---
title: "Steps of sewer construction"
subject: "Sanitary Engineering"
chapter: 3
key: "semester_6/sanitary_engineering:sewer-construction-steps"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/sewer-construction-steps
site: BCT Engineering
---

# Steps of sewer construction

Sanitary Engineering · Chapter 3: Design and Construction of Sewers

- Asked in: 2071 Magh, 8 marks
- Video: [Construction of Sewer](https://www.youtube.com/watch?v=62dloT4viS8) — Dr. Arpan Laskar
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/sewer-construction-steps
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-3

## Question

With the help of neat sketches, describe in detail the various steps of sewer construction.

## Short answer

Sewer construction steps: surveying and setting out, trench excavation with timbering and dewatering, bedding preparation, pipe laying from downstream with sockets upstream, jointing with cement mortar or rubber rings, testing, backfilling, and manhole construction. Gradient control with boning rods ensures self-cleansing flow.

## Answer

Sewers are constructed in the following steps (for pipe sewers in an urban area).

```text
 Ground level
 |<---- trench width ---->|
 |   \                  /   |   Sheeting & strutting
 |    \   backfill      /    |
 |     \______________/      |
 |      |  bedding   |       |
 |      |  ( O pipe )|       |
 |      |____________|       |
        gravel / concrete bed
```

#### 1. Survey, setting out and alignment
The centre line and the levels (invert levels) of the sewer are marked on the ground from the design drawings with a theodolite and levelling instrument. Manhole locations are fixed. Sight rails (boning rods) are set up at intervals to control the gradient.

#### 2. Excavation of trench
- The trench is excavated by hand or machine (excavator) to the required depth and width. Width = pipe diameter + 0.3-0.6 m working space on each side.
- The excavated soil is placed on one side of the trench at least 0.6 m from the edge, and the other side is left free for pipes.
- The depth must give a minimum cover of about 1 m to 1.2 m over the pipe (to protect from traffic).

#### 3. Timbering (sheeting and shoring)
In loose soil or deep trenches (more than 1.5 m), the sides are supported by timber planks and struts or steel trench boxes to prevent collapse and accidents.

#### 4. Dewatering
Water in the trench is removed by sump and pump, or well points, so that the pipe can be laid in dry condition.

#### 5. Preparation of bed (bedding)
A bed of sand, gravel or 10-15 cm of concrete (1:3:6) is laid and levelled exactly to the slope. Holes are dug for the socket of the pipes. Rock is removed so the pipe rests uniformly.

#### 6. Handling and laying of pipes
- Pipes are inspected for cracks and lowered into the trench by ropes, a chain pulley or crane.
- Laying starts from the downstream end, with the socket facing upstream, and each length is checked for level and line with a boning rod, so that the gradient is exact.

#### 7. Jointing
- *Spigot and socket joints:* the annular gap is filled with tarred gasket (yarn), then cement mortar 1:1 or lead. A fillet is formed at the mouth.
- For concrete and PVC: collar joints, rubber ring joints or solvent joints. Joints must be watertight.

#### 8. Testing of the sewer
Pipe lines are tested (see testing) before backfilling.

#### 9. Backfilling
Selected soil, free from stones, is filled around and 30 cm above the pipe by hand, and compacted in layers (15-20 cm), then the rest of the trench is filled and road restored.

#### 10. Manholes and house connections
Manholes are built at specified points, and the house connections are made with Y-junctions.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Pipes are laid starting from
   A. The downstream end, with sockets facing upstream
   B. The upstream end, with sockets facing downstream
   C. The middle of the line
   D. Any end; direction does not matter
   Answer: A. Laying upstream-ward with sockets facing upstream keeps joints properly formed and gradient controlled. (Key point: Order of laying)

2. What is used to check the level and gradient of the laid pipes?
   A. A plumb bob only
   B. Sight rails and boning rods
   C. Water hose levels only
   D. Visual inspection from the surface
   Answer: B. Boning rods against sight rails keep the sewer true to line and gradient. (Key point: Gradient control)

3. When is timbering (sheeting and shoring) needed?
   A. Only for rock trenches
   B. Only for shallow trenches in dense clay
   C. In loose soil or deep trenches (deeper than about 1.5 m) to prevent collapse
   D. Never; backfilling supports the sides
   Answer: C. Trench supports protect workers and prevent collapse. (Key point: Timbering)

4. When is the sewer tested?
   A. After jointing and before backfilling
   B. After the road is restored
   C. After house connections are in use
   D. Testing is not part of construction
   Answer: A. Joints must be accessible to repair leaks, so testing precedes backfill. (Key point: Testing and backfilling order)

## Related in Sanitary Engineering

- [Sewer construction and testing](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asewer-construction-testing) — Sanitary Engineering, chapter 3
- [Working of two sewer appurtenances](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Atwo-sewer-appurtenances-sketch) — Sanitary Engineering, chapter 4
- [Manhole: necessity and construction](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Amanhole-necessity-construction) — Sanitary Engineering, chapter 4, asked 3×
- [Grit chamber: purpose, construction, design](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Agrit-chamber-purpose-design) — Sanitary Engineering, chapter 7, asked 4×
- [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×
- [Skimming tank: purpose and construction](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Askimming-tank-purpose-construction) — Sanitary Engineering, chapter 7

## Related in other subjects

- [Construction steps of cement concrete pavement](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Acc-pavement-construction-steps) — Transportation Engineering II, chapter 3
- [Excavation equipment and sketches](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Aexcavation-equipment-sketch) — Construction Management, chapter 4, asked 6×
- [Earth compaction equipment with sketches](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Aearth-compaction-equipment) — Construction Management, chapter 4, asked 3×
- [Detail estimate report for sewerage project](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Aestimate-report-sewerage) — Estimating and Costing, chapter 5, asked 2×