Polyurethane resin injection to stabilize sinking highway
Contractor: Atlanta Boring and Tunneling
Subcontractor: Barton Southern
Client: Georgia Department of Transportation
PROBLEM: A pipe contractor was attempting to bore under a five-lane highway carrying commuters to and from the eastern suburbs of Atlanta in winter 2000. The bore was to be about 12’ below the surface of Highway 78 and nearly 200’ in length.
At first the 36” 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.
The initial portion of the bore was enlarged to 48” to accommodate hand digging, transforming the job into a tunneling operation. 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 distance, it was clear that the asphalt above the tunneling operation was sinking dramatically, creating a dangerous dip that affected two westbound lanes of the highway. Tunneling was halted immediately and officials with the Georgia department of transportation were called in to assess the situation.
SOLUTION: After the initial inspection, Barton Southern (then a sister company of Prime Resins), a contractor experienced with soil stabilization using chemical grout injection was called in to advise. They drilled several holes through the 12” thick asphalt and found that the substrate had settled about 8” beneath the asphalt directly over the centerline of the tunneling operation. The settlement was likely due to existing voids created by decaying biodegradable materials in the soft, sandy substrate that then collapsed during the blasting.
To stabilize this situation, 1” holes were drilled in a grid, five feet on center, beginning over the centerline of the tunnel and offset to either side. Prime Flex 920, a hydrophobic, water-activated polyurethane grout, was injected through 3⁄4” black iron pipes driven to an elevation just over the top of the tunnel casing. The Prime Flex 920 easily penetrated the void areas and poor soil. Water in the substrate activated the grout causing it to expand and form a rigid foam that filled the voids and consolidated the loose soil to hardness similar to that of sandstone.
The grout was placed in lifts (vertical intervals) from the top of the tunnel casing up to the top of the substrate. As each of the 1” holes were grouted, the gap between the substrate and the asphalt decreased as the voids were filled and consolidated. The expansion of the grout into a rigid foam rebuilt solid mass back into the substrate.
The process took just five days and the highway remained open the entire time. After the soil stabilization process, a paving contractor milled and inlayed the remaining dip in the pavement. The work passed D.O.T. inspection and the tunneling project was completed shortly thereafter.
BENEFIT: Soil stabilization by chemical grout injection saved the pipe contractor time, money and public scrutiny. An open ditch restoration––the alternative solution––would have taken considerably more time and money to complete and the necessary road closure would have created major inconveniences for commuters, aggravating already heavy traffic congestion.