In 2007, California rebuilt 160 feet of sliding Marina Drive with 133,000 discarded tires; the lighter repair saved Mendocino County about $740,000 in costs

The last time you changed your tires, you may well have spent your wait scrolling through your phone and paying to drive off with a brand new set. Your old tires were then disposed of and may not have even crossed your mind. However, if you live in California, you could find yourself driving over them. In Mendocino County, 160ft of Marina Drive was patched in 2007 using around 133,000 tires, according to CalRecycle, the state’s recycling agency, which resulted in a total saving of $740,000 for the county. This equates to over 800 tires per foot of road being used. Although this seems like an odd idea, it was actually a well-thought-out engineering solution. Let’s delve into the details of this road patch, what the numbers really mean, and why it makes sense to use recycled rubber.

Tires in the dirt?

Not quite. TDA stands for Tire-Derived Aggregate. This is created by cutting up scrap tires into smaller pieces of rubber. The main benefit of this is the weight. It has an in-situ density of between 45-50lbs per cubic foot, according to CalRecycle’s 2022 report titled “Best Management Practices for Using Tire-Derived Aggregate (TDA) in Civil Engineering Applications.” This is roughly a third to a half of the density of typical soil or rock fill, which commonly runs from about 110 to 130 lbs per cubic foot, so the comparison is closer to "under half as heavy" than a precise one-third ratio. This means that when working on a slope, you want to reduce the weight as much as possible. According to the U.S. Environmental Protection Agency (EPA), tire shreds can be effectively used as fill in embankments due to their lightness.

The California Integrated Waste Management Board (CIWMB), the state agency that managed tire recycling programs before it was dissolved and folded into CalRecycle in 2010, collaborated with the Mendocino County Department of Transportation on this project. The existing soil fill was stripped and replaced with the lighter TDA. This was laid in two layers on top of strengthened backfill soil. The TDA and finished grade were then covered with 3 ft layers of low-permeability soil. The surface drainage was also designed with the terrain, soil type, and hydrology in mind, so drivers on the finished road were not actually driving directly over exposed rubber.

The $740,000 saving

The project's $740,000 savings figure, drawn from CalRecycle's project summary, reflects what TDA tends to deliver more broadly: per the agency's Best Management Practices report, most projects using tire shreds as lightweight fill see significant cost savings compared with conventional fill materials, which is part of why this approach has been used elsewhere in the state since.

The small print

TDA is not a magic solution. Some applications for using TDA in road repairs and embankments need overbuilding and settling. This means that the material will settle over time, according to data in CalRecycle’s Best Management Practices report. TDA has historically been used above the water table.

From an environmental perspective, a literature review undertaken by the University of Maine found that tire-derived aggregate has minimal impact on drinking water. Levels of iron and manganese are likely to increase; however, these have limited leaching potential. Impacts will depend upon local factors such as pH.

Why this is interesting

This was a good solution for certain applications. While not a miracle fix, the Marina Drive patch showed, back in 2007, how a lighter, recycled material could be engineered to help stabilize a slope-damaged road, a repair that has now been holding for nearly twenty years.