Sewer construction and testing

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.