When I first started in the landfill business, one of the first bits of insider information I received was that tires float ... right up out of the landfill. And I suppose that if you've been in the landfill business for any length of time, you've probably heard a similar tale. Floating tires. Yeah sure.
Yet after working at the landfill for a while, I saw it myself. Tires in a landfill were like Joe Lewis in the ring, they just wouldn't stay down.
Now this isn't to suggest that it's some kind of alien plot. It isn't some kind of mystery that goes against the rules of modern science. Nor is it, as I heard one person suggest, that the tires eventually fill up with landfill gas and float to the surface like a hot-air balloon. But the fact is, if they are placed near the surface of the landfill, some of them will, in time, emerge.
It's simply that tires, being flexible and rubbery (duh, Bolton, they're made out of rubber), will spring back into shape after being compressed. And, if they're near the surface and surrounded by materials that are less springy, they can tend to work themselves upward.
Build a "Floating Tire" Simulator
Picture this ... or if you're a hands on kind of person, go ahead and do it: Place a soft rubber ball (like a tennis ball) in a bucket, then cover it with little scraps of paper, leaves and dirt. Cover the ball to a depth of 3-4 inches. Then tamp everything down with the end of a 2 x 4. Guess what, after a little "compaction" the ball will pop to the surface. So will tires.
No, tires won't climb 10 feet through compacted garbage like baby turtles headed for the ocean. But depending on the type of waste, type and size of tire, depth of burial, and amount of compaction/traffic, tires can certainly come to the surface.
Based on these factors, tires often emerge on sideslopes. On sideslopes, waste is often less compacted, thus tires have an easier time getting out. Also, the process of placing, grading, and compacting cover soil will increase the chance of tires emerging.
Similarly, tires will often emerge through haul roads that cross the landfill. The repetitive tamping or "pumping" caused by traffic will often bring tires to the surface. Heavy vehicles (i.e., scrapers) are more likely to expose tires than light traffic.
Minimizing the Problem
If you want to minimize the problem with floating tires, here are some ideas:
First, you can choose not to place tires in the landfill. Or, if you accept them, require that they be cut. Cutting them, takes away some of their bounce.
Place tires deep within the cell and cover them with at least 4 feet of waste. It's a sure bet that if you can see any part of a tire prior to placing cover soil, it's likely that it will emerge even after it's covered. With that in mind, some landfills have a policy of hand picking all exposed tires before placing cover soil. Unfortunately, while this method gets rid of the tires, it also creates a safety concern by requiring that a person walk across raw waste.
Restrict vehicle traffic to established roads and don't allow random driving across the landfill. Vehicle traffic on areas that have only 6-12 inches of cover soil can expose tires.
Finally, if all else fails, you could just shoot holes in all the tires and let the gas leak out. Everyone knows that tires with holes in them won't float.
Neal Bolton, a registered engineer, author of "The Handbook of Landfill Operations," and principal of Blue Ridge Services, provides operational consulting for landfills in the U.S. and abroad. He can be reached at firstname.lastname@example.org.