--- title: "Drop manhole: need and construction" subject: "Sanitary Engineering" chapter: 4 key: "semester_6/sanitary_engineering:drop-manhole-purpose-sketch" source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/drop-manhole-purpose-sketch site: BCT Engineering --- # Drop manhole: need and construction Sanitary Engineering · Chapter 4: Sewer Appurtenances - Asked in: 2080 Chaitra, 1+3 marks; 2072 Magh, 4 marks; 2071 Magh, 4 marks; 2070 Magh, 4 marks (asked 4 times) - Video: [Drop manhole | All About Drop Manhole in Environmental Engineering IN HINDI](https://www.youtube.com/watch?v=S3kLBzle8sw) — EASY CIVIL - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/drop-manhole-purpose-sketch - Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-4 ## Question Define drop manhole. Explain the necessity (need) of providing a drop manhole in a sewer line with a neat sketch, describing its purpose and construction. ## Short answer A drop manhole uses an external vertical pipe to safely carry sewage from a higher branch sewer to a lower main sewer, avoiding splashing and damage. It is necessary when the level difference exceeds 0.6 m, to limit excavation, scouring, and worker risk. Construction includes a drop pipe encased in concrete with a cleaning eye. ## Answer A **drop manhole** is a manhole in which the incoming sewer enters at a higher level than the outgoing sewer, and the sewage is led down to the lower sewer through a vertical pipe (drop pipe) built on the outside or inside of the manhole. #### Necessity (need) of drop manhole 1. **Steep ground:** when a branch sewer joins a main sewer at a much lower level (difference more than about 0.6 m), a direct fall would splash the sewage, cause erosion of the manhole base and release foul gas. 2. **Avoid high velocity:** sewage falling freely produces a high velocity and scouring of the lower sewer. 3. **Level difference at junction:** when a shallow sewer must join a deep main sewer, the drop pipe avoids deep excavation of the entire branch and reduces cost. 4. **Gradient control:** it lets the sewer follow a flatter gradient than the ground slope. 5. **Protection of workers:** the sewage does not fall on a person who enters the manhole. #### Purpose To bring the sewage safely from a higher sewer to a lower sewer without splashing and damage, and to avoid deep trenching. #### Construction ```text Ground level ________________ Cover | | | | <- Manhole shaft | | Inlet | | | | sewer-->| | | |__ ____ | | Vertical drop pipe | | | | (outside the shaft, in | | | | concrete encasement) | v | | | Outlet ---> main sewer |____ Channel ___| ``` - It is built like an ordinary brick/RCC manhole (brickwork in 1:3 cement mortar, circular or rectangular chamber with steps and a cast iron cover). - A **vertical drop pipe** of the same diameter as the branch sewer (CI or stoneware) is fixed on the outside wall of the manhole and connected to the branch sewer through a tee/bend at the top. A cleaning eye with a cover or plug is provided at the top for rodding. - The bottom of the drop pipe discharges into the manhole base through a bend, in a half-round channel (benching) leading to the main sewer. - The drop pipe is encased in concrete 1:2:4 to protect it from load and impact. - The base is built of concrete with a cushion to take the impact of falling sewage. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What defines a drop manhole? A. The incoming sewer enters at a higher level and the sewage is led down to the lower sewer through a vertical drop pipe B. The outgoing sewer is higher than the incoming sewer and sewage is pumped up C. A manhole with a deeper shaft than normal but no pipe D. A manhole with a sloping cover to drain rainwater Answer: A. Incoming sewer is high; a vertical drop pipe carries sewage down to the lower sewer. (Key point: Definition) 2. Why is a drop manhole needed when a branch sewer joins a main sewer much lower (more than about 0.6 m)? A. Sewage would flow too slowly in the branch B. A free fall would splash sewage, erode the manhole base and release foul gas C. The main sewer would be overloaded with water D. The cover would become too heavy Answer: B. Direct fall causes splashing, erosion of the base and foul gas release. (Key point: Need) 3. How does a drop manhole reduce cost? A. It removes the need for a manhole cover B. It allows thinner pipes C. It avoids deep excavation of the whole branch sewer D. It removes the need for benching Answer: C. A shallow branch can join a deep main sewer via the drop pipe, so the branch need not be laid deep. (Key point: Economy) 4. How is the vertical drop pipe protected in construction? A. Left bare for inspection B. Wrapped in bitumen only C. Buried in sand D. Encased in 1:2:4 concrete Answer: D. The drop pipe is encased in concrete to protect it from load and impact. 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