The concrete just collapsed!

So we are working on converting a screened in patio into a four season room.

The Plan was to build up the outside perimeter with one course of block on top of the existing pad and and attach ledger boards and joists to meet the floor height of the existing house. Basically deck over concrete.

The engineer required us to add footers around the perimeter, to support the block and additional framing.

Here’s when the fun began:
While digging out the perimeter we found an existing footer. It was in bad shape, top course of cinderblocks were disintegrated noting but dirt. Once we dig out and expose the full footer the concrete pad cups/collapse in the middle. Not the perimeter just the middle.

After cleaning the Sh*t out of our pants, we discovered the was no dirt under the pad. The voids are any where between 5”, 2’ and 3’ in some places

My guess is water over time washed the dirt out. But where did it go?? In one of the crawl spaces in the basement has a lentil in the back right hand corner that creates and opening under the pad (5’ down). You see the dirt but no signs of wash out.

As of now we are planning on removing the pad, pouring a new footer/stem wall and then pouring a rat pad to cover where the pad was.

Any thoughts if that’s a solid plan? The change order will not be cheap because it’s a tone of manual labor. Chipping the pad with nothing under will be a pain not to mention dangerous.