Assessment of the hazard from rock fall on a highway

Abstract

Rock falls can cause delays, damage, injury, and death to highway users. In 1982, a rock fell on a vehicle in the Argilite Cut on British Columbia (B.C.) Highway 99 killing a woman and disabling her father. The father successfully sued the provincial Ministry of Transportation and Highways for damages. The Supreme Court of Canada found that the Ministry could readily foresee the risk that harm might befall users of the highway if it were not reasonably maintained and that maintenance could be found to extend to the prevention of injury from falling rock. A risk analysis methodology can assess the probability of loss of life due to rock fall. Rock fall impact-mark mapping supplemented by documented rock fall records was used to establish a rock fall frequency. The risk of rock fall on this highway in 1982 can be compared with accepted societal risks. For the Argillite Cut, the probabilities of death of a one time user and a daily commuter on the highway were 6 X 10-8 and 3X10-5 per year, respectively. The annual probability of a rock fall causing a death in the exposed population was 8 X 10-2 in the Argillite Cut. This was higher than the level of risk now commonly accepted in some industries and large engineering projects. (A)