Not a new idea, although you're much better off with the content of the bag including a cementing agent to help it hold it's shape after some kind of treatment is applied.
Concrete is a good example. It can be largely sand or rock by volume, but the cementing agent will harden it to provide structural stability.
That is also what I have used when building or improving the bunker / underground storage - storm shelter.
I would use sand or earth bags but first of all I have only a couple bags and sand is not too easy to acquire. Any rock - stone free earth I have is for a garden, mainly container gardening.
Here is one pic of some hardened 80 pound bags of concrete which I have stored about 10 feet in front of the steel front door of the bunker. >
I hate to have let those 12 good bags get hardened, which they did when I had to leave them over the winter until the next June. But I did not have time to use all of the bags that summer a few years ago. I can use them, if necessary, as more good protection for the slightly vulnerable front door.
Or if I need them elsewhere, such as for walls, I will use them. I have also obtained quite a few hardened bags of concrete from the Encampment, WY landfill. And one more pic showing a wall I made the past couple summers and the huge amount of dirt and rock from the large hole dug for the new cabin. I used 5 eighty pound bags of mortar to hold these bags together. Only the top of the bags are shown with maybe 25 bags total, many of them buried. The hole that I can look and shoot out of the bunker is shown in the middle of this pic >