substation stones

The purpose of crushed stones in power substations, usually about 0.08–0.15 m (3–6 in) in depth, is to retard the evaporation of soil moisture especially during dry seasons. Also, 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 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 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.

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, studies indicated that the resistivity of the water with which the rock is wet has considerable influence on the measured resistivity of the surface material layer. Thus, surface material subjected to sea spray may have substantially lower resistivity than surface material utilized in arid environments.

Also local conditions, size, and type of stone, etc., may affect the value of resistivity. Thus, it is important that the resistivity of rock samples typical of the type being used in a given area be measured.

So you see, the presence of crushed rocks are essentially for protecting the lives of the substation personnel and for soil moisture retention.

Refer to IEEE 80-2000 for more detailed explainations.