Sanitary manhole
[6] Sanitary manholes should be constructed at locations where there is a change from a simple straight sewer line.
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 large circular concrete was built at the bottom to form smooth channels for sewage to pass through.
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 construction methods, materials should conform to precast concrete manhole standards such as ASTM C478.
In a masonry manhole, the bricks form a corbel inward to reduce the opening to a desired size.
An eccentric cone has the top opening shifted off center all the way to one side of the manhole wall.
The heights of the chimney can vary and as they are used to make the manhole cover to flush with the ground.
[7] A common method to add a new lateral pipe to an existing sewer line without any service interruption is to use a doghouse manhole.
A special precast concrete structure has two upside-down U shape openings at the bottom directly opposite of each other.
To install this, the doghouse manhole is lowered to have the two openings to slide over the existing pipe.
[21] Sanitary manholes located in wetlands, salt marshes, or floodplains are susceptible to inflow issues.
[22] Floodplain manholes should be elevated to have their rim of the top frame above the maximum flood level to prevent the inflow.
Plastic or stainless steel bowls can be used to insert between the cover and the frame to prevent inflow to get down to the chamber.
The first method is to make the inlet sewer pipe inclined from the previous manhole.
This will allow the elevation of the inlet sewer to be lower and closer to the manhole channel.
A disadvantage of this method is a requirement to have deep trenching to lay the inclined pipe which may not be practical.
[26] In a vortex drop manhole, the flow from a higher elevation enters a vertical shaft that has a spiral structure.
[28] An alternative to having people to go down scraper manholes to remove sewer blockage is to use jet rodders and vacuum trucks.
A simple design is a manhole that allows a sanitary worker to open and close a value from the ground level to perform water filling and flushing operations.
Infiltration is when water from the ground enters sewers through defective pipes, joints, and manholes.
Inflow enters directly through opening parts of the sewer system such as vents and manhole covers.
Exfiltration testing uses water to fill up a manhole and measurements are taken to detect leaks.
In a modern time, vacuum testing is performed to check integrity of manholes and identify for leaks.
In this method, sandbags are used to seal a sewer section, then smoke is pumped into a manhole.
This damages sewer infrastructure, creates road obstacles, and prolongs a disaster recovery process.
Saturated loose soils can loses strength and stiffness in response to an applied stress such as shaking during an earthquake.
Manholes, which are sealed hollowed objects with lower apparent weights, will be forced to push upward after a soil liquefaction.
Once the pore water pressure is increased to higher than that level due to an earthquake, the plugs will be released into the manhole and the water from the surrounding soils can be drained into the manhole, preventing soil liquefaction.
A solution to this to have a flexible and watertight sheet attached over each joint on the inside wall of the manhole.
