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Laying technique


Trench classification

One of the most important matter when installing a conduit line is the type of trench you must use. This fact is strictly linked to the kind of ground and to the stresses to which the conduits is subjected to.
The trench classification can be made according to geometrical dimensions, such as depth, H, and width, B, of the trench in connection with the diameter of the conduit to be installed (see table).
By these two methods you can determine the type of trenches usually made for conduit installation, such as narrow trenches, wide trenches, and embankment.
The following table shows the width of the excavation, B, according to the diameter, D, of the conduit or the depth, H, for any type of trench.

Table "Trench classification"
Trench type B
Narrow trench = 3 DN < H/2
Wide trench > 3 DN < H/2
  < 10 DN < H/2
Embankment = 10 DN = H/2

Legenda:

ND = nominal diameter of the pipe
B = width of the trench measured at the level of the pipe crown.
H = height of the backfilling from the pipe crown.


TRENCH

Narrow trench
This is the best situation for laying a conduit since part of the load is discharged on the trench walls.

This type of trench must be used as often as possible, according to the ground condition.
   
     
Wide trench
The wide trench is adopted when the ground is mainly gravel and sand.
The load to which the conduit is subjected is higher than the load of a narrow trench as the load is not discharged on the trench walls.
For this reason, during the design, we suggest, for safety reasons, to take this fact into consideration.    
   
     
Trench width
It is characterised by the bedding depth and the conduit diameter; in fact, the trench width must be such as to allow to settle the bottom, to connect the conduit, and be enough place for the operators to operate.
In any case, the trench is more effective when the width decreases.
If more than one pipe must be installed in the trench, the trench width must also consider this situation.

Trench bottom
It is composed of the filling, usually sand, that offers the cable duct a constant and flat support.
Due to its resistance to mechanical stresses, the Polieco cable duct does not need a trench bottom made of casting or similar. Instead, we suggest preparing special niches on pre-arranged distances to make bar or coil jointing easier for cable installation.


TRENCH BOTTOM AND SIDEFILL

When necessary, bedding must be prepared before the trench settlement. The material suitable for bedding must be, if possible, of sand mixed with gravel or gravel mixed with 10 to 15 mm crushed stones. Due to resistance to mechanical stresses, the Polieco conduits does not need to have concrete bottom.
Bedding must be carefully compacted so that the loads can be regularly distributed along the duct.
The conduit sidefill must be carried out using easy to compact materials, such as sand, while all organic, peat, muddy, clay must be avoided due to their high water content which hinders compaction.
The trench backfilling, and in general the backfilling of all types of excavation, is the most important operation for cable duct installation. In fact, this operation must be properly carried out to ensure a perfect interaction between the cable duct and the ground and allow the cable duct to react to the ground deflections caused by the ground settlement as well as the loads which the excavation is subjected to.
The interaction between the cable duct and the ground can be assured by a layer backfilling of the trench (see picture).


TRENCH BACKFILLING

   
The trench backfilling, and in general the backfilling of all types of excavation, is the most important operation for cable duct installation. In fact, this operation must be properly carried out to ensure a perfect interaction between the cable duct and the ground and allow the cable duct to react to the ground deflections caused by the ground settlement as well as the loads which the excavation is subjected to.
The interaction between the cable duct and the ground can be assured by a layer backfilling of the trench (see picture).

The first layer is the conduit sidefill which reaches the pipe crown; here, the material used is generally the same as for bedding. Compaction is carried out on the conduit sides only.

In the second layer, 15-20 cm approximately, which is prepared using the same material of the bedding, compaction must be carried out only at the cable duct sides and not vertically to avoid useless dynamical stresses.
For the following 30 cm thick layers, the same material of the excavation is used free from all stones with a diameter greater than 10 cm and from all vegetal fragments.
Compaction must always be very careful and all material which cannot be compacted must be eliminated.
Remember to leave room for the last layer of loam soil.