Access and Maintenance of Sewers: Manholes
What is a Manhole?
Manholes, maintenance holes, sewer access points, sewer access chambers……. the list to describe them can be endless. Regardless of what they’re called, let’s take a look inside them and what their basic function is?
These are the assets that provide a surface connection to the underground sewer network below. Manholes split gravity sewer mains at regular intervals, often at changes in direction. Their primary function is to provide access to pipes for cleaning, removing blockages, and allow condition assessment inspections by maintenance crews.
Within the manhole there are a range of sub-components; it’s much more than just a hole. So now we know the basics, let’s jump in (not literally).
Parts of a Manhole
Manhole Cover and Ring
Generally round and constructed from cast iron with a concrete inset, the cover sits tightly inside the ring with the intention of creating an airtight seal to stop odours escaping, and surface inflow entering. In most cases, the cover should be flush with the surrounding ground surface level.
Typically made from steel, although more recently non-corrosive materials, step irons are installed to enable maintenance personnel to climb down. In some cases, these are being removed, or not installed in new builds due to safety implications with workers lowered in using dedicated confined space entry equipment.
Cone/Straight Back Taper
Located just under the cover there is a taper or cone increasing in diameter to transition the internal diameter to a larger size at the sub-surface level.
Below the taper, sits the shaft. This leads down to the base of the manhole. In significantly deeper manholes there will be a notable separation between the shaft and chamber. The diameter of the shaft varies between 1m to 1.8m.
This is a flatter concrete surface prepared with the intention of providing an area where maintenance crew can position themselves and stand within the manhole above the pipe to perform any tasks that are required .
The channel is located at the center of the benching and is where the sewage/wastewater flows through the manhole.
The invert is located within the channel and is the lowest part of the manhole. It is a crucial level which is set with reference to a specific datum, specifically to allow the water to flow by gravity to its intended location.
These are the flow entry and exit points of the manhole. There can be multiple inlets to the manhole but almost always only one outlet. In special instances a bifurcation manhole may be installed that splits the flow into separate downstream pipes.
Location of Manholes
Their locations can vary from in the roadway, within properties and almost anywhere that sewer pipes are located (which some of the time isn’t the most practical). With relation to the sewer network, they are generally located at changes in direction, grade/slope, invert level or intersection of another sewer branch.
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