Decks, Patios, Porches and Docks - footers - what is "solid ground"

My county requires deck footers to be a minimum of 24" deep AND to "solid ground". With two of my footers, I hit hard clay very early and it took a digging bar to chip my way down to the minimum required depth.

However, with the next two I dug, I got to much softer clay again well before the 24" minimum, and I am easily able to go deeper & deeper with only light pressure on my post hole digger.

So my question is, what does "solid ground" mean? If I keep digging through this softer clay will I hit the hard stuff again, or do I just need to tamp the bottom?

Thanks.

I stand corrected - it was the county I used to live in that said "solid ground". My current location says "solid undisburbed ground". OK, so it cant be backfill. But my house was built on 30 feet of "controlled backfill", and I can tell you for a fact that my house footers aren't more than 24" deep. So am I good at the minimum 24" deep?

If you are in the controlled backfill, you have met intent of the requirements. If you are using concrete you will be OK. If just wood posts without concrete, someone could possibly make an arguement, but for a deck you should be O.K.

The difference in the two clays may have been due to the moisture content. Clay is not as reliable as more granular materials that lend themselve to compacting easier.

Dick