ROCKFALL HAZARD ASSESSMENT OF HIGHWAYS IN VERMONT

Highway construction across the rugged topography of Vermont has required the excavation of numerous rock cuts, many of which extend for hundreds of meters and are commonly 10s of meters in height. Each year, the combination of steep slopes, adverse geologic structures, and ongoing weathering processes leads to rockfall events. As those reaching the highway may pose a hazard to travelers and are very expensive to clean up, a rockfall hazard rating system has been developed to enable the Vermont Agency of Transportation (VTrans) to prioritize limited maintenance funds to address these hazards. The system is modified from one developed by the Federal Highway Administration (FHWA) and other DOTs.

A preliminary field survey of all Interstate, U.S., and State highways in VT identified 3,647 cuts greater than 1.5 meters high. Based on roadway and geological characteristics and known rockfall history, 77% were ranked as low hazard (rockfall not likely), 6% as moderate (slight chance of rockfall that reaches road), 10% as elevated (rockfall possible and may reach road), 2% as significant (rockfall likely and may reach road), and 5% as high (rockfall expected to occur and to reach road).

Detailed ratings of about 174 high hazard cuts have been completed based on height and length of rock slope, ditch effectiveness, speed limit, traffic count, sight distance, road width, water/ice conditions, known rockfall history, and geologic factors. Geologic factors include rock type, discontinuity characteristics (orientation, length, spacing, openness, roughness, wetness, and infilling of joints and faults), block size, and volume of rockfall. Field work included slope profiles and preliminary remediation estimates. Data for the high hazard rock slopes was entered into spreadsheets in the field and is being migrated to a database and GIS. A scoring system modified from the FHWA system is being used to produce a numerical estimate of the overall hazard posed by each slope. High hazard slopes will be periodically resurveyed by VTrans to track deterioration and emerging hazards.