As a technical consultant I am able to document significant repairs in action while advising customers on on polyurethane application. In this case we were dealing with a water filled void.
Water filed voids can be filled with two component polymer foam designed to work in wet conditions. AP 475 foam will cure to 90% full strength in 15 minutes. Polyurethane foam conforms to void shape more accurately than cement grout, and - unlike cement - it doesn't shrink or sink over time.
Take a look at the video to see for yourself...