When carrying out the laying of cable ducts, it is of fundamental importance to establish what type of excavation to carry out and thus determine at what depth the underground electrical cables are to be installed.
Excavations can be classified according to the geometric dimensions and diameter of the cable to be laid. Based on these two data, it is possible to define the type of excavations normally used for laying cable ducts, such as narrow, wide or endless trenches (typical of earthwork excavations).
| TYPE OF TRENCH | B | |
| Narrow trench | ≤ 3 DN | < H/2 |
| Wide trench | > 3 DN | < H/2 |
| < 10 DN | < H/2 | |
| Endless trench | ≥ 10 DN | ≥ H/2 |
Key
DN = nominal pipe diameter
B = trench width measured at the level of the upper generatrix of the pipe
H = cover height from the upper pipe generatrix
The laying depth and the diameter of the cable duct determine the width of the trench, which must be wide enough to allow for the bedding of the bottom, the connection of the conduits with the junction sleeves, and the possibility of operation by personnel. In any case, trenching is all the more effective the smaller its width. If it is necessary to lay more than one pipe in the trench, the width of the trench must also allow for the installation of any connecting saddles.
The bottom of the trench must consist of backfill material (sand, sand mixed with gravel or small gravel) so as to provide a continuous and level support for the cable duct.
The laying bed must be carefully compacted so that the loads are evenly distributed along the pipeline. The backfill of the cable duct should be carried out in the best possible way, using perfectly compactable materials such as sand or gravel. If gravel is chosen, the size of the gravel must be between 3 and 5 mm. Organic, peaty, muddy, clay soils are to be excluded due to their high water content, which prevents constipation.
The backfilling of the trench and in general of all types of excavation is the most important operation for laying cable ducts. This must be done correctly in order to achieve perfect interaction between the cable duct and the ground and thus allow the cable duct to react to the deformations of the ground caused by both its settlement and the loads on the excavation. The correct way to realise this system of interaction between cable duct and ground is to backfill the trench in successive layers. The first layer consists of backfilling the conduit up to the upper generatrix of the pipe, using the same material used to form the bedding. Constipation is only performed on the sides of the cavity. The second layer of approximately 15-20 cm, still made of the same material as the laying bed, should only be compacted on the side of the cable duct, and not on the vertical of the duct. This avoids unnecessary dynamic stress on the cable duct. For subsequent layers 30 cm thick, material from the excavation is used, cleared of stones larger than 5 cm in diameter and plant fragments. Compaction of the layers must always be carried out with the utmost care, taking care to eliminate materials that are difficult to compress.