Quantitative Risk Assessment Of Rock Slope Instabilities PDF

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Canadian Geotechnical JournalQuantitative Risk Assessment of Rock Slope InstabilitiesThat Threaten a Highway Near Canmore, AB, Canada:Managing Risk Calculation Uncertainty in PracticeJournal: Canadian Geotechnical JournalManuscript ID cgj-2018-0739.R2Manuscript Type: ArticleDate Submitted by the24-Feb-2019Author:DrComplete List of Authors: Macciotta, Renato; University of Alberta, School of Engineering Safetyand Risk ManagementGräpel, Chris; Klohn Crippen BergerKeegan, Tim; Klohn Crippen BergerDuxbury, Jason; Klohn Crippen BergerSkirrow, Roger; Government of Canada, Alberta TransportationQuantitative Risk Assessment, Uncertainty, Risk Criteria, Rock Fall,Decision-makingaftKeyword:Is the invited manuscript forconsideration in a Special Not applicable (regular submission)Issue? :https://mc06.manuscriptcentral.com/cgj-pubs

Page 1 of 62Canadian Geotechnical JournalQuantitative Risk Assessment of Rock Slope Instabilities That Threaten a Highway NearCanmore, AB, Canada: Managing Risk Calculation Uncertainty in PracticeRenato Macciotta (Corresponding Author)School of Engineering Safety and Risk Management, University of Alberta12-324 Donadeo Innovation Centre for Engineering, Edmonton, AB T6G 1H9Email: [email protected]: 1 (780) 862-4846Chris GräpelKlohn Crippen Berger301, 2627 Ellwood Dr SW, Edmonton, AB T6X 0P7Email: [email protected] KeeganKlohn Crippen Berger301, 2627 Ellwood Dr SW, Edmonton, AB T6X 0P7Email: [email protected] DuxburyKlohn Crippen Berger301, 2627 Ellwood Dr SW, Edmonton, AB T6X 0P7Email: [email protected] SkirrowAlberta Transportation2nd Floor, 4999-98th Avenue, Edmonton, AB T6B 2X3Email: tral.com/cgj-pubs

Canadian Geotechnical JournalAbstract:We present a quantitative risk assessment (QRA) to guide decision-making for selection of rockfall protection strategies. The analysis corresponds to a section of highway near Canmore,Alberta, Canada; where rock falls are common. Environmental concerns, tourism and economicactivities overlap the project area and increased the complexity of the decision-making process.QRA was adopted to improve highway user safety and minimize effects on natural, social andeconomic environments. Uncertainty was associated with hazard and consequencequantification, 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 anyDrparticular individual. Current total risk was calculated at 2.9 x 10-4 probability of fatality and aaftplausible range between 2.0 x 10-5 and 5.5 x 10-3. The slope protection configuration selectedhad a residual total risk between 9.0 x 10-4 and 2.9 x 10-6, and a best estimate of 4.5 x 10-5. Therisk levels were evaluated against criteria previously used in Canada and were considered aappropriate balance between project costs, public safety, environmental concerns, tourism,and economic activities after mitigation.Key Words:Quantitative Risk Assessment, Uncertainty, Risk Criteria, Rock Fall, /cgj-pubsPage 2 of 62

Page 3 of 62Canadian Geotechnical JournalIntroductionThe adoption of quantitative risk assessments (QRA) for landslide management decision-makinghas increased over the last few decades (Morgenstern 1997; ERM 1998; Kong 2002; Mostynand Sullivan 2002; El-Ramly et al. 2003; Bonnard et al. 2004; Fell et al. 2005; Lacasse et al. 2008;Cassidy et al. 2008; Lu et al. 2014; Corominas et al. 2014; Vranken et al. 2015; Uzielli et al. 2015;Macciotta et al. 2016). Particularly in Western Canada, quantified risk has become the basis fordecision-making regarding development upon landslide-prone areas (VanDine 2018). Examplesinclude the District of North Vancouver (Hungr et al. 2016) and the proposed Cheekeye FanDrDevelopment (Clague et al. 2015) in the province of British Columbia, and the debris flow/floodprotection works for Canmore, in the province of Alberta (Hungr et al. 2016).aftHowever, landslide QRA is associated with a number of challenges (Ho et al. 2000; Crozier andGlade 2005). These include: 1) the required input of judgment to elicit the probabilities neededto populate the analysis of landslide occurrence and/or its consequences, and 2) the adoptionof risk evaluation criteria. The first challenge is associated with increased uncertainty in thecalculated risk. Ways forward include: adopting consequential risk assessments; relativedecrease in risk after mitigation, and; the use of calculated ranges of risk values. The secondchallenge is associated with social, political and economic implications and associatedexpectations, policies and consequences. Ways forward include robust risk understanding,communication and public consultation, and adoption of a country-wide set of ubs

Canadian Geotechnical JournalThis paper presents the risk-based decision-making process used to select the mitigationstrategies along a section of highway threatened by rock falls. The site is regarded for itssignificance to wildlife (as a wildlife movement corridor), recreational activities (as a rockclimbing area), scenic tourism corridor and is adjacent to a reservoir perched high aboveCanmore. QRA was adopted to better balance the safety of highway users with minimizing theimpacts to the environment and recreational activities. Decision-making followed a QRA for theexisting conditions and for different mitigation options, evaluated against risk criteria used inCanada. This paper focuses on the QRA for the site and detail discussion on the social andenvironmental aspects is out of the scope of the paper. QRA was the tool used to evaluate thatrisks were within tolerable thresholds for the mitigation options proposed, after anyDrmodifications that would follow social and environmental considerations. The landslide riskaftassessment process followed the outline and recommendations in Fell et al. (2005) andCorominas et al. (2014). This process defines the scope of the assessment, calculates failurevolumes and their likelihood as a measure of hazard. The process then calculates landslide runout, velocities, impact energies, impact likelihoods, and vulnerability as a measure ofconsequences. Hazard and consequences combine to provide the levels of risk that areevaluated against the selected criteria. The following sections present the characteristics of thestudy area, the calculations and required judgment to elicit the probabilities to establish thehazard, consequence and risk values. In addition, an evaluation of the current conditions andproposed mitigation against risk criteria previously used in Canada is provided. This paperhighlights the uncertainty associated with risk estimations and the approach adopted to4https://mc06.manuscriptcentral.com/cgj-pubsPage 4 of 62

Page 5 of 62Canadian Geotechnical Journaladdress these uncertainties and mitigate their impact in the decision-making process formanaging risks.The S042 Rock SlopeLocationThe S042 site is located on Alberta Highway 742 (also known as Spray Lake Trail), at theentrance to the Spray Lakes valley approximately 5 km southwest of Canmore, AB (Figure 1).Figure 2a presents a plan view of the rock slope showing slope Sectors A through F forDrreference through this paper, and rock debris window mapping Locations 1 through 7. Figureaft2b shows a representative photograph of the rock slopes. This figure shows the loose talusslope that extends from the edge of the gravel highway (there is no ditch) at approximately 40degrees and to a near vertical rock cliff face approximately 80m high. Alberta Transportation(AT) manages this section of Highway 742, identifying this rock slope as the S042 site in theirGeohazard Risk Management Program.Geologic Context and ClimateThe S042 site is at the southeast end of the Mount Rundle range which is a part of the SouthBanff Ranges oriented southeast-northwest. Its steep, rocky and bare nature contrasts with thegentle and forested physiography of the Bow valley to the east. There are seven distinctsummits along the Mount Rundle range, the highest is at an elevation of 2,948 m. The highwaythrough Whitman’s Pass, immediately beneath East Rundle is at elevation 1715m, while the5https://mc06.manuscriptcentral.com/cgj-pubs

Canadian Geotechnical Journaladjoining Bow valley is between elevation 1,300 m and 1,400 m. Mount Rundle consists ofPaleozoic limestones, dolomitic limestones, dolostones and shales thrusted onto Mesozoicsedimentary rocks outcropping in the Bow valley. Bedding planes dip at 41 towards thesouthwest (Price 1970). The S042 rock slope has a general orientation in the southeastdirection, with slope sections dipping within a range between east and south.Based on data collected between 1981 and 2010 at a station 29 km to the southeast, theaverage daily temperature ranges between -6.1 C in January and 14.5 C in July (EnvironmentCanada 2016). The average daily maximum temperature ranges between -1.0 C and 22.1 C,while the minimum ranges between -11.7 C and 6.8 C. Extreme temperatures can reach 34.5 Cand –45.6 C during the Summer and Winter, respectively. Monthly precipitation rangesDrbetween about 19 mm in December and 119 mm in June, for a total annual precipitation of 647aftmm per year. Between October and April, most precipitation consists of snow.Previously Proposed MitigationWork in the area consists of site investigations, detailed structural mapping, ground and airphotogrammetry, and rockfall trajectory modeling (KCB 2016). These studies led to proposedmitigation options that included the use of catch fences at the toe of the talus slope (6 m to 9 min height and 800 KJ impact energy), ditch widening, upslope catch fences / attenuator systems,visual inspections for loose blocks, long-term monitoring and establishment of trigger responseprotocols for potential large instabilities (KCB 2016). The catch fence / attenuator systemconsists of a high strength steel wire mesh curtain hanging at certain elevation from the rockface and with the capacity to capture falling blocks from higher elevations and attenuate age 6 of 62

Page 7 of 62Canadian Geotechnical Journalenergies as the blocks are diverted towards the toe of the slope by the hanging mesh. Thesesystems were required at a 70 m-long section mid-slope north of Sector D and Sector C inFigure 1 and a 70 m-long section in Sectors D and E. AT’s option selection corresponded totechnical feasibility and efficiency in hazard reduction. A sketch of the location of these systemsin shown in Figure 3.Consultation with Alberta Environment and Parks (AEP) was done through a joined fieldassessment. AEP observations and information exchange in the field highlighted that the S042site is a primary wildlife passage corridor through the valley and that it would be detrimentallyaffected by the proposed mitigation. Moreover, the proximity of this site to the active town ofCanmore and the scenery of the area have led to a variety of activities, including wildlifeDrphotography, hiking, and rock climbing. These considerations added a layer of complexityaftregarding potential engineered structures for rock slope safety. In this regard, adoption ofmitigation strategies to reduce risks to highway users required minimizing the potential impactsto wildlife and recreational activities. A risk-based decision-making approach was adopted tothis end.Rock Slope Hazard bs

Canadian Geotechnical JournalSite Investigations and ObservationsSite InvestigationsInvestigations focused on identifying potential failures and estimating rock fall debris volumedistributions. This was done through observations of the rock slope and debris windowmapping at 7 locations (Figure 2). Manual and virtual structural mapping through terrestrialphotogrammetric models were also undertaken. The average volume of blocks at the base ofthe talus slope was found to be 0.3 m3, with a maximum volume of 3.8 m3.Large blocks were observed at the base of a 2013 rock fall event and adjacent to the highwayDr(Figure 4b). However, large blocks generally do not reach the base of the talus slope. It wasassessed that large blocks falling individually would fragment upon impact on the talus slope oraftwould not bounce or roll due to their shape and weight. Consequently, these larger blockswould likely slide on the talus slope and stop uphill from the highway (Figure 4a). Based on thisobservation, it was judged that 0.3 m3 is a representative volume for individual rock fallsreaching the road.Results of structural mapping are shown in Table 1. A crude estimate of potential rock fallvolumes can be obtained by multiplying discontinuity spacing (Macciotta and Martin 2015).Spacing for J1, J2 and S0 renders a maximum volume of 2.7 m3, and an average of 0.15 m3,values smaller than those observed but consistent with their order of magnitude.Kinematic analyses based on information in Table 1 pubsPage 8 of 62

Page 9 of 62Canadian Geotechnical Journal-Planar sliding is marginally possible on east facing rock slopes and possible alongnortheast facing rock slopes.-Wedge failure is possible.-Flexural toppling is marginally possible on northeast facing rock slopes but unlikely tohappen because the required joint set is not ubiquitous and with close spacing.None of the failure mechanisms highlighted above were observed at the S042 site in largevolumes, likely because measured persistence is short (Table 1) and persistent discontinuitiesare relatively infrequent. This suggests that rock detachments require cohesion loss to makeblock movement kinematicaly feasible, following precipitation, temperature, seismic, wind,wildlife, or anthropogenic triggers.aftDrField ObservationsBlocks at the toe of the talus slope varied from weathered, sub-angular blocks to fresh (noperceptible weathering) angular blocks. The angular blocks were considered to have detachedfrom the rock face more recently than the weathered blocks, however a quantitative estimatefor their time of detachment is indeterminate with the available data. Some angular, freshblocks, equal and less than 0.3 m in equivalent diameter were found along the west side thehighway at the toe of the talus slope and along the east side of the highway, adjacent to atraffic barrier, as shown on Figure 5. A subset of these blocks might have been encountered onthe road and moved to the side by maintenance staff or highway users. Approximately 50 rockblocks with equivalent diameters between 0.3 m and 0.6 m were found to have been ubs

Canadian Geotechnical Journalby an existing 2 m

risk levels were evaluated against criteria previously used in Canada and were considered a appropriate balance between project costs, public safety, environmental concerns, tourism, and economic activities after mitigation. Key Words: Quantitative Risk Assessment, Uncertainty, Risk Criteria, Rock Fall, Decision-making Page 2 of 62