I'd really appreciate a bit of advice here...

I built a basic intermittent mist system and got some sand at Lowes. This is the stuff: http://www.quikrete.com/ProductLines/SandAllPurpose.asp

It says "coarse" and "sand" on the bag so based on the threads I'd been lurking I hoped it'd be fine. It looks fine on the surface but if I cut into it with a putty knife or pat the flat surface with my hand, it becomes soup...

The sand is in a wooden box with a 1/24 slope with weed-stop fabric covering a drainage slot on the low edge.

Is my issue the box? Should I put a gravel layer under the sand to help it all run to the edge?

Or is this sand just the wrong stuff?

Thanks!