bphgravity said:In addition to the comments given by haskindm, the idea is to ensure the conductor has sufficient surface area contact with earth. By installing the conductor at least 18" below grade, you establish uninterruptible soil contact from all directions; in effect the weight of the 18" of soil is sufficient to compact around the conductor at that depth. While this may be accomplished at lesser depths, there is a chance that the conductor will not maintain good contact with the earth when relatively close to the surface of the earth.
bphgravity said:Probably not. There is a huge amount of research and documentation on the effectiveness and performance of grounding systems used on lightning protection systems. For the most part, they only include systems that are installed to a minimum standard or code.
I personally doubt you would be able to calculate or determine the performance of the entire system has been or will be compromised if the burial depth of the electrode does not exactly meet the minimum requirements.
Too many variables.
Keep in mind that many installations on rock or other solid ground have grounding systems that sit on the surface of the earth. These systems have been shown to be very effective when installed properly.