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The purpose of a back water (check) valve is to prevent a sewage back up outside the house from overflowing inside the house. These valves must be accessible for cleaning and maintenance. Whoever buried it in the slab apparently didn't know this. If the broken flapper is catching solids and causing a back up then it will have to be dug up and removed. Replacing it however may or may not be worthwhile. Their only benefit is to prevent a sewage back up down stream from overflowing inside the house. Also when they are working and it closes to stop a back up it also stops any waste water inside the house from draining thus causing a back up on the house side of the valve. If you decide to replace rather than remove it must remain accessible. You might want to get more bids for the work.
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