Why is stone used in sub-stations?

First of course to minimize GPR (ground potential rise).
Because of this, the resistance of the ground increases, the step potential (step potential) and (touch potential) limits are low and the possibility of electric shock is reduced for those who work in the substation.

It also comes with the following benefits as a bonus:
No grass or weeds grow, so it stays clean.
Movement of animals such as snakes, frogs, chameleons, rats etc. is reduced, thereby reducing the risk of short circuit, rainwater falls quickly and hence does not become wet or slippery.
If oil spills from transformers or circuit breakers etc., it does not accumulate.
It is cheaper than cast concrete and there is no cleaning hassle. Now let’s talk about step potential and touch potential. Whenever any sub-station designing is contemplated. Then these two things are noticed first. Because “Safety First!” Electric shock is inevitable if these two potentials are not properly designed.

Step potential:
The potential between two/four legs of an animal is called step potential. If the ground is not at equal potential, if a man stands with his legs apart, there must be a potential difference between his legs. This is the step potential.

Touch potential:
Touch potential is the potential between the hands and feet of a person standing on the ground while touching the body of a device.