Quantitative risk assessment of rock slope instabilities that threaten a highway near Canmore, Alberta, Canada: managing risk calculation uncertainty in practice

Abstract

We present a quantitative risk assessment (QRA) to guide decision-making for selection of rock fall protection strategies. The analysis corresponds to a section of highway near Canmore, Alberta, Canada; where rock falls are common. Environmental concerns, tourism, and economic activities overlap the project area, which increased the complexity of the decision-making process. QRA was adopted to improve highway user safety and minimize effects on natural, social, and economic environments. Uncertainty was associated with hazard and consequence quantification, and the study elicited plausible ranges of input variables for risk calculation. Expected and range in risk were calculated for current conditions and after mitigation. Individual risk to highway users was found to be low, following the limited exposure of any particular individual. Current total risk was calculated at 2.9 × 10−4 probability of fatality and a plausible range between 2.0 × 10−5 and 5.5 × 10−3. The slope protection configuration selected had a residual total risk between 9.0 × 10−4 and 2.9 × 10−6, and a best estimate of 4.5 × 10−5. The risk levels were evaluated against criteria previously used in Canada and were considered an appropriate balance between project costs, public safety, environmental concerns, tourism, and economic activities after mitigation.

Remote Sensing Applications for Landslide Monitoring and Investigation in Western Canada

“…Rock falls and erosional processes have created a talus slope of 40 • between the slope and the highway. Rock falls mostly are captured on the talus slope; however; blocks of less than 0.3 m in equivalent diameter are commonly found on the road [55].…”

Fragmented Rockfall Volume Distribution from Photogrammetry-Based Structural Mapping and Discrete Fracture Networks

“…This site is monitored and managed by AT as part of the department's Geohazard Risk Management Program (GRMP). This site was selected due to the availability of information which had been gathered to inform rockfall mitigation strategies [20]. Figure 3b shows a view of the Site S042 rock slope looking towards the southwest direction.…”

“…Figure 3b shows a view of the Site S042 rock slope looking towards the southwest direction. This figure shows the loose talus slope that extends from the edge of the gravel highway (there is no ditch) at approximately 40 • to a near vertical rock cliff face approximately 80 m high [20]. The rock face mainly consists of limestones.…”

“…Quantifying rockfall hazard and risk, and the design of rockfall mitigation structures, require information about the falling block volumes (and mass), their fragmentation during motion, and 2 of 18 their frequencies [13,[20][21][22][23][24]. Information on rockfall volumes and fragmentation can be gathered from comprehensive records or estimated through surveying fallen blocks along known rockfall trajectories [6,11,13,16,20,23]. However, these methods present challenges where rockfall records are scarce and block surveys are not possible due to the steepness of the terrain or falling blocks entering waterbodies downslope from the rock fall sources.…”

Managing Risk in Geotechnical Engineering – From Data to Digitalization

“…For example, Schuppener and Heibaum (2011) remarked that "soil excavations and tests of the mechanical properties of soil never provide enough data to enable a probability calculation to be performed". Macciotta et al (2019) argued that there was not enough information for quantitative risk assessment to guide decision-making for adoption of rock fall protection strategies. Chilès and Delfiner (1999) noted that volume fractions for cores, cuttings, and logging at a Brent Field site in the North Sea are 1 10 -9 , 7 10 -9 , and 1 10 -6 , respectively.…”