During the winter of 2000, a pipe contractor was attempting to bore nearly 200 linear feet under a major five-lane highway that serves as an artery carrying commuters to and from the eastern suburbs of Atlanta. At first, the boring auger made effortless progress through the wet, sandy, clay like substrate that was full of partially decomposed biodegradable materials. The contractor’s hopes of completing the boring ahead of schedule were dashed when the auger struck a large vein of Georgia Blue Granite.

Explosives were used to break the rock into small pieces that could be removed by hand. Due to the small diameter of the bore and the density of the granite, progress under the highway was slow. After completing about half of the total distance, the contractor noticed that the asphalt above the tunneling operation was sinking, creating a dangerous dip that affected the two westbound lanes of the highway.

After the DOT’s initial inspection, Barton Southern Company was brought in to determine what had caused the dip in the asphalt and propose a solution. After drilling several inspection holes through the 12-inch thick asphalt highway it was found that the substrate had settled about 8 inches directly over the centerline of the tunneling operation. The settlement was caused by existing voids created by decaying biodegradable materials in the soft, sandy substrate that had collapsed during the blasting.

To stabilize the soil, three rows of 1inch holes were drilled through the roadway 3 feet on center over the tunnel. _ inch black iron pipes were then driven through these holes to a depth of about 10 feet (just above the top of the tunnel casing). A water activated polyurethane grout was injected through the pipes and easily penetrated into the void areas and wet sandy soil. The moisture in the soil activated the grout causing it to expand to a rigid foam that filled the voids and consolidated the loose soil to a hardness similar to that of sandstone. As each of the one inch holes were grouted the gap between the substrate and the asphalt began to decrease. This indicated that the voids were being filled and the soil was being consolidated into a solid mass.

The stabilization was completed in five days without shutting down the highway.