We have used standard size polyprop sandbags (synthetic rather than hessian) for a number of years for spoil disposal and wall building. They stack very well, depending on what you put in them. Make sure you tie up the end well. Mud will probably ooze out, but sand, gravel and small rocks should be OK. Shop around, though, as the price can vary greatly. Filled and stacked carefully, they can provide a degree of support for the roof as well. The only drawback is that we have only used white and they don't really blend well in the underground environment. I suppose they do at least make it clear to future explorers that the spoil dump is not a natural feature! They do make quite a striking feature when caught in the beam of a caving lamp contrasting with the drabness of the mines where we have used them.