Actually sand is one of the better soils for compaction when done properly. It would be my choice of backfill material if 3/4 clean stone was not available. The issue with using any soil that requires compaction to backfill a liner pool is it is impossible to get mechanical compaction around all of the supports. Flood settling is a accepted practice but needs to be done with the correct material and in stalled in lifts just like mechanical compaction. So you would need to meticulously install the lifts around the supports an flood settle and pack which requires a lot of time and a lot of water. You also need to be very careful to be filling pool at the same time for proper support of walls. 3/4 clean stone is a bulletproof and cost saving in the end. So the moral of this story is you probably have the best material to try and deal with settling a incorrectly backfilled area to support your patio. The key is flooding the area as many times as possible. The absolute best approach now (short of digging it all out and properly compacting back into place) would be to make a dam around the entire area where the deck will be.Theres no way to properly compact it.