Flowable fill questions

I have a customer who called me the other day about a little bridge on her property washing out. The "bridge builder" set 2x2x6 concrete blocks (mafia blocks) for abutments and then just poured 4" slabs for approaches and a suspended 4" slab to cross. Now the water has eroded under one of the approaches. Ideally would be to break out the slab, cut it down solid and backfill with compacted gravel and repour the slab, costwise thats not feasible. My thought is to cut out either side of the approach slabs and set blocks to reinforce and protect them, which will also give me a "diversionary structure" to channel the water. Then cut a hole in the washed out approach slab and run flowable fill under it to fill the void and make it solid. Here's the hard part... I've never dealt with flowable fill and don't know anything about it. Will it set up under an existing slab given time to set and cure? I'm open to suggestions.



Thanks

John