We understand by underground cables those installed below ground level. These cables may or may not be tubed.
In the case of being tubed, the characteristics do not differ much from cables that are tubed in other places.
Perhaps rodent attacks just have to be considered as a certain possibility.
Cables can be laid by hand or pneumatically. In long runs, pneumatic laying is usually used, known as the “blowing system”.
Basically the process runs using a hydraulic motor to pull the cable, while injecting pressurized air through the channel at the same time. This way, the friction between the cable and the conduit decreases and the operation is simple and fast. That is, we do not launch the cable with compressed air, the air only facilitates the sliding inside the tube.
This system is used when really long sections are installed, more than 1 km long, trying to do it without this help would significantly complicate the laying.
Installation material must be provided for this system. That is, there must be an adequate size ratio between the conduit and the cable, the cable must be rather rigid than used in other circumstances, and it must guarantee sufficient mechanical robustness.
It is also necessary to take into account that the compressed air outlet temperature is very high, around 80ºC or more. This aspect must be considered when choosing the cable or, more often, an air cooler must be installed at the compressor outlet.
Frequently for installations of this type, PkP-type cables are used, that is, double-jacketed with aramid reinforcement between the two jackets (polyethylene-kevlar-polyethylene). The aramid reinforcement provides sufficient strength to the mechanical tension and the jackets assure durability during and after the blowing process.
Fiber cables are sometimes directly buried, without being housed inside any tube. This case is frequent in deployments in the middle of nowhere, such as the cables that link the generators of a wind farm together. A trench is simply made, the cable is inserted and covered with the same removed soil. The cost of a complete trench would be much higher than the cost of the cable per meter, so it is a system that, if the cable is good enough for this kind of installation, may report important savings.
In this case, the cables, which are called “direct buried” have somewhat thicker covers and are provided with steel armor, which mechanically protects them from external agents and rodents. In case we are working in extremely acidic soils, it should be taken into account too.