Manhole ? Purpose, Types, Features and Construction
Manholes allow for vertical pipes to flow between main sewers and branch sewers. The inlet pipe of this type of manhole is higher than the outlet pipe when the inlet pipe is in steep terrain. Stop inverts are located at the same level as outlets invert in a manhole.
It is uneconomical to construct drop manholes due to their steep sloping gradient. To access utilities such as sewers, drainage systems, etc., manholes and inspection chambers are prevalent underground. Manholes are used to inspect, modify, clean, and maintain underground utilities.
Purpose of Manhole
Manholes can be used for joining sewers, rerouting sewers, and aligning sewers. Foul gases can escape through a perforated cover. As a result, it is an effective means of ventilation for the underground sewage system. It is necessary to inspect the sewer line, clean it, and remove obstructions. A manhole is necessary for laying sewer lines in conventional lengths.
Construction of Manhole
Excavation
Manhole construction starts here. Excavation occurs as indicated on the plan and in the drawing concerning the dimension and depth of the manhole. It is important to allow some digging width so that workers can work freely.
Concrete Bed
For manhole depths from 4.25 meters to above 4.25 meters, you should have a minimum of 20 cm to 30 cm of concrete in the bed. It is necessary to build a special foundation in areas with loose soil. In place of the concrete paste, the bed of the manhole is covered with a 1:4:8 concrete pastes.
Foundation
Manholes are constructed using regular bricks of strength 7.5N and mortar with a ratio of 1:4. The ratio of cement mortar in an arched structural manhole is 1:3. For manholes deeper than 4.2m, a wall of 10 inches is provided if the wall thickness is greater than 5 inches. Manholes should be smooth on the outside, and their masonry pipes should be leak proof and specially treated.
Plastering
A manhole's construction cannot be completed without this step. A masonry manhole should have a Master 12mm in thickness and a mortar paste of 1:3. Decorative plaster is required for crown portions of arched manholes, along with 1:2 cement mortar pastes.
The outsides of manholes need not be plastered, but if the soil is saturated, 12mm of plaster might be needed, along with waterproofing of the inner manholes.
Channeling
The manhole channel consists of a path that is constructed out of cement mortar, which allows wastewater to flow in a specific direction and prevents the manhole interior from being damaged. Manhole channels are constructed by mixing cement and concrete in the ratio of 1:2:4; the channel and benching must be neatly finished.
Covering of Manhole
To guard against unauthorized access, the manhole cover acts as a plug. There are many types of manhole covers, including rectangular, square, and circular ones. Covers may be made of concrete precast panels, fiberglass-reinforced plastics, or composite materials. They must be both airtight and waterproof.
Types of Manhole
Normal Manhole
A heavy cover sits on top of these at the sewer line. The depth of the cover is 150 cm. Normal manholes are square in shape.
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Shallow Manhole
The depth of a shallow manhole ranges between 75 and 90 centimeters. A branch sewer is built at the beginning of the sewer or in an area where there is not much traffic. Light is provided by the inspection chamber in the shallow manhole.
Deep Manhole
A very heavy cover is provided over the top of a deep manhole with a depth greater than 150cm. This cover can be increased in size and access can be increased.
