Factors in excavating trenches for pipes

Factors to be kept in mind when excavating trenches for pipes:

1. Alignment and gradient: Correct line and slope as in the drawing, set with sight rails and boning rods, so that the pipe drains properly (sewers need a fixed gradient).
2. Depth: Enough cover over the pipe to protect it from loads (about 1 m for water pipes; below frost and traffic effects), and adequate depth for the gradient.
3. Width of trench: Pipe diameter plus 300 to 600 mm working space, as narrow as practical to reduce the load on the pipe and the cost.
4. Nature of soil: Soft soil needs side slopes or shoring; rock needs blasting or chiselling.
5. Ground water: Dewatering by pumping, sumps or well points.
6. Side support (shoring): Timber or steel sheeting for deep trenches in loose soil, for the safety of workers.
7. Safety and traffic: Barricades, signs, lights; spoil kept at least 0.6 to 1 m from the edge.
8. Existing underground services (cables, pipes) should be located and protected.
9. Bed preparation: A level trench bottom with a bed of sand or concrete, and joint holes at the sockets.
10. Backfilling: In layers with compaction, after the pipeline has been tested.
11. Disposal of excavated material and restoration of the road surface.