One of the issue I face with the bases i make using ballast and sand is that once my models are ready, and need to be joined with the base, I have to drill through sand and ballast to make holes for the brass pins I have attached to model legs.
The problem with this is that drilling through sand and rock is the best way to destroy those nice 1mm or 1.5mm drill bits.
There is a very simple solution for this problem though :)
Make little mounds of Milliput around where the feet of the model will connect with the base, press the mini on them to make light impressions, and let milliput dry before adding ballast and sand to the base.
The reason for using milliput is that it cures rock hard, and is a lot easier to drill through than green stuff.
When making impressions in milliput, make sure that either milliput or model feet are lubricated so they dont stick. I use petroleum jelly (Vaseline), but you can use anything that works for you as long as you are sure it can be easily washed off.