During the early spring of 2012, a large, deep-seated landslide threatened the southbound lanes of Interstate 75 near Jellico, TN. TDOT geotechnical engineers designed a repair using large rock buttress, but recognized the need for rapid temporary shoring during the excavation phase of the project. Based on the success of a similar repair less than five miles away where over 500 launched soil nails were installed in less than 4 days, a similar temporary shoring plan was included in the 2012 design. During the excavation, however, the prime contractor encountered an unexpected “trough” in the bedrock as well as significant subsurface water. Those unexpected site conditions led to a massive failure of the existing southbound lanes that literally developed overnight. The slide measured over 263 vertical feet from scarp to toe and was over an acre in area. During the initial phases of the failure, the slide was moving more than 1 inch every 2 minutes.
GSI® engineers were on-site within 12 hours to assess the situation and quickly developed a design/build mitigation plan that included launched soil nails, self-drilling SuperNails® up to 120-feet long, and 200,000-pound capacity six-strand anchors. Less than 24 hours after the initial failure, GSI®’s crews and equipment (as well as a senior geologist) arrived at the site. During the initial 18 hour period of construction, 258 launched prevented the regression of the headscarp into the remaining northbound lanes, and interstate traffic resumed. Over the course of the following 26 days, GSI® crews worked 3 shifts to install over 6,500 SF of temporary slope stabilization that included 11,510-feet of Self-Drilling SuperNails®, nearly 1,000-feet of strand anchors, and over 1,000-feet of horizontal drains to increase the slope factor of safety from less than 1.0 to 1.1 (with launched soil nails) to 1.3 (with self-drilling SuperNails® to 1.5 (with strand anchors). Despite the massive and unexpected failure during construction, GSI®’s emergency design/build solution enabled TDOT to complete the overall buttress construction 6 weeks ahead of the original construction schedule.
Swinging Bridge Anchors, Virginia Department of Transportation