In 1999, Missouri drove recycled-plastic pins into a failing I-70 slope; the site remained stable after four years, and the method cost 25% less than conventional repairs

In an unremarkable section of Interstate 70 just outside the small town of Emma, Missouri, engineers in 1999 tried an unusual method to address a landsliding hillside. Thousands of plastic pins made from recycled bottles were driven into the ground; specifically, a 2000 constructability report by the University of Missouri–Columbia for the Missouri Department of Transportation noted that 3,117 recycled plastic pins were driven into a slope at milepost 62 that had repeatedly failed. The movement stopped completely. No steel bars or imported rock were used; compressed plastic pins made from recycled soda bottles did the job instead.

The problem baked into every hillside highway

Shallow slope failure is one of the least glamorous problems in American highway infrastructure. When water builds up inside an embankment, the soil can grow slippery and send a section of land sliding onto the shoulder or roadway. Although it rarely makes the news, the issue occurs often. The traditional fix is to regrade the hillside and secure it with imported rock, or, alternatively, to drive steel pins into place, a slow, expensive process that grows costlier still if the embankment keeps failing.

The pins themselves are very simple. The recycled plastic (mainly from used bottles) is compressed under high pressure along with sawdust and byproducts. This creates a solid plastic pin approximately 3.5 inches square and 8ft long. These are then driven into the slope in a dense pattern. The pins are driven into the slope at a right angle to the direction of the slope. This helps secure the slope, as the pins are like staples. The pins help keep the unsecured soil in place. The method works much like soil nails or rock bolts, except recycled waste stands in for steel.

The Missouri test that actually worked

Crews drove the pins at a rate of 80 ft per hour in November 1999, at a cost of about $3.90 per square foot, according to the same University of Missouri–Columbia report, roughly 25% cheaper than the traditional method. A 2004 field report by the Federal Highway Administration found the Emma site still stable after four years of monitoring, making it the longest-running success documented in the program at that time.

Since then, the idea has spread to other states. A 2015 University of Texas graduate thesis titled “Slope Stabilization and Performance Monitoring of I-35 and SH-183 Slopes Using Recycled Plastic Pins,” published in Civil Engineering Theses, traced the method’s history in more detail. Recycled plastic pins have since been used to stabilize highway slopes in Iowa and Texas, including near the Dallas–Fort Worth airport.

The catch

Despite being around for more than two decades, the pins still aren’t widely used. That span covers only part of a structure’s expected service life, no published research yet tracks what happens to the plastic after 30 to 40 years underground, including whether it eventually sheds microplastics into the soil. It’s a creative fix for a real, costly problem, but not yet one proven over the long haul.

The Emma hillside remained stable through at least the four years it was formally monitored after 1999, making it a rare, well-documented case of recycled plastic standing in for steel in highway repair. It’s a modest, unglamorous fix, but one that deserves more attention as transportation agencies look for cheaper ways to keep aging roads from sliding out from under them.