You probably need to build a form out of lath mess or even several layers of chicken wire. You can add some strength with the canned, exanding spray foam insulation, then pat your cement on top of your form. Lots of threads on this, so a search would give you loads of helpful information.

If you still want to build a rock with the hole in the ground, then be sure to add something like joint tape mesh pieces between layers.

I agree with madhabitz

I used lath mesh to build an entire waterfall. The waterfall is comprised of several boulders which were built separately, then stacked together.

The boulders are about 4x3x3, and I could carry them easily, though I wouldn't try dropping them too hard on the floor (they could resist it, but why risk?)

Use the mesh as an inner mold, and apply about a 1 inch thick layer of tufa.

Look up my post called "Why Square". I used it to create a boulder-planter, but it's the same technique... only difference is I didn't flip over and do the other side ;-)

Hope this helps

I made a giant Hypertufa boulder here while back, & found that if you just make a big pile of dirt, and cover the surface with it, is much better than the hole method.
For one thing you can pile the dirt as high as you need it, plus if you wet the dirt prior to applying the tufa, it will help the curing process. Not only that but doing it this way makes it a lot easier in pre-determining how its going to look in the end. This will give you the "tip of the iceberg effect" where most of it appears to be still underground.
I just placed Chicken wire over the dirt, then packed the Hypertufa on it about an inch thick, then sculpted it to my likens, then went out the next day & brushed it down good with a wire brush. I think i would wait at least a week or more before trying to remove it though. Good luck.. Fenicus