From my observations of the game, it seems that eventually the game world will be flooded by an endless amount of dirt hills from digging mines. As one digs a tunnel, you take various types of ground materials (soils, clay, rock, etc. hereafter collectively called dirt) into your inventory that you then have to dump somewhere. After a few days, the tunnel "collapses" back to it's original ground height, thus generating dirt. I have personally dug up the same square of ground several times over the last several real world days and each time, I get the same amount of dirt I have to go dump atop my ever growing dump hill.
My easy fix suggestion is to eliminate the soil and any other unlimited resource one would encounter while digging tunnels, still dumping any limited resource into inventory. Keep those unlimited resource items still collectible when using a shovel.
I get the want for realism, and yes realistically I have to move huge amounts of dirt when excavating a tunnel. However, realistically, when my tunnels collapse, they don't return to the exact same ground height they were before I dug them out. They collapse downward, bringing adjacent land with them forming a little valley.
Bringing me to my hard fix suggestion, realistic tunnel collapse. Make the ground behave like a fluid when tunnels collapse, leaving behind depressions where the squares adjacent to the tunnel shift material into the tunnel spaces, keeping the total amount of "dirt" in the world a constant.