Stearns County DOT Finds a New Solution to an Old Problem

By: Jim Penman

The Stearns County Department of Transportation relied on the Spectra Roadway Improvement System to treat a 2.5-mile-long section of weak subgrade. County State Aid Highway (CSAH) 115 had been repaved and patched repeatedly over a 20-year period. Each time the repairs quickly degraded, causing the roadway to fail. Adding to the challenge was the fact that sections of the route traverse swamp-like soils with low shear strength (CBR=1.5%). County Engineer Jodi Teich noted, "The site contained the worst soil conditions in Stearns County. There is always water pooled up on both sides of the road.”

The conventional solution would be to remove the existing soft subgrade and replace it with good quality fill material. However, in this case, the thickness of the soft soil meant that this was not a viable option. The design that contractor Minnerath Construction used to address the project called for under cutting the roadway to a depth of 48 in. The contractor’s crew then leveled and rolled the subgrade and covered it longitudinally with Tensar Biaxial (BX) 1200 Geogrid sourced locally from The Brock White Co.

The layer of BX1200 Geogrid was covered with 36 in. of select embankment fill, followed by a second layer of BX1100 Geogrid . The construction crews completed the project with 12 in. of aggregate and 5 in. of asphalt paving.

Teich acknowledged that Tensar’s analysis suggested that the road thickness could have been reduced substantially, perhaps to as little as 12 in., but that Stearns County preferred to “err on the side of caution” and overbuild when addressing particularly difficult site conditions. A year after installation, CSAH 115 is in great shape even though the conditions around the road remain wet. It has not subsided or cracked—although nearby culverts, which were not reinforced with the Spectra System, have begun to subside.

More owners are selecting the Spectra System to:

• Simplify and speed up construction while increasing the performance of pavement surfaces;
• Decrease labor and equipment requirements;
• Reduce aggregate fill thickness;
• Reduce under cut and over-excavation requirements;
• Improve durability by spreading the applied load more evenly, reducing the pressure applied to the subgrade; and
• Enable construction to proceed even in difficult working conditions.