Pebbles in switchyard

From IEEE 80

7.4 Effect of a thin layer of surface material

A 0.08–0.15 m (3–6 in) layer of high resistivity material, such as gravel, is often spread on the earth's surface above the ground grid to increase the contact resistance between the soil and the feet of persons in the substation. The relatively shallow depth of the surface material, as compared to the equivalent radius of the foot, precludes the assumption of uniform resistivity in the vertical direction when computing the ground resistance of the feet. However, for a person in the substation area, the surface material can be assumed to be of infinite extent in the lateral direction. If the underlying soil has a lower resistivity than the surface material, only some grid current will go upward into the thin layer of the surface material, and the surface voltage will be very nearly the same as that without the surface material. The current through the body will be lowered considerably with the addition of the surface material because of the greater contact resistance between the earth and the feet. However, this resistance may be considerably less than that of a surface layer thick enough to assume uniform resistivity in all directions. The reduction depends on the relative values of the soil and the surface material resistivities, and on the thickness of the surface material. Covering the surface with a material of high resistivity is very valuable in reducing shock currents. The value of this layer in reducing shock currents is not always fully realized. Tests by Bodier at a substation in France showed that the river gravel used as yard surfacing when moistened had a resistivity of 5000 Ω·m. A layer 0.1–0.15 m (4–6 in) thick decreased the danger factor (ratio of body to short-circuit current) by a ratio of 10:1, as compared to the natural moist ground. Tests by Langer in Germany compared body currents when touching a hydrant while standing on wet coarse gravel of 6000 Ω·m resistivity with body currents while standing on dry sod. The current in the case of dry sod was of the order of 20 times the value for wet coarse gravel. Tests reported by others provide further confirmation of these benefits .

In basing calculations on the use of a layer of clean surface material or gravel, consideration should be given to the possibility that insulation may become impaired in part through filling of voids by compression of the lowest ballast layers into the soil beneath by material from subsequent excavations, if not carefully removed, and in some areas by settlement of airborne dust. The range of resistivity values for the surface material layer depends on many factors, some of which are kinds of stone, size, condition of stone (that is, clean or with fines), amount and type of moisture content, atmospheric contamination, etc.

If you need the equations and details, I suggest you read IEEE 80. When I did substation design I would spec granite fractured on three sides, at what ever size the client preferred. Some liked large some small. There are limits in size though. It is where the points of the rocks touched other rocks in the layer that caused the high resistance areas. This is definitely not considered to be decorative rock. I always looked at railroad track ballast rock as something that would have been good substation cover, probably because it is fractured on all sides. Be careful to inspect what you get sometimes it will be different from what you ordered. Remember each time it goes through the crusher it costs the vendor more. If you are not paying attention then it is more profit for him