Manhole

A manhole is a specialized opening that provides access to a confined space underground.

These openings lead to various structures like shafts, utility vaults, or large vessels. They serve as vital access points for many public utilities. Workers use them to perform inspections, carry out maintenance, or implement system upgrades. Most underground services rely on manholes for these tasks. This includes water, sewers, telephone, and electricity. They are also used for storm drains, gas, and district heating systems.

Manholes are most frequently found in urban areas. They are typically located in streets or occasionally under sidewalks.

In contrast, rural or undeveloped areas often handle services differently. In those places, telephone and electricity lines are usually carried on utility poles or pylons. In Australia, the term "manhole" has an additional meaning. It can refer to a square access hatch used to enter the ceiling cavity of a building from a room or hallway.

To protect the opening, a manhole cover or grating is used.

This flat plug is designed to prevent accidental or unauthorized access. Traditionally, these covers were made of metal. However, they can also be constructed from precast concrete or glass reinforced plastic. Some are made of other composite materials, especially when there is a concern about cover theft. In Europe, there is a trend toward using lighter-weight composite materials. These materials are easier to handle manually due to weight restrictions. They also offer better slip resistance and electrical insulating properties.

Shape plays a very important role in manhole design. Many manholes are round because a circular shape resists the compression of the earth most effectively.

Round covers are also practical because they can be moved by rolling them. Furthermore, a round cover cannot fall into the opening. However, shapes vary by region. In the United Kingdom, covers are nearly always square or rectangular at the street level. In some areas, such as Cambridge and its surrounding villages, manholes can be triangular.

Some specialized manholes are built entirely from composite materials like fiberglass.

These composite manholes are used when there is a risk of corrosion from hydrogen sulfide, which is found in sewer gas. They are also useful when infiltration or exfiltration must be prevented. These structures can be factory integrated with many different components. For example, they can include flow inverts or flumes to manage water. They may also contain drop structures, weirs, storm water screening structures, or sewage grinders. Some even use energy absorbing structures to dissipate turbulence or velocity in the flow stream.

Engineers must follow strict design standards when placing manholes to ensure effective drainage and easy maintenance. The distance between two manholes depends on the specific setting of the project. In cities, manholes are typically placed 20 meters apart. In industrial or commercial settings, the distance is usually between 10 and 15 meters. For manholes located near homes or restaurants, the distance is typically between 7 and 10 meters. These precise measurements help ensure the entire underground system functions correctly.

While essential, manholes can present significant hazards if they are damaged or uncovered.

In urban areas, stray voltage and broken covers are serious concerns. For instance, on January 16, 2004, Jodie S. Lane was electrocuted by a metal cover in New York City. In December 2020, Justin Carey was injured in Smithtown when a split cover gave way, causing him to suffer a strained Achilles tendon and a bruised knee. Additionally, failures in underground electrical lines can cause fires or sudden releases of energy. These events can cause manhole explosions that blow covers into the air. To mitigate this, some utilities use safer covers that feature metal rails to vent hot gas while remaining attached to the ground.