money by cutting down on theft and also cut down on the risk of injury to pedestrians and animals from missing manhole covers.
Keep Out Inflow and Storm Water – While cast iron manhole covers are made to have a loose fit, manhole covers crafted from composites, plastics, and fiberglass tend to have a more secure fit with a seal. This, in turn, keeps contaminants [8] such as runoff and oil from making their way into our sewer or stormwater systems, and eventually our local bodies of water.
More Secure – With manhole cover theft as a concern, as well as public safety, it is important that manhole covers are secure. Fortunately, these plastic manhole covers can be fitted with specialized security systems that will keep them locked in place, preventing tampering, theft, and unintentional removal.

Manhole Accessories

Manhole Locks, Security Systems, and Barriers

When using cast iron manholes, many cities around the world rely on bolts [9] to cut down on manhole cover theft issues. However, when opting for a plastic manhole, fiberglass manhole, or composite manhole, it is still important to have a lock or security system in place to ensure your manhole remains secure [10] and safe in the case of a sewer or stormwater backup, vandalism, or even a truck tire knocking it out of place. There are a variety of systems [9] out there to ensure you manhole stays in securely in place.

Manhole Maintenance and Rehabilitation

According to the Environmental Protection Agency, there are approximately 12 million sewer or stormwater manholes across the nation. These manholes typically correspond with the lengths of blocks within cities and suburbs, being spaced anywhere from 100 to 500 feet apart. However, you may be surprised to find that of these nearly 12 million manholes, the Public Works Magazine estimates 80% need some level of maintenance or rehabilitation [11].

Common Manhole Maintenance Issues

In order to keep your city’s sewer or stormwater system functioning properly, and in order to maintain public safety, regular maintenance on manholes is critical. Common manhole maintenance tasks [12] include:

Ensuring no trash or debris is on the grate opening, in front of the manhole, or inside of the manhole
Making sure the inside of the structure is clear of any dead animals, vegetation, roots, or sediment
Checking both inlet and outlet pipes for any trash or debris
Ensuring the manhole cover does not have any cracks or damage
Verifying that the frame of the manhole is “sitting flush on the riser rings or top slab and firmly attached”
Ensuring that the structure itself is sound and that all pipes are securely connected with no cracks present
Checking that there are no pollutants present in the manhole (including gasoline or oil)
Making sure that the manhole cover is in place, locked securely, and easy to remove
Ensuring that the ladder within the manhole 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 [13] 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 – Epoxies can be used 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.

Before Rehabilitation

After Rehabilitation

Manhole FAQs

As manhole manufacturers, we often receive questions about manholes. In order to most effectively answer these questions, we created a frequently asked questions section to address as many as possible.

How deep are manholes?

As previously discussed manholes vary in depth based on their function. Most manholes are categorized as shallow normal or deep. Shallow manholes are typically only two to three feet deep, while deep manholes are any manhole greater than 150 cm deep. Due to this variation, there is no one-size-fits-all answer to the question “How deep is a manhole?”

How heavy are manholes?

The weight of manhole covers varies depending on the material that is used in the manufacturing process. Historically, manhole covers have been extremely heavy due to their construction from cast iron and concrete. These heavy manhole covers typically weigh around 249 pounds. Although this is useful when it comes to keeping them in place, it can lead to injury and other issues when they need to be removed. Fortunately, there are now lightweight alternatives for manhole covers, which include plastic, fiberglass, and composites.

What are manhole covers made of?

There are many types of manhole covers which can be made from a wide variety of materials including concrete, cast iron, composites, fiberglass, and plastic. Lightweight manhole covers are typically plastic, fiberglass, or composite drain covers. Meanwhile, precast concrete manhole covers and cast iron covers are the heaviest options.

What metal is used for manhole covers?

The metal typically used for manhole covers is cast iron. Cast iron is created by melting iron, which is poured into a mold. This process results in a strong and durable finished product. There are two main types of cast iron used to create precast manhole covers which include gray cast iron and ductile cast iron.

What are manholes used for?

There are many purposes for manholes. The main manhole functions include facilitating sewer inspections and maintenance, sewage ventilation, the joining or changing direction of sewers, and allowing manholes to be installed in convenient lengths.

WANT TO LEARN MORE ABOUT PLASTIC MANHOLES?

Plastic manholes have many benefits…Want to learn more about how they can benefit you?

Traditional materials for manholes and manhole covers have been used effectively for years. However, breakthroughs in the materials used to engineer manholes, such as plastics, have created an entirely new playing field when it comes to choosing the manholes and manhole covers that will work best for your purposes.

If you are interested in learning more about the many benefits of plastic manholes in comparison to traditional models, visit our RHINO Manholes page by clicking below.

Learn More Here!

References

[1] http://precast.org/wp-content/uploads/2014/08/Precast-concrete-manhole-tech-brochure.pdf

[2] https://www.openchannelflow.com/blog/fiberglass-vs-precast-manholes

[3] http://www.madehow.com/Volume-7/Manhole-Cover.html

[4] https://www.aquatechnologygroup.com/category/applications/plastic-manhole-covers/

[5] https://www.osha.gov/OshDoc/data_Hurricane_Facts/hydrogen_sulfide_fact.pdf

[6] http://americas.ejco.com/composite-manhole-cover-benefits/

[7] http://www.nytimes.com/2012/05/04/nyregion/thieves-take-con-eds-manhole-covers.html?mcubz=3

[8] http://fiberflexfactory.com/blog/details/composite-manhole-cover-vs-traditional-manhole-cover—–/HaMhFh5dkf38sHicoLqn_h4m10RedUlSjA3j0xT9m3I

[9] http://www.mswmag.com/online_exclusives/2015/02/5_ways_cities_are_clamping_down_on_manhole_cover_theft

[10] https://www.aquatechnologygroup.com/category/applications/plastic-manhole-covers/

[11] http://www.pwmag.com/roadways/so-many-manholes-so-little-time_o

[12] http://www.stormwaterpartners.com/maintenance/documents/Manhole.pdf

[13] http://www.mswmag.com/editorial/2014/04/manhole_equipment_and_rehabilitation

Photo Credits

Precast concrete – http://www.stanton-bonna.co.uk/drainage-systems/standard-manholes-and-slabs/

Plastic manhole – http://www.euroem.com/producator-camine-plastic.php

Rehabilitation – http://www.madewell.net/manhole-rehabilitation/manhole-rehab-resources/45-manhole-rehab-process.html