There are projects where reserve cables or reserve pipes/ducts are provided (empty or with a cable in them). Of course, this complicates the project and makes it more expensive, but maybe it’s worth it? It looks like we are talking about “insuring” a cable line in case of trouble, but the problem is that even if trouble does occur, it can be difficult to use the reserve provided.

There can be:
✔️reserve CABLE (without joints/terminations installed, or with them);
✔️reserve PIPE/DUCT (without a cable, or with it).

A CABLE reserve may be useful in case the cable is damaged and needs to be replaced quickly in order to restore power supply. This type of reserve is rare, and only if the cables are single-core, that is, in addition to the phases ABC, a 4th phase is laid nearby, acting as a reserve. Problems of a reserve cable:
✅ leads to an increase in the project cost and the cable route expansion;
✅ increases the number of thermal calculation options for cable selection;
✅ requires space for reserve joints/terminations;
✅ creates an asymmetry of the longitudinal parameters of the line (R and L);
✅ not applicable if the screens have cross-bonding;
✅ not applicable if the cables are transposed.

A PIPE/DUCT reserve with a cable laid inside is provided in a number of countries for extremely responsible 110-500 kV lines made with single-core cables. But in practice, the author has not seen this.

A PIPE/DUCT reserve without a cable inside, on the contrary, is very common and is may be needed in case of:
➡️ damage to the pipe (during its installation or, say, deformation due to ground/transportation);
➡️ damage to the pipe in case of a short circuit of the laid cable;
➡️ laying additional cables in the future (well-known for the case of pipe blocks, where about 25-30% of the pipes are left empty for the future).

Pipe blocks are typical only for 6-35 kV cables, and pipe reserves are made there regardless of the type of cable (single-core or three-core). In other cases, except for the pipe block, if reserve pipes are made, it is usually only if the line is made with single-core cables (as shown in the diagrams), and classes 110 kV and higher.

Talking about empty pipe reserves, the problems are the same as those discussed in the cable reserves discussed above. It should also be added that:
✅ the presence of an empty pipe filled with air prevents the three working pipes from dissipating the heat of the cables, and must be taken into account in thermal calculations;
✅ it is better to place empty pipes from below, and pipes with cables should be placed higher, closer to the ground surface.

So, of all the reserve options (cable or pipe), the most common case is laying an empty reserve pipe (in a trench or HDD), and not a reserve cable.

Reserve pipes are well described in the norms. It would be also good if the norms had clear requirements for those rare cases when reserve cables are needed.