Site Characterization and Geotechnical Roadway Design over Karst: Interstate 70, Frederick County, Maryland

Geotechnical engineering efforts must address the geological findings from site characterization, as exampled in the roadway widening and frontage road improvements along Interstate 70 (I-70), Frederick County, Maryland. The project site traverses a karst area that exhibits an aggressive sinkhole occurrence rate that averages roughly 8 sinkholes per year. Approximately 138 sinkholes have been reported in 2001 within an 8± km² area. A sinkhole risk assessment identified potentially hazardous areas of sinkhole occurrence and predicted the risk levels of sinkhole formation within the project corridor. Since completion of the sinkhole risk assessment analysis, approximately 63 percent of the new sinkholes developed in areas identified as moderate to high risk for sinkhole occurrence. A significant task in the geotechnical engineering efforts involves the control of stormwater runoff utilizing a geosynthetic membrane along drainage ditches and within three stormwater management ponds; creating the largest known impermeable stormwater facility in the State of Maryland. Two pumpstations pump the stormwater through a 1.8± km long force-main system and into an existing creek north of the project site. The pumpstations require a deep foundation system utilizing micro-piles. Proven installation techniques and innovative grout mix make micro-piles well suited for the anticipated cutter and pinnacle subsurface conditions. Geophysical and test-boring data lead to the development of subsurface contours along the top of epikarst and top of rock. The bedrock topography and the characteristics in the epikarst zone provide information on the groundwater flow patterns. Based on this information, the merits of grouting versus an inverse graded aggregate filter system that allows for the movement of water and minimizes the transport of fine materials are compared. Although based on proven geotechnical engineering principles, permitting restrictions prohibited the advancement of select treatments. As such, roadways traversing high-risk areas are grouted. Time Domain Reflectometry (TDR) systems will be used to monitor potential post-construction movement beneath the embankments. The instrumentation system will automatically notify appropriate personnel when a threshold value is exceeded.