[suck my wake]

You should not be scared with cutting a 12x12 hole in the corner of your basement floor. I should do no harm. All along the outside of the floor where it touches the walls should be expansion joint which for sure is not water proof if there was running water under you floor it sure as well would be flowing out from this joint if anything the joint already has as much open surface area as the hole you would be cutting. My Mothers house is within 10 feet for a small stream. There is a 2 foot hole in the basement with a sump pump in it if it wasnt for that sump pump, every time it rained hard, there would have been inches of water in the basment. As long as you pipe the water a decent amount away from your house that sump pump will do no harm.