On April 5, 1991, inspectors walked a short stretch of gravel road beside the Dane County landfill near Madison, Wisconsin, and scored it with PASER, a visual road-rating system. Under parts of that road sat shredded car tires instead of soil, and the embankment came out of the inspection with strong marks for drainage. It was one of the first American attempts to treat scrap tires as building material rather than waste, and versions of the same idea now hold up highway ramps and rail lines.
The experiment
Civil engineers Peter Bosscher, Tuncer Edil and Neil Eldin of the University of Wisconsin-Madison built the embankment in 1990, with the Wisconsin Department of Transportation and the state Department of Natural Resources as partners. The state had a reason to care: about 20 million waste tires were sitting in stockpiles, with roughly 4 million more arriving every year.
The team built eight sections, each about 20 feet long and 6 feet high. Six used tire chips, either alone, mixed with soil or layered with it, and under different depths of soil cover. Two used reinforced soil. Garbage trucks drove over the lot every day, which is about as harsh a test as a gravel road gets.
It was not smooth at first. The surface broke up within days, crews added crushed rock and regraded it, and rutting and potholes kept them busy through the summer. Calcium chloride went down for dust. The road shut for the winter in December 1990 after heavy snow. When the inspectors came back the following spring, the scores were good, though the engineers cautioned that the regrading before the visit flattered them.
Their published verdict, in the Transportation Research Record in 1992, was measured. Once the chips settled, the road behaved much like an ordinary gravel road. Chips under 3 feet of soil did better than chips under 1 foot. Rollers and vibrating plates barely compacted the rubber, and steel wire poking from the cut edges could puncture vehicle tires. Lysimeters buried beneath the chip sections collected water from April 1990 to June 1992; the pH held near 7.5, and the authors judged it unlikely that the tires would affect groundwater, although manganese and zinc showed some leaching.
Why rubber chips drain so well
A pile of tire chips is mostly gaps. The pieces are irregular and springy, so they never pack as tightly as sand or clay, and water runs straight through the voids. Federal Highway Administration guidance puts their permeability at 1.5 to 15 centimeters per second, in the same range as clean gravel, while a compacted fill of chips weighs only one-third to one-quarter as much as ordinary earth. That combination, drains like gravel but weighs like a fraction of soil, is the whole reason engineers keep coming back to shredded tires. Load squeezes some of that advantage away: in landfill tests, water flowed through unloaded chips at 0.87 centimeters per second and through heavily loaded ones at 0.13.
Then and now
Wisconsin was early but not alone. Minnesota had been laying shredded tires under logging roads on soft ground since 1985, and by the late 1990s at least 15 states had used the material in fills. The Wisconsin numbers show why the idea appealed. Federal guidance estimates about 75 car tires per cubic yard of compacted chips. Working from that figure, Wisconsin’s 20-million-tire stockpile would have filled roughly 267,000 cubic yards of embankment (20,000,000 divided by 75). At about 100 tires per ton and the reported processing cost of $30 to $65 a ton, shredding the whole pile would have cost somewhere between $6 million and $13 million (200,000 tons times $30, and times $65).
The practice also brought a lesson the 1990 test did not reveal. In 1995 three tire-shred fills, two in Washington State and one in Colorado, had combustion problems inside them, and federal guidance now calls for geotextile wrapping, limits on layer thickness and a soil cap to keep oxygen out. Placing tires in open water is a different matter: a review by Edil concluded that tires in surface water are toxic to aquatic life and that the practice should be abandoned.
Today the material carries a name, tire-derived aggregate, and California is its biggest showcase. The state’s recycling agency, CalRecycle, lists uses that include lightweight fill behind retaining walls, vibration damping under rail lines, landslide repairs and stormwater infiltration galleries. A 2004 rail project for the Valley Transportation Authority used 1,000 tons of it and saved more than $1 million, and more than 3 million tires have gone into California road repairs. For a sense of scale against Wisconsin’s old problem, California alone generates more than 51 million waste tires a year. Badiyaa covered another recycled road material, crushed glass in asphalt, in a recent piece.
The Wisconsin embankment’s full record runs to nine pages, 44 to 52, in volume 1345 of the Transportation Research Record.
Further reading
- FHWA: Scrap Tires, Embankment or Fill (User Guidelines for Waste and Byproduct Materials)
- CalRecycle: Tire-Derived Aggregate
- UW-Madison Recycled Materials Resource Center: Project 79 summary
- Minnesota DOT: Using shredded waste tires as a lightweight fill material for road subgrades (1994)
Image: TeWeBs, CC BY-SA 4.0 via Wikimedia Commons
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