POMEROY, Wash.-Nobody knows for certain what ignited the tire chips now smoldering beneath two public roadways in the state of Washington. But the prevailing theory points to some type of spontaneous combustion in which rain water appears to have played a significant role. Michael Blumenthal, executive director of the Scrap Tire Management Council, points to several ``unique'' conditions under which tire chips were used in the two Washington road beds where the underground fires occurred.
He said this may help explain why other similar fires have not been reported in the six years since tire chips began being used in road construction.
In the case of the Garfield County site, he said, the area filled with tire chips was 45 feet deep. ``Nowhere have tire shreds been placed that deep before,'' Mr. Blumenthal said. In most cases, such fill rarely exceeds five feet.
Secondly, he said, the tire chips or shreds used were produced using a hammermill rather than a shear-type shredder, which turns out a smoother, more uniform product. As a result, they emerged as ``long, stringy pieces with a lot of exposed metal.''
One theory holds that oxidation of the exposed metal may have contributed to the heat build-up deep within the pile of shreds, ultimately helping ignite the rubber.
In any case, most observers agree, the ragged nature of the shreds discouraged their compaction within the fill, creating air pockets to help feed the fire.
Another possible factor was that the chips in both cases were covered with highly organic soil, rather than the inorganic material such as clay or gravel that usually is used, according to University of Maine Professor Dana Humphrey. He has toured both sites and is preparing a report on the subject for the Federal Highway Administration.
Mr. Humphrey said both sites are covered with four to five feet of highly organic topsoil. Observers theorize that rain water may have washed some of that organic material into the chips, creating an atmosphere in which bacteria could successfully exist.
Aggravating this situation in the case of the Garfield County fire may have been the fact that the ravine containing the tire shreds became flooded with water, some of it rich in nitrogen, phosphorous and potassium-all basic agricultural chemicals used to fertilize the soil in nearby farms.
Though it was not flooded, the site of the Ilwaco fire is located in southwestern Washington where it receives a lot of rain, Mr. Humphrey pointed out.
Mr. Blumenthal, for one, speculates that bacteria present in the shreds may have produced one or more gases capable of igniting in the presence of oxygen.
``We believe that when the water receded and the air came back into the fill, the oxygen ignited the gas that was in the fill,'' he said.
Hopefully, more will be known after April 8, when Mr. Blumenthal's group hosts an industry meeting on the subject in Washington, D.C. Out of that meeting, the group hopes to develop a working hypothesis, under which similar fires can be produced under controlled conditions.
From such research, the council wants to develop guidelines for eliminating the threat of such fires, thereby helping to preserve the market for tire chips in civil engineering applications.