Idaho Transportation Department (ITD): Emergency Rockfall Assessment, US-95, Riggins, ID, December 2012

摘要

During the evening of 2 December, 2012, a large rockslide occurred that blocked US-95 at MP 188 about-5 miles south of Riggins, Idaho. No accidents were reported as a result of the rockslide. The next day, Idaho Transportation Department (ITD) noted a large, unstable, 200-ton block, hanging precariously on the face about 180 feet above the road that required immediate assessment. The rockslide originated from an ultramafic rock massif cropping out about 250 feet above the road. The team employed rappelling techniques to map vertical scanlines and access the unstable block. While on rappel, the team collected physical and engineering characteristics of the rock mass including information on discontinuities. The field data were used to assess the kinematic relationships between the structure of the rock mass and the rockslope face and establish failure mechanisms. During the assessment, a large and deep tension fracture was observed between the critical block and the hanging wall of the main rock mass. The kinematic analysis demonstrated that the critical block and slope were unstable and wedge failures were dominant. To mitigate the initial, unstable, slope conditions and critical block, a rock scaling contractor was immediately mobilized to the site. After assessment, the team established that the block should be removed by trim blasting. The team developed a trim blasting design that would bring the block down yet preserve the back wall. They worked closely with the blaster-in-charge and contractor to drill, load, and shoot the critical block. The block was removed safely by presplit blasting on 23 December. US-95 highway was then reopened to the public to accommodate the Christmas traffic.