Steps of sewer construction

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

 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.