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Sanitary manhole
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Sanitary manhole
A sanitary manhole (sewer manhole, sanitary sewer manhole or sewer maintenance hole) is a manhole that is used as an access point for maintenance and inspection of an underground sanitary sewer system. Sanitary manholes are sometimes used as vents to prevent the buildup of pressurized sewage gas. Additionally, they are used for debris removal, and application of chemicals such as degreaser and insecticide.
When underground sewer lines are laid close to the ground level (this may be at the depths of less than 0.75 metres (2.5 ft) or 2 metres (6.6 ft) depending on local regulations), an inspection chamber is used as an access point instead. Inspection chambers allow access to sewer lines without requiring a person to get in them.
Sanitary manholes should be constructed at locations where there is a change from a simple straight sewer line. These include all junctions that combine multiple lines into one or split from one, bends, changing in elevation, changing in pipe size, and changing in pipe type.
For straight sewer lines, there is no universal agreement on spacing requirements of sanitary manholes for such lines. Historically, in business cores of urban areas in the United States, manholes may be placed at each and every street block, 300 to 400 feet (91 to 122 m) apart. A more modern guideline in the United States suggested to have them every 400 feet (120 m) for small diameter sewers, and every 500 feet (150 m) for larger sewer mains. Similar spacing guidelines of 120 m and 150 m can be seen in Canada with specificities on sewer diameters. England sets maximum spacing at 90 metres (300 ft). The only exception is for sewers with diameters larger than human heights. Those can have spacing up to 200 metres (660 ft). Maximum spacing of 100 metres (330 ft) is used in Scotland. The 120-metre (390 ft) requirement is used in Brazil, and 150-metre (490 ft) in South Africa.
Historically, sanitary manholes were built in one piece by bricks. A large circular concrete was built at the bottom to form smooth channels for sewage to pass through. Brick wall was laid to raise up to be closer to the ground level. In this section, the wall diameters could be gradually smaller. Near the top, the brick wall diameters were rapidly reduced, forming a cone shape to meet a smaller diameter cover. A ladder of iron rungs was built on to the wall.
Modern sanitary manholes have five main sections from the bottom to the top. The base section is where the channels are located. The barrel section, also known as chamber, sits on top of the base. It is a straight section instead of being gradually narrower like in older manholes. The heights of this section vary depending on the depths of the manholes. Near the top is the cone section where it reduces from the size of the chamber to the size of the cover. Chimney section, also known as shaft, is a straight and narrow section that sits on top of the cone section but below the cover. The last is the cover section.
A main component of the base section is a channel that provides a smooth conduit for wastewater to flow from inlet pipe to outlet pipe. The channel is in a U shape formed by a "half pipe" as if a pipe is cut in half and left with the bottom half. The width of the channel should match the inside diameter of the inlet and outlet pipes. The lowest point of the channel is called the invert. The invert should have a slope to allow wastewater to pass through easily. The channels should be straight between the inlet and the outlet. If a direction change is needed, a channel should be continuously curved instead of having sharp corners. When joining from multiple inlets, the joint should provide a smooth transition with curves. The surface area from the manhole wall to the top of the channel is a bench. The bench should have a slope to direct water back down to the channel. The bench can be in one of the two configurations based on the lowest point of the bench that meets the channel. If the depth of the channel from the that point to the invert is half of the channel's width, it is a half bench. If the depth is the channel's width, it is a full bench. In the full bench configuration, the point from lowest part of the bench (the top of the channel) to the point that the channel shape is changed to form a U shape is a straight drop. Designing and constructing the channel and the bench should be done in high quality to reduce scouring which is a result of abrasive solids in wastewater that pass through with high velocities causing scour abrasion to degrade invert materials.
The chamber section can be built in various materials. In the 19th century, manholes in the United States were built with bricks and mortar or stone. In the mid-20th century, precast concrete was introduced. In the late-20th century cast-in-place concrete was used. Newer materials such as HDPE, concrete with corrosion reducing additives and plastics are used. Currently, precast concrete is the most common material for new manholes, but bricks and concrete blocks are still used in special situations such as when the work sites are in small area which are unsuitable for precast concrete lifting operations, or for chemical waste lines. When constructing sanitary manholes with precast concrete, the joints that connect between precast concrete pieces should have gaskets. The construction methods, materials should conform to precast concrete manhole standards such as ASTM C478.
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Sanitary manhole
A sanitary manhole (sewer manhole, sanitary sewer manhole or sewer maintenance hole) is a manhole that is used as an access point for maintenance and inspection of an underground sanitary sewer system. Sanitary manholes are sometimes used as vents to prevent the buildup of pressurized sewage gas. Additionally, they are used for debris removal, and application of chemicals such as degreaser and insecticide.
When underground sewer lines are laid close to the ground level (this may be at the depths of less than 0.75 metres (2.5 ft) or 2 metres (6.6 ft) depending on local regulations), an inspection chamber is used as an access point instead. Inspection chambers allow access to sewer lines without requiring a person to get in them.
Sanitary manholes should be constructed at locations where there is a change from a simple straight sewer line. These include all junctions that combine multiple lines into one or split from one, bends, changing in elevation, changing in pipe size, and changing in pipe type.
For straight sewer lines, there is no universal agreement on spacing requirements of sanitary manholes for such lines. Historically, in business cores of urban areas in the United States, manholes may be placed at each and every street block, 300 to 400 feet (91 to 122 m) apart. A more modern guideline in the United States suggested to have them every 400 feet (120 m) for small diameter sewers, and every 500 feet (150 m) for larger sewer mains. Similar spacing guidelines of 120 m and 150 m can be seen in Canada with specificities on sewer diameters. England sets maximum spacing at 90 metres (300 ft). The only exception is for sewers with diameters larger than human heights. Those can have spacing up to 200 metres (660 ft). Maximum spacing of 100 metres (330 ft) is used in Scotland. The 120-metre (390 ft) requirement is used in Brazil, and 150-metre (490 ft) in South Africa.
Historically, sanitary manholes were built in one piece by bricks. A large circular concrete was built at the bottom to form smooth channels for sewage to pass through. Brick wall was laid to raise up to be closer to the ground level. In this section, the wall diameters could be gradually smaller. Near the top, the brick wall diameters were rapidly reduced, forming a cone shape to meet a smaller diameter cover. A ladder of iron rungs was built on to the wall.
Modern sanitary manholes have five main sections from the bottom to the top. The base section is where the channels are located. The barrel section, also known as chamber, sits on top of the base. It is a straight section instead of being gradually narrower like in older manholes. The heights of this section vary depending on the depths of the manholes. Near the top is the cone section where it reduces from the size of the chamber to the size of the cover. Chimney section, also known as shaft, is a straight and narrow section that sits on top of the cone section but below the cover. The last is the cover section.
A main component of the base section is a channel that provides a smooth conduit for wastewater to flow from inlet pipe to outlet pipe. The channel is in a U shape formed by a "half pipe" as if a pipe is cut in half and left with the bottom half. The width of the channel should match the inside diameter of the inlet and outlet pipes. The lowest point of the channel is called the invert. The invert should have a slope to allow wastewater to pass through easily. The channels should be straight between the inlet and the outlet. If a direction change is needed, a channel should be continuously curved instead of having sharp corners. When joining from multiple inlets, the joint should provide a smooth transition with curves. The surface area from the manhole wall to the top of the channel is a bench. The bench should have a slope to direct water back down to the channel. The bench can be in one of the two configurations based on the lowest point of the bench that meets the channel. If the depth of the channel from the that point to the invert is half of the channel's width, it is a half bench. If the depth is the channel's width, it is a full bench. In the full bench configuration, the point from lowest part of the bench (the top of the channel) to the point that the channel shape is changed to form a U shape is a straight drop. Designing and constructing the channel and the bench should be done in high quality to reduce scouring which is a result of abrasive solids in wastewater that pass through with high velocities causing scour abrasion to degrade invert materials.
The chamber section can be built in various materials. In the 19th century, manholes in the United States were built with bricks and mortar or stone. In the mid-20th century, precast concrete was introduced. In the late-20th century cast-in-place concrete was used. Newer materials such as HDPE, concrete with corrosion reducing additives and plastics are used. Currently, precast concrete is the most common material for new manholes, but bricks and concrete blocks are still used in special situations such as when the work sites are in small area which are unsuitable for precast concrete lifting operations, or for chemical waste lines. When constructing sanitary manholes with precast concrete, the joints that connect between precast concrete pieces should have gaskets. The construction methods, materials should conform to precast concrete manhole standards such as ASTM C478.
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