I like the idea in general, but in terms of gameplay, I think realistic cave-ins and more difficult farming need to come first.
You can never quite fill in a solid rock mountain to be as strong as it was before digging it out. At least not with any pre-1400 technology that I'm aware of, and certainly not with any material that you wish. Recreating a natural wall that is indistinguishable from any other natural wall would however allow this; these filled in walls would be strong enough to prevent cave-ins just like ordinary natural walls, and this seems inappropriate. (Obviously, there is already a large amount of inappropriateness, such as building an entire fort of obsidian off the side of a single floor tile of sand. But let's allow that issue to be resolved first, rather than making it even more severe than it is now.)
Secondly, being able to dump sand/soil wherever you want would make underground farming way too easy. It already is way to easy. Glassmaking would also become easier. These tasks need to become more appropriately difficult first. Farming needs a repeated supply of water; one application of soil at the beginning of time shouldn't be enough to grow crops forever. Similarly, sand tiles need to lose mass over time when used for glass. Hauling down a few tiles worth of sand shouldn't last very long. Really, it shouldn't last any longer than if the sand gatherers were just hauling it directly from its original source to the smelter (or sandbag stockpile). To do otherwise would imply that the haulers can haul significantly more sand at once when moving it from one tile to another than they can when putting it in a bag and moving it like they currently do.