Rockslide Impacting the Banks-Lowman Highway

The Banks Lowman Highway is a National Scenic Highway through the Boise National Forest in southwestern Idaho.  In February 2004, 50,000 cubic yards of rock failed onto the highway.  The rock slide was approximately 350 feet wide and extended over 300 feet above the road.  Landslide Technology was selected by the Idaho Department of Transportation to perform an emergency site reconnaissance and develop a rockfall protection strategy to reopen the highway by the time slide debris was removed.  The size of the rockfall source area and the low quality, highly altered granite bedrock made complete elimination of the rockfall potential infeasible. 

LT worked within the available construction budget to develop rockfall mitigation recommendations to protect highway users from the anticipated routine type of rockfalls at the site.  LT staff directed high scaling efforts to remove unstable blocks in the source area while the DOT’s earthwork contractor worked to remove the 50,000 cubic yards of material covering the highway.  Slope data was collected and actual rockfalls were observed during high scaling and trim blasting work.  Data and observations were used in performing rockfall computer simulations to quantify expected rockfall behavior before the scaling contractor demobilized. 

Mitigation included scaling unstable rock blocks, regrading portions of the slope to remove launch features, improving fallout area geometry, draping rockfall protection mesh over highly unstable portions of the source area, and installing a 184-foot-ton rockfall barrier system near the edge of the road.