Is this rat slab as useless as it seems?
I would leave the slab in since there is water down there and moving. This kind of slab in Oregon is to reduce moisture or make a storage space not rodents and I would believe this would be the case since it only partly covered. If you remove the slab all the wet earth will be exposed and the home humidity levels may drasticly increase and possible musty smell, if the slab is removed a new vapor barrier should be put down.