Overcoming an Environmental Surprise with Innovative, Fast-Track Technology

As the Tennessee Department of Transportation (DOT) was finalizing the future route of I-69 through northwest Tennessee, a previously unknown, unregulated landfill was discovered directly beneath the proposed route near its juncture with a continuing segment into Kentucky. The Tennessee DOT, cooperating with the Tennessee Department of Environment and Conservation, implemented a rapid-response, technology-driven strategy to assess the potential costs and liabilities of crossing the landfill versus avoiding it. After a concerted site-history investigation, an electromagnetic geophysical survey mapped the landfill boundaries, established the area of fill, and identified areas of higher metal content within the fill. A concurrent soil–gas survey showed only isolated patterns of volatile organic compounds, often a driver for environmental mitigation costs. These results were used to optimize locations for subsurface drilling that employed the waste-minimizing sonic drilling technique. Depth of fill from exploratory drilling was combined with the geophysical delineation of landfill boundaries to yield an estimated waste volume of 65,000 m³ (85,000 yd³) within the proposed I-69 footprint. Chemical analyses were done for subsurface sonic rig soil cores, surface soil samples, the contents of drums scattered across the surface, and surface water. These analyses showed relatively modest environmental risk; it was suggested that if the landfill were excavated, most of the soil could be handled as nontoxic special waste. Nevertheless, considering both design–build factors and environmental issues, the cost of excavating and building through the landfill ($33.5 million) exceeded that of rerouting slightly east of the landfill ($29.5 million), and the route was diverted to avoid the landfill.