--- title: "Manhole: necessity and construction" subject: "Sanitary Engineering" chapter: 4 key: "semester_6/sanitary_engineering:manhole-necessity-construction" source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/manhole-necessity-construction site: BCT Engineering --- # Manhole: necessity and construction Sanitary Engineering · Chapter 4: Sewer Appurtenances - Asked in: 2075 Baisakh, 1+4 marks; 2073 Bhadra, 4 marks; 2068 Bhadra (old course), 6 marks (asked 3 times) - Video: [What’s Inside a Manhole?](https://www.youtube.com/watch?v=1ztGpGjO60o) — Practical Engineering - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/manhole-necessity-construction - Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-4 ## Question Define manhole. Explain the necessity of providing manhole in a sewer line and the construction (components) of a manhole with the help of a neat sketch. ## Short answer A manhole is an access chamber for inspection, cleaning, and repair of sewers. It is provided at every change in direction, gradient, or size, at junctions, and at 30-90 m intervals for straight lines. Components include concrete base, brick or RCC walls, channels with benching, access shaft with steps, and a cast-iron cover on top. ## Answer A **manhole** is an access chamber, built on a sewer line, that allows a person to enter the sewer for inspection, cleaning, testing and repair. #### Necessity of manholes 1. For **inspection** and **cleaning** of the sewer (removing silt and blockage). 2. For **joining** two or more sewers (junctions) and for changes in **direction, gradient or size** of sewers. 3. For **ventilation** of the sewer through the cover. 4. For **testing** (smoke, mirror and lamp) of the sewer. 5. To allow sewers to be laid in straight lengths between manholes, which makes laying, inspection and cleaning easy. 6. For **flushing** and for house connections. Manholes are provided at every change of alignment, gradient and diameter, at junctions, and at intervals of 30-90 m on straight lengths (smaller sewers need closer spacing, e.g. 30 m for 300 mm). #### Construction (components) ```text Cast iron cover & frame ________________ |_______________| <- Cover slab / ring | | | | | | Neck | | <- Shaft (min 0.9 m dia.) Steps |-| | | with CI/GI steps at 0.3 m | | | | / \ <- Chamber widens (working / Chamber \ chamber about 1.2-1.5 m) | Inlet-->____ | | ( channel )--> outlet |__Benching____Base_| concrete base ``` 1. **Base (foundation):** cement concrete 1:3:6, 15-30 cm thick, laid on a firm bed to carry the manhole. 2. **Walls (chamber):** brickwork in cement mortar 1:3 (or stone masonry, or RCC), 20 cm thick for shallow and 30-45 cm for deep manholes, plastered inside and out with 1:2 mortar to make it watertight. 3. **Channels and benching:** semicircular channels of half-round pipes (or cement concrete) in the base carry the flow smoothly from the inlet to the outlet. The sides (benching) are sloped at about 1 in 6 towards the channel, in 1:2:4 concrete, so that the workers can stand and solids do not stay. 4. **Shaft (access shaft):** above the chamber, the manhole narrows to a shaft of 0.9 m diameter or 0.9 x 0.6 m rectangle, to take a cover (corbelling at the top). 5. **Steps:** CI or galvanised iron steps (foot rests), fixed 30 cm apart in the wall, for going down. 6. **Cover and frame:** heavy cast iron (or RCC for light traffic) cover, with frame set in the top slab, circular of 50-60 cm diameter, kept flush with the road. 7. A **drop pipe** is added where an incoming sewer is much higher than the outgoing one (drop manhole). Depth classification: shallow (up to 0.9 m), normal (0.9-1.5 m) and deep (>1.5 m) manholes. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is a manhole? A. A pipe that lifts sewage to a higher level B. An access chamber on a sewer allowing a person to enter for inspection, cleaning, testing and repair C. A tank that stores sewage D. A vent that releases gas only Answer: B. A manhole is an access chamber on the sewer line. (Key point: Definition) 2. Where must manholes be provided? A. Only at the treatment plant B. Only at the end of the sewer C. At changes in alignment, gradient and diameter, junctions, and at intervals of 30-90 m on straight runs D. Only at every house Answer: C. They are placed at changes and junctions and at regular intervals on straight lengths. (Key point: Location) 3. What is the purpose of benching in the manhole chamber? A. To store sewage for flushing B. To increase velocity by narrowing the pipe C. To hold the cover frame D. Slopes (about 1 in 6) towards the channel so workers can stand and solids do not stay Answer: D. Benching gives a standing place and directs flow to the channel so solids do not settle. (Key point: Benching) 4. Which shaft dimension and step spacing are given for the manhole? A. Shaft 0.9 m diameter; steps 30 cm apart B. Shaft 0.3 m diameter; steps 1 m apart C. Shaft 2 m diameter; steps 10 cm apart D. Shaft 0.9 m diameter; steps 2 m apart Answer: A. The access shaft is 0.9 m diameter with CI/GI steps fixed 30 cm apart. (Key point: Components) ## Related in Sanitary Engineering - [Drop manhole: need and construction](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Adrop-manhole-purpose-sketch) — Sanitary Engineering, chapter 4, asked 4× - [Steps of sewer construction](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asewer-construction-steps) — Sanitary Engineering, chapter 3 - [Sewer construction and testing](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asewer-construction-testing) — Sanitary Engineering, chapter 3 - [Sewer appurtenances and their necessity](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asewer-appurtenances-necessity) — Sanitary Engineering, chapter 4, asked 2× ## Related in other subjects - [How symmetrical components help analysis](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Asymmetrical-components-unbalance-help) — Power System Analysis II, chapter 4