Substation Grounding System

Arm yourself with facts, not opinions.

Scholarly articles for Determination of soil resistivity for substation grounding

...and after you have the facts, get the standards that are applicable to the installation and its locale, then have your design approved by an electrical engineer who has experience dealing with the AHJ.

Here try this... and next time do your own research!
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The resistance value you measure may well be accurate on the day you measured it. But the resistance value limit is whatever the standard the substation grounding is to be built to says it needs to be, and if you cannot achieve it with the existing soil resistivity then you dig earth pits and use additives and extra earth rods and otherwise lower the resistivity of the site until you can achieve it.

This needs to be included in the design and costs when building the substation.

...and soil resistivity varies with a number of factors, not least of which is the amount of moisture in the soil. Among the techniques for reducing it is the pouring of a tanker-load of seawater into the soil immediately where it is to be measured, though this may have unwanted impact on the vegetation in the area. If none is available, bovine urine would seem to make a reasonable substitute.

In the limit, the soil resistivity has to be low enough to operate any applicable circuit protective device in the event of a fault, and equipment needs to be selected accordingly. There are national standards for such things, which one would be well advised to follow.