For a substation and a substation grounding grid, all the conductive fixtures in a substation and the surrounding area must be directly grounded or bonded. Ideally, the main grounding rods should penetrate down to the water table but this is not always possible. The resistance of the soil should be tested before determining the dept to set the grounding rods and the distribution of the grid.
For example, if a cow was standing just between two parts of the grid that did not overlap and there was an earth fault current which made a potential difference between those two parts of the grid, then the cow would be electrocuted.
So, in a substation grounding grid, the grid must be designed with total overlap to prevent any potential difference between any parts of the grid. The resistance of the ground must also be determined for dry conditions when the moisture in the soil is low and the water table at a lower level than normal.