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concrete floor cracks - earthquake

I have a problem in the bathroom of the house I purchased. Water has got into the concrete floor of the bathroom (from outside the house) and I have just ripped up a patch of vinyl to see the concrete is very badly cracked and has lost all structure. EQC have acknowledge the leak problem is a poor earthquake repair originally. I need advice on how to repair. I think it is beyond epoxy and may need digging out. There is about 4 square meters of concrete affected.