Design Guidelines to Minimize Internal Heating of Tire Shred Fills

Background

Since 1988 more than 70 tire shred fills with a thickness less than 1 m and an additional ten fills less than 4 m thick have been constructed. In 1995 three tire shred fills with a thickness greater than 8 m experienced a catastrophic internal heating reaction. These unfavorable experiences have curtailed the use of all tire shred fills on highway projects.

Possible causes of the reaction are oxidation of the exposed steel belts and oxidation of the rubber. Microbes may have played a role in both reactions. Although details of the reaction are under study, the following factors are thought to create conditions favorable for oxidation of exposed steel and/or rubber: free access to air; free access to water; retention of heat caused by the high insulating value of tire shreds in combination with a large fill thickness; large amounts of exposed steel belts; smaller tire shred sizes and excessive amounts of granulated rubber particles; and the presence of inorganic and organic nutrients that would enhance microbial action.

The design guidelines given in the following sections were developed to minimize the possibility for heating of tire shred fills by minimizing the conditions favorable for this reaction. As more is learned about the causes of the reaction, it may be possible to ease some of the guidelines. In developing these guidelines, the insulating effect caused by increasing fill thickness and the favorable performance of projects with tire shred fills less than 4 m thick were considered. Thus, design guidelines are less stringent for projects with thinner tire shred layers. The guidelines are divided into two classes: Class I Fills with tire shred layers less than 1 m thick and Class II Fills with tire shred layers in the range of 1 m to 3 m thick. Although there have been no projects with less than 4 m of tire shred fill that have experienced a catastrophic heating reaction, to be conservative, tire shred layers greater than 3 m thick are not recommended. In addition to the guidelines given below, the designer must choose the maximum tire shred size, thickness of overlying soil cover, etc., to meet the requirements imposed by the engineering performance of the project. The guidelines are for use in designing tire shred monofills. Design of fills that are mixtures or alternating layers of tire shreds and mineral soil that is free from organic matter should be handled on a case by case basis.

General Guidelines for All Tire Shred Fills

All tires shall be shredded such that the largest shred is the lessor of one quarter circle in shape or 0.6 m in length; and at least one sidewall shall be severed from the tire shred.

The tire shreds shall be free of all contaminants such as oil, grease, gasoline, diesel fuel, etc., that could create a fire hazard. In no case shall the tire shreds contain the remains of tires that have been subjected to a fire because the heat of a fire may liberate liquid petroleum products from the tire that could create a fire hazard when the shreds are placed in a fill.

Class I Fills

Material guidelines. The tire shreds shall have a maximum of 50% (by weight) passing the 38-mm sieve and a maximum of 5% (by weight) passing the 4.75-mm sieve.

Design guidelines. No design features are required to minimize heating of Class I Fills.

Class II Fills

Material guidelines. The tire shreds shall have a maximum of 25% (by weight) passing the 38-mm sieve and a maximum of 1% (by weight) passing the 4.75-mm sieve. The tire shreds shall be free from fragments of wood, wood chips, and other fibrous organic matter. The tire shreds shall have less than 1% (by weight) of metal fragments which are not at least partially encased in rubber. Metal fragments that are partially encased in rubber shall protrude no more than 25 mm from the cut edge of the tire shred on 75% of the pieces and no more than 50 mm on 100% of the pieces.

Design guidelines. The tire shred fill shall be constructed in such a way that infiltration of water and air is minimized. Moreover, there shall be no direct contact between tire shreds and soil containing organic matter, such as topsoil. One possible way to accomplish this is to cover the top and sides of the fill with a 0.5-m thick layer of compacted mineral soil with a minimum of 30% fines. The mineral soil should be free from organic matter and should be separated from the tire shreds with a geotextile. The top of the mineral soil layer should be sloped so that water will drain away from the tire shred fill. Additional fill may be placed on top of the mineral soil layer as needed to meet the overall design of the project. If the project will be paved, it is recommended that the pavement extend to the shoulder of the embankment or that other measures be taken to minimize infiltration at the edge of the pavement.

Use of drainage features located at the bottom of the fill that could provide free access to air should be avoided. This includes, but is not limited to, open graded drainage layers daylighting on the side of the fill and drainage holes in walls. Under some conditions, it may be possible to use a well graded granular soil as a drainage layer. The thickness of the drainage layer at the point where it daylights on the side of the fill should be minimized. For tire shred fills placed against walls, it is recommended that the drainage holes in the wall be covered with well graded granular soil. The granular soil should be separated from the tire shreds with geotextile.

General Guidelines for all Tire Shred Fills (July 1997)
All tires shall be shredded such that the largest shred is the lessor of one quarter circle in shape or 0.6 m in length; and at least one sidewall shall be severed from the tire shred
Tire shreds shall be free of contaminants such as oil, grease, gasoline, diesel fuel, etc., that could create a fire hazard
In no case shall the tire shreds contain the remains of tires that have been subjected to a fire
Class I Fills (< 1 m thick) Class II Fills (1-3 m thick)
Maximum of 50% (by weight) passing sieve Maximum of 25% (by weight) passing 38-mm sieve
Maximum of 5% (by weight) passing 4.75-mm sieve Maximum of 1% (by weight) passing 4.75-mm sieve
Tire shreds shall be free from fragments of wood, wood chips, and other fibrous organic matter
The tire shreds shall have less than 1% (by weight) of metal fragments that are not at least partially encased in rubber
Metal fragments that are partially encased in rubber shall protrude no more than 25 mm from the cut edge of the tire shred on 75% of the pieces and no more than 50 mm on 100% of the pieces
Infiltration of water into the tire shred fill shall be minimized
Infiltration of air into the tire shred fill shall be minimized
No direct contact between tire shreds and soil containing organic matter, such as topsoil
Tire chips should be separated from the surrounding soil with a geotextile
Use of drainage features located at the bottom of the fill that could provide free access to air should be avoided

These guidelines were prepared by the Ad Hoc Civil Engineering Committee, a partnership of government and industry dealing with reuse of scrap tires for civil engineering purposes. More information can be obtained by calling RMA at (202) 682-4800.