Excavation of Fallen Rock in Wedge Failure A wedge failure developed in a new rock cut slope along U.S. Highway 2 west of Libby soon after it was constructed in 1992. By 2005, the wedge failure had retrogressed beyond the crest of the original cut slope to form a trough, extending approximately 130 feet into the slope that was over 150 feet wide and nearly 200 feet high. Due to the slope height, continuing rockfall activity, concerns for worker and traffic safety, and a history of vehicle involvement; MDT retained Cornforth Consultants through its division, Landslide Technology (LT), to develop mitigation measures to reduce rockfall hazards. Tasks included detailed geologic and rock structural mapping, rockmass characterization, stability analyses, computer rockfall simulations, and development of rockfall mitigation measures.
Mitigation measures included: scaling, shotcreting, rock bolts, draped ring nets, and a rockfall barrier fence. Final design plans and special provisions were developed for MDT to include in bid package documents. Following the bid letting, LT provided continuous technical support during construction activities.