Botched Cistern fill- How to fix?

I bought a house toward the end of last year. There was a cistern approximately 5'X5'X8' that we asked the seller to fill. He used construction fill consisting of mostly mid-sized rocks and sand. Looked OK at the time, but apparently the fill was very loose, and the top level is now about a foot below where it was when we moved in. I want to concrete over it all and put my grill on it, but I don't want to have it sink further, and digging it out is probably not an option at this point. I've heard of using some kind of grout that will sink to the bottom and push up any water while cementing down any solid material at the bottom... but no idea how to figure out what to get or how much of it. The only other option is that the walls of the cistern are about 8" wide concrete. they are starting to deteriorate, but I could probably drill down, put in some rebar, then pour a new 'cap' around the rim and build a wooden top instead of a solid concrete platform. Any suggestions as to which is the better option, or additional ideas on how to make this work?
I appreciate it!

 

What size is the intended concrete slab? That type of grout is normally brought out in a concrete truck. Installing rebar in the slab will help keep it stable but can only do so much.
Pics could be helpful - http://www.doityourself.com/forum/el...-pictures.html

 

It's roughly a 5' square. What I would do with the rebar is I would chip off the unstable concrete, then drill down for rebar, spaced about 8"-12" and put the rebar into those holes, so that the ends protrude about 3 or 4 inches from the top, then build forms around the remaining concrete walls and fill in in with concrete. The idea would be to recreate the top portion of the concrete, that is crumbling now due to years of weather exposure. The rebar would be thereto give strength to the replacement at the top of the walls. Kind of like when the dentist installs a crown at the end of root canal, but hopefully with a lot less pain! Once the walls are reinforced at the top, I would pour a cap on top. My only concern is how to deal with the sinking backfill that's there now. I could always get gravel for the top, but even running a compactor over this, how would I keep the interior from collapsing and having the slab crack and break?

 

Backfilling and compacting (with gravel) will take care of the settlement issues, and the reinforced concrete top slab will seal things up nicely. Heavy wire mesh will work just fine instead of rebar for the slab reinforcement. Don't forget to ask for air entrainment in the concrete.