There appears to be a bit of misinformation about dealing with the old tip. From days before I was at work I can remember using the old tip. From memory it was uncontrolled waste including dust carts. When the filling ended the area was capped with a clay material. Now it would be described as contaminated fill of poor structural strength. It can be dealt with for any type of construction. As you said piling can be used to support buildings. Piling can also be used to support slabs, (aprons and taxiways). A membrane would be needed to prevent any rainwater getting into the ground. A system to control and extract methane would be needed. The area can be built on, in my opinion.