If the 'ground' is concrete, it will work fine.. if the ground is dirt, grass or other,, then the lower 4-6" will become mud, and the mud and coal will mix. The water will not drain and you will have a mess when you try to shovel near the bottom of the coal bin. You will lose about 6" of coal in the bottom of the bin.
You want to be able to shovel down to a hard surface. The fines in the coal settle to the bottom, and you need to shovel these up and use them.. If not, after several years of re-filling the coal bin, the bottom foot or so of the bin becomes mostly fines, and you have a real chore to use the coal near the bottom..
Concrete, or pressure treated plywood would be my choices.