When a Missouri River backwater stream near Washington, Mo., in Franklin County, nearly destroyed old Route 100 during floods that struck the St. Louis area in 1993, the water washed out soil to a depth of 16 feet, destroying two lanes of the highway. City crews tried dumping asphalt and concrete riprap on the remainder of the embankment to stabilize it, but erosion continued, threatening to destroy the rest of the highway. That's when use of an unlikely combination of materials--permeable concrete and geotextile fabric--saved the embankment and the highway.

The design required forming the embankment with 2-foot-thick cells of soil compacted in three 8-inch- thick lifts and wrapped with Geotex 4x4, a woven reinforcement geotextile made by Synthetic Industries Inc., Chattanooga, Tenn. Eight of these geotextile-wrapped cells brought the embankment back to its original elevation. To anchor the permeable-concrete slope cover, crews dug a 3-foot-wide, 3-foot-deep ditch in the original soil at the base of the new embankment and lined with the geotextile fabric. This ditch was filled with permeable concrete when the slope cover was placed.

To drain water that would flow through the slope cover, ditches were also dug perpendicular to the base of the new embankment and sloped to daylight at the creek bank. These ditches also were lined with the woven geotextile and partially filled with permeable concrete. Finally, they were covered with soil to form French drains.