Investigation and Mitigation Design for a Highwall Rock Slope in Southwest Virginia

In 2015 - 2016, a solid waste facility in southwest Virginia mitigated rockfall hazards associated with an old mine highwall adjacent to a lined leachate pond by removing loose rocks from the highwall, stabilizing large metastable blocks in place, and designing and installing a wire mesh rockfall drape. The work was completed in response to frequent rockfalls and raveling of portions of the highwall that could potentially injure personnel working next to the leachate pond, damage a perimeter drainage ditch at the toe of the highwall or damage the pond liner. A portion of the highwall was constructed during past open-pit coal mining, and past blasting exhibited backbreak up to 30 feet (ft) into the highwall. The maximum highwall height is on the order of 320 ft, and the highwall slope is 1,000 ft long, with eight benches and an overall slope angle of 50 degrees. Rockfall mitigation design was initiated in late 2014 consisting of geologic/geotechnical data collection, ground-based light detection and ranging (LiDAR) topographic surveying, and rockfall trajectory modeling to establish a basis for design. Slope mitigation was started in mid-2015 and consisted of hand and mechanical scaling to remove loose rocks from the slope, along with stabilization of large rock blocks and dental/structural shotcrete placement to fix blocks too large to be safely scaled down to the leachate pond in place. Scaling was performed with a fifteen-foot-high, temporary, movable rockfall barrier placed along the edge of the lined leachate pond to protect the pond liner. Analysis of controlled rockfall trajectories during scaling indicated two rockfall mitigation approaches could be considered: a rockfall drape placed over the entire slope, and a hybrid rockfall barrier placed at the toe. In September 2015, 50% design packages were developed for each option to allow pricing of each alternative. While the hybrid system was about half the cost and could ostensibly be constructed faster, it did not provide as much worker protection from a large rockfall event as the drape alternative. Based on the reduced performance of the hybrid, a need for barrier post foundation support in blast-damaged ground, and the operator’s desire for more comprehensive worker protection, the drape system was selected. GeoBrugg’s 4mm high corrosion resistant Supercoating® high-strength steel Tecco™ wire mesh material was chosen for the drape. This heavier-gauge mesh provides the strength to control larger rock blocks, has mesh spacing sized for smaller rock pieces, obviated the need for a smaller secondary mesh, and could be deployed in one pass. From late November 2015 when materials were ordered through June 2016, the drape system anchors and mesh were installed. The completed system includes 144 wire rope anchors and about 372,000 ft² of wire mesh drape over the slope. The wire mesh panels were installed using a heavy lift helicopter over ten days in April 2016 and were finish-seamed together after placement. The length of the 11.5-ft wide panels hung with the helicopter ranged from 25- to 225-ft long, averaging about 200-ft. The panels were hung in 187 flights (i.e., “picks”), and weighed between 430 and 1,617-lbs. The helicopter averaged panel placement cycle times on the order of 13 minutes, a testament to the teamwork of the ground crew and the military training of the pilot. Approximately 500,000 clips, placed by hand on rope rappel, were used to finish seam the panels to one another.

Language

  • English

Media Info

  • Media Type: Digital/other
  • Features: Figures; Photos; References;
  • Pagination: pp 275-291
  • Monograph Title: Proceedings of the 68th Highway Geology Symposium (HGS 2017)

Subject/Index Terms

Filing Info

  • Accession Number: 01642823
  • Record Type: Publication
  • Files: TRIS
  • Created Date: Jul 15 2017 2:38PM