--- title: "Sewer construction and testing" subject: "Sanitary Engineering" chapter: 3 key: "semester_6/sanitary_engineering:sewer-construction-testing" source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/sewer-construction-testing site: BCT Engineering --- # Sewer construction and testing Sanitary Engineering · Chapter 3: Design and Construction of Sewers - Asked in: 2070 Magh, 4 marks - Video: [Testing of Sewers | Wastewater Engineering | Explained Concepts of Civil Engineering](https://www.youtube.com/watch?v=w3tdPqBympo) — Simplified Learning Civil Engineering - 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-testing - Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-3 ## Question State the steps involved in construction of sewers in urban area. Briefly describe the testing of sewer line. ## Short answer Construction steps: setting out, excavation, timbering, dewatering, bedding, pipe laying from downstream, jointing, testing, backfilling, manholes. Testing includes water test with 1.5 m head for leakage, air test (pressure drop limit), smoke test for leaks, mirror-and-lamp for alignment, and ball test for obstructions. ## Answer #### Steps in construction of sewers in an urban area 1. **Setting out:** survey; mark the centre line and levels of the sewer and manhole positions from the drawings. Place sight rails over the line. 2. **Excavation:** dig the trench by machine or hand, wide enough for pipes (diameter + 0.6 to 1.0 m) and deep enough for the invert level, with a minimum cover of about 1 m. Keep excavated soil away from the edge. 3. **Timbering and dewatering:** support trench sides with sheeting and struts, and remove water by pumping. 4. **Bedding:** lay a bed of sand, gravel or concrete, levelled to the design gradient. 5. **Laying of pipes:** lower the pipes carefully and lay from downstream to upstream, with sockets facing upstream, checking the line and level with boning rods. 6. **Jointing:** make water-tight joints with cement mortar, tarred yarn, rubber rings or solvent cement. 7. **Testing:** test the joints before backfilling. 8. **Backfilling and compaction** in layers, then restoration of the road surface. 9. **Construction of manholes** and house connections. #### Testing of sewer line Sewers are tested to ensure they are watertight and straight. - **Water (hydrostatic) test:** a length of sewer between two manholes is plugged at the lower end, and filled with water to a head of about 1.5 m above the crown at the upper end. After allowing absorption, the drop in water level in a standpipe is measured over 30 min. The loss of water (leakage) must not exceed the limit given in the specification; otherwise the joints are repaired and the test repeated. - **Air test:** the pipe is sealed and air at a pressure of about 100 mm of water is pumped in; the pressure should not fall by more than 25 mm in 5 minutes. - **Smoke test:** smoke is blown in and any leak shows as smoke coming out (used for house drains). - **Mirror and lamp test:** a mirror is held at one manhole and a lamp at the next; the circle of light shows the line and the clear bore of the pipe is checked for obstruction or sagging. - **Ball test:** a ball of diameter about 13 mm less than the pipe is rolled through to check that there is no obstruction or projecting joint material. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which tool-based activity controls gradient during sewer construction? A. Random excavation depth B. Sight rails and boning rods set over the line from the drawings C. Manholes placed after laying with no levels D. Measuring tape alone Answer: B. Levels and sight rails ensure the invert follows the design slope. (Key point: Setting out) 2. In the hydrostatic test, what is measured? A. Water pressure at the pump B. Temperature of the water C. Drop of water level in a standpipe over about 30 min, compared with the permitted leakage D. Time for water to flow out of the pipe Answer: C. The pipe is filled to about 1.5 m above the crown; excess loss means leaking joints. (Key point: Water test) 3. The air test pass condition is A. Pressure of about 100 mm of water falls by no more than 25 mm in 5 minutes B. Pressure rises by 25 mm in 5 min C. Pressure falls to zero in 30 min D. Pressure falls by 100 mm in 25 min Answer: A. A small fall indicates an airtight, hence watertight, line. (Key point: Air test) 4. What does the mirror and lamp test check? A. Watertightness of joints B. Straightness of the line and clear bore (no obstruction or sagging) C. Strength of the pipe D. Quality of backfill Answer: B. A full bright circle of light between manholes shows a straight, clear sewer. (Key point: Mirror and lamp test) ## Related in Sanitary Engineering - [Steps of sewer construction](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asewer-construction-steps) — Sanitary Engineering, chapter 3 - [Short note: Testing of sewers](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asewer-testing-note) — Sanitary Engineering, chapter 3 - [Sewer diameter for a 2.5 sq km area](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asewer-diameter-2p5-sqkm) — Sanitary Engineering, chapter 3 - [Sewer diameter for a 12 sq km area](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asewer-diameter-12-sqkm) — Sanitary Engineering, chapter 3 - [Combined sewer section, 85 ha area](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Acombined-sewer-85ha) — Sanitary Engineering, chapter 3 - [Sewer for 200 ha catchment at 0.6 depth](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asewer-runoff-0p6-depth-200ha) — Sanitary Engineering, chapter 3, asked 2× ## Related in other subjects - [Quality control in urban road construction](https://www.bctengineer.com/question.md?key=semester_7%2Fproject_engineering%3Aquality-control-urban-roads) — Project Engineering, chapter 4 - [T/F: loop arrangement in urban area economical](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-loop-urban-economical) — Transmission and Distribution System Design, chapter 5 - [Hypothesis testing steps; computer assemblies](https://www.bctengineer.com/question.md?key=semester_5%2Fprobability_and_statistics%3Asteps-hypothesis-testing-computers-readjust) — Probability and Statistics, chapter 7 - [Speed-time curves: urban, sub-urban, main line](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aspeed-time-curves-urban-mainline) — Utilization of Electrical Energy, chapter 4