California buried 662,700 waste tires under I-880 on-ramp

The brief

Context

In 2000, engineers upgrading the Interstate 880 and Dixon Landing Road interchange in Milpitas, near San Francisco, had to build a 26-foot-high embankment for a southbound on-ramp over roughly 30 feet of soft, water-logged San Francisco Bay mud. Conventional earth fill would have pressed the weak soil down, risking uneven settlement and forcing a wait of about 12 months for the ground to stabilise. Caltrans and state waste authorities instead used tire-derived aggregate (TDA) — shredded scrap tyres that weigh about one-third as much as ordinary soil — as lightweight fill. The project simultaneously addressed California's late-1990s problem of growing scrap-tyre piles.

Key facts

• 662,700 shredded waste tires, totalling 6,627 tons of rubber, were buried beneath the new on-ramp at the I-880/Dixon Landing Road interchange in Milpitas in 2000.
• The site had about 30 feet of unstable bay mud; the embankment needed was about 26 feet high, 700 feet long and 50 feet wide.
• A standard earth embankment would have loaded the foundation with about 3,750 pounds per square foot; TDA cut this to 2,250 pounds per square foot, a roughly 40 percent reduction.
• CalRecycle reported savings of about $477,000 compared with commercial lightweight aggregate.
• The alternative conventional option required waiting about 12 months for the mud to compress before paving.
• Rubber was placed in two layers, each up to 10 feet thick, separated by a three-foot layer of low-permeability soil, with heavy geotextile fabric on the sides.
• California generated about 40 million scrap tires a year at the time; uncontrolled piles caused fires, toxic smoke and oil runoff threatening groundwater.
• CalRecycle's case study reported 22 years of continuous traffic with no maintenance problems, ground movement or structural damage.

Timeline

1. Late 1990sCalifornia faces growing scrap-tyre piles, generating about 40 million scrap tires annually, with fire and groundwater contamination risks.
2. Design phase, before 2000Engineers weigh two options: conventional fill with about 12 months of soil settlement, or a lightweight engineered fill.
3. 2000The California Integrated Waste Management Board agrees with transport officials to supply shredded tyres; 662,700 tires (6,627 tons) are placed under the I-880 on-ramp in Milpitas.
4. After the project's successTransportation agencies across North America adopt similar lightweight-fill methods; California uses TDA for slopes, retaining walls and light rail vibration control.
5. About 22 years laterCalRecycle records show the tyre fill still performing without structural repairs.

Who has a stake

• California Department of Transportation (Caltrans) — Needed a stable on-ramp foundation on bay mud without schedule delays or long-term settlement maintenance costs.
• California Integrated Waste Management Board / CalRecycle — Diverted scrap tyres into engineering use and documented performance and cost data for future projects.
• Motorists using I-880 and Dixon Landing Road — Benefit from a ramp that has stayed even and repair-free, with the rubber fill invisible beneath pavement.
• DingXin Cheng, civil engineering professor, California State University, Chico — Researched the layered design and helped develop technical guidelines for the state agency.
• Taxpayers and the state budget — Saved about $477,000 versus commercial lightweight aggregate, plus avoided settlement-related maintenance.
• Communities near tyre dumps — Reduced exposure to tyre-pile fires, toxic black smoke and oil runoff contaminating groundwater.

Why it matters

The Milpitas ramp shows how a waste stream can become a structural material, solving two problems at once: weak soil foundations and mounting scrap-tyre stockpiles. Its documented 22-year, repair-free record and $477,000 saving turned an experiment into a model now used for highway slopes, retaining-wall backfill and light rail vibration control. For countries with soft coastal soils and large end-of-life tyre volumes, it is a low-cost, circular-economy template rather than a laboratory idea.