IEEE Std 80 deal with surfacing material resistivity to maintain the allowable step and touch potential at desirable level as a function of the resistivity and thickness of a crushed stone layer .
From the civil engineering standpoint, crushed rock may be use as soil stabilizer, internal roadway, sub drainage, etc.
Crushed stone # 2 without fines material is expected to have a wet resistivity around 2000 to 3000 Ohm-meter. A layer of 4 inches inside the substation and 3 ft beyond the fence usually provide may have good results.
