Proceedings, 2012 International Science Workshop, Anchorage, Alaska

EVALUATION & MITIGATION OF ICEFALL HAZARDS FOR CIVIL ENGINEERING WORKS

David J. Scarpato*, P.E. Haley & Aldrich, Inc., Boston, MA., USA Martin J. Woodard, PhD, P.G. Haley & Aldrich, Inc., Washington, D.C., USA

ABSTRACT: The implications of build-up on surface rock excavations can prove to be costly over the design lifetime of a slope. In areas subject to significant precipitation and cold temperatures, ice accumulation can unknowingly wreak havoc on surface rock excavations and lead to an increase in the frequency of rock and icefall events. Ice build-up can destabilize a rock slope by expansive action (ice- wedging), by surcharging portions of the slope face, and by inducing an ice-dammed condition where -pressures are allowed to build-up on discontinuity surfaces as a result of inadequate drainage during periods of thaw. Although icefall may logically be treated as a variation of a classic rockfall problem, there are some significant differences between rockfall and icefall evaluation. These differences are primarily related to variations in ice density and the transient nature of ice accumulation thickness and distribution. High-energy icefall impacts can also result in a significant amount of shatter, which can result in the release of ice projectiles. Ice build-up mitigation techniques can take the form of simple drainage elements and periodic cold-weather maintenance efforts, or can incorporate more advanced treatments like engineered topographic enhancements and bio-stabilization. In cases where source zone treatment is not permissible, engineered barriers may be incorporated for mitigating the risk of icefall impact to the traveling public where appropriate. This paper and presentation will describe some of the technical challenges associated with icefall evaluation, the importance of long-term monitoring and maintenance programs, and mitigation strategies for dealing with the under-represented problem of icefall at both the source and impact zone. re-crystallization of snow. If unaccounted for, such accretion can wreak havoc on engineering 1. INTRODUCTION structures and result in hazards such as icefall. The incidence of icefall is one of the most This hazard can occur near structures such as underrepresented and underappreciated of all the pipelines, building structures, electrical natural hazards. Following the Varnes (1978) transmission towers and lines, wind turbines, landslide classification system, the term “icefall” is airplanes, roadways, and even earth excavation a general term used to describe the travel of a features such as rock slopes (Figure 1). mass of ice under the influence of gravity by falling, bouncing or rolling. Ice loading and icefall from roof structures are well-documented and accounted for in the literature and public domain, but cases of icefall emanating from rock excavations, at least until recently, are relatively rare. Recent increases in documented cases of icefall may be attributed to regionally increased precipitation due to climate change, and anthropogenic construction in remote terrain. This paper was drafted to help start chart a path toward defining icefall hazard, risk to public safety and other stakeholders, and to initiate mitigation measures to reduce the potential impacts from falling ice. Figure 1 – Ice accretion on a rock slope in Bethel, 2. GENERAL AFFECTS OF ICE ACCRETION Maine. ON ENGINEERING WORKS A critical part of engineering evaluation is to Ice accretion or build-up can result from freezing evaluate a structure with respect to specific precipitation and/or consolidation and subsequent design cases or “events”. Prolonged or episodic * Corresponding Author’s Address: David J. Scarpato, P.E., 3 Bedford Farms Drive, Bedford, NH, 03120; (603) 361-0397, [email protected]

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ice accretion is one such design case that should iii. Elevated water-pressures on discontinuity be considered for the following reasons: planes, due to ice-dammed conditions can result in increased rock slope i. Structural Surcharging – Additional ice loads instability as described by Hoek (1981). to be considered during design. 2. Bedrock generally consists of geomaterials ii. Potential Non-Uniform Load Distribution – that are lithified and/or mineralized, resulting Vertical surcharge loads in addition to in higher intact material strength than that of circumferential loading. soil. Where slope behavior is controlled by iii. Variable Load Duration – Loads can be discontinuity orientation and strength, this considered cyclical with respect to the season results in: or lifetime of the structure. i. Rock slopes that are designed and iv. Accelerated Mechanical and Chemical constructed at steeper face angles; Degradation – Ice expansion, traction forces, ii. Steeper slope angles can result in “back- and substrate differential thermal coefficients slopes” (i.e. flatter areas beyond the slope can lead to increased stress release by crest) that are closer to engineered fracturing, followed by chemical alteration features, such as roadways. This (e.g. weathering, corrosion) due to increased geometry can result in increased ice and surface area exposure. rockfall impacts. Stakeholders and to some extent even the engineering community, have been reluctant to 4. ROCK SLOPE ICEFALL HAZARDS consider constructed slopes as engineering structures. Excavated slopes or natural slopes Icefall hazards can consist of direct ice particle subject to principles of geotechnical engineering impact, impact shatter, or secondary debris design and remediation should be considered splattering. engineered structures. 4.1. Direct Impact Direct icefall impact hazards can be most 3. ICE ACCRETION ON CIVIL ENGINEERING significant, and result from point-to-point contact EXCAVATIONS with pavement, pipelines, utilities, or vehicle (Figure 2). Direct impact hazards for icefall can be Significant ice accumulation affects all similar to direct impact from rockfall with respect excavations, including rock slopes as well as to energy and collision damage. some tunnels. However, in addition to the general loading cases outlined in Section 2, the prolonged and cyclical nature of ice accretion on rock excavations results in more significant detrimental effects for the following reasons: 1. Rock mass behavior is frequently controlled by “discontinuities” in the rock mass such as joints, faults, bedding planes, or fractures. Discontinuities control the size and modes of failure in a rock mass and serve as the primary conduits for water flow. In climates subject to cold-weather conditions, water within discontinuities can result in: i. Ice formation along discontinuities where water flow is present, resulting in ice over- hangs or classic “” formations; ii. Ice-jacking of rock blocks, whereby expansion mechanics results in prying Figure 2 – Direct icefall impact damage to bus action within discontinuities resulting in near Terrace, British Columbia, courtesy of increased rockfall; Terrace Daily Online, February 5, 2011.

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4.2. Impact Shatter 5. ICEFALL HAZARD ASSESSMENT FOR Impact shatter results when an ice particle breaks- ROCK SLOPES up upon initial contact with a substrate, like In regions subject to extended winter conditions, pavement, walls, rock outcrops, or a roadside specifically the Northern Tier States, Alaska, and ditch. Similar to “flyrock”, an unintended Canada, icefall hazard assessment should consequence from rock blasting, smaller ice generally consist of the following steps: projectiles can be liberated even if direct impact is within a dedicated rockfall area (Figure 3). Such i. Hazard acknowledgement; projectiles could enter, for example, a roadway ii. Observation of ice build-up conditions during and cause a hazard to the traveling public. winter and early spring months (in the northern hemisphere); iii. Observation of rock slope conditions in spring and summer months, to look for signs of bedrock scour (e.g. polished surfaces), absence of overburden soil, rock slope surface irregularities where ice can accumulate, and evidence of vegetation damage (e.g. trees with sharp bends, loss of vegetation); iv. Where potential source areas for icefall have been identified, a monitoring program should be established, whereby a trained geotechnical engineering professional will periodically inspect the slope. It is entirely Figure 3 – Impact shatter from icefall event in reasonable to assume that such a program Gilead, Maine. could be incorporated within the framework of a state-specific Rockfall Hazard Rating 4.3. Secondary Impact Splatter System (RHRS) and rock slope inventory Impact splatter hazards could be considered a program (RSIP). In states where no such subset of shatter and results upon initial ice programs exist, such programs can be particle contact, where the substrate material initiated or a state-specific Icefall Hazard yields and is sent travelling away from the point of Rating System can be established. Icefall impact. An example of this could entail an ice hazard studies can be initially focused on block impact in a wet, soil-filled rockfall ditch, areas where ice accretion is documented or where soil, water, and small fragments of rock are where icefalls have historically occurred; cast horizontally, resulting in debris entering the roadway (Figure 4). v. Depending upon site-specific observations, site history of producing icefall events, level of risk to asset holders or public safety, and the climate in a specific region, specific slopes can be ranked, monitored, and if warranted, maintained. Defined icefall hazard areas can be tracked and updated using GIS-based asset management tools.

6. ICEFALL MECHANICS Initiation of ice block failures which lead to icefalls are subject to the same mechanics as rock block failures (e.g. sliding and toppling). Ice block sliding can be assessed, at least preliminarily, with limit equilibrium slope stability analyses. These types

of failure mechanisms in the source zone are Figure 4 – Impact splatter from icefall impact in applied at the moment of incipient failure; Gilead, Maine. Note mud and gravel in road. however, a block of ice observed at the side of a

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road may be perceived as an “icefall” event. using such an approach. Based on our rockfall Management controls dictate how a failure is analyses evaluations, we have developed the recognized. Source zone controls make use of following conclusions regarding the use of design elements that are intended to manage the conventional rockfall analysis software for icefall block in the source zone. Conversely, impact area evolutions: controls are intended to allow for block failure and i. Conventional rockfall models can be used to subsequent fall, but limit the horizontal distance complete sensitivity analyses and develop that the block can travel. preliminary design evaluations; After initiation and movement of the ice block the ii. Normal and tangential coefficients of failure can be analyzed in a similar manner as restitution, ice block shape, and block unit rockfalls to understand the potential trajectories, weight variations did not result in significant rollout distances, and energies of an icefall event. changes in block bounce height or horizontal Traditionally used rockfall analysis programs such travel; as the Colorado Rockfall Simulation Program (CRSP) and RocFall (Rocscience©) can be used iii. As is the case with modeling rockfall on rock to model potential impacts of icefalls. These slopes, variations in slope surface roughness programs are based on empirically derived (average vertical deviation from a single plane algorithms of rockfalls tested in controlled within one block diameter) resulted in the environments. They model individual blocks as most significant variations in bounce height falling particles that impact the slope with varying and horizontal rollout distances. properties such as geometry, energy attenuation, Disadvantages to using a conventional rockfall and roughness (Figure 5). analysis approach include: i. Reliability of material substrate conditions based on climatic variations. Ice falling on loose dry-pack snow will produce different results than ice falling on rock and so forth; ii. Reliability of slope substrate geometric properties, including surface roughness. An undulating ice surface will produce different results that a flat planar ice surface; iii. Quantifying spatial and temporal variability of source areas and substrate conditions, all of which are tied to climatic variability. iv. Probabilistic ice block material properties in the source zone, including thickness, density, and gradation. Our sensitivity analyses indicate that variations in substrate slope surface roughness exhibit the most pronounced affect on icefall energy and Figure 5 – Output from icefall analysis using horizontal travel distance. CRSP modeling software. Industry-available rockfall modeling software, 7. ICEFALL CASE HISTORIES although not without its disadvantages, can be utilized for preliminary icefall evaluations. Documented cases of icefall events impacting civil Numerical geomechanics modeling programs that engineering structures are hard to come by. There make use of discontinuum or distinct element are numerous cases of icefall impact injuries in methods, like particle flow/tracking codes (e.g. city environments, with the ice falling from bridges ItascaTM PFC2D) can also be utilized for icefall or tall buildings in cities like Chicago, Illinois (Willis modeling studies. Tower) and Dallas, Texas (Cowboy Stadium). There are also documented cases where ice The authors have completed site-specific icefall climbers have been killed or injured by falling ice studies for the Route 2 project in northern Maine, in alpine environments. However, there are only a

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handful of specific cases where icefall emanating evaluation and developed feasibility-level from a slope has directly impacted a roadway or mitigation recommendations for the project. walking trail resulting in an injury: i. A falling piece of ice near Terrance, British 8. INDUSTRY STATE-OF-KNOWLEDGE & Columbia hit a Greyhound bus on February 4, DESIGN CRITERIA 2011, resulting in an injury to the bus driver and the local highway and railway being The engineering community in the United States temporarily shut-down (Figure 2 above). (and Canada), particularly the civil and geotechnical engineering community, has ii. An 11-year-old girl was killed and her mother historically been remarkably quiet regarding icefall injured on July 31, 2010, when a chunk of ice hazards. During our evaluations of project-specific struck them in Mount Baker-Snoqualmie work, we wanted to define the nationwide level of National Forest, in Washington State. industry knowledge regarding icefall hazards. We reached-out to the FHWA and various state The authors have dealt with the impacts of ice DOT’s in the northern tier states, including New accumulation on rock slopes throughout New England. The consensus was that icefall hazards England, most recently on Route 2 in Bethel, were either not considered or not acknowledged, Maine, where two ice shedding events resulted in due to the following factors: icefall both during and after construction. The first event was in 2008 during construction, when an i. Documented icefall events are rare; ice block measuring approximately 3 m in maximum dimension landed on a temporary ii. Evidence of injury resulting from icefall events bench. The second event(s) occurred in 2010, just is very rare; after construction, when two to three smaller . No reliable industry-wide engineering design fragments, measuring between approximately criteria that considers icefall; 0.15 m and 0.7 m landed in the roadside ditch, with one block resting on the pavement at the iv. Icefall is difficult to record and monitor due to white paint striping (Figure 6). potential for rapid phase change (i.e. melting). Furthermore, the definition of icefall is also open to interpretation, by those in the civil engineering community and by public officials and the media. True icefall events may be initially described as “avalanches”, “falling ice”, “falling snow”, or “ice slides”. With such variation in how an event is initially defined and characterized, the quantity of true icefall events is likely under-reported. Rockfall Hazard Rating Systems (RHRS) have been adapted for use by select DOT’s nationwide, and provide a method for slope “ranking” based on roadway geometric characteristics and slope geotechnical properties. Most RHRS are state- specific due to regional geologic features, with the first state-side RHRS being pioneered by Oregon Figure 6 – Icefall block adjacent to roadway in DOT in 1989. The value of such a system is that it Bethel, Maine. allows agencies to relatively rank slopes given state-specific geologic and climatic Further down the project alignment, there was an considerations, and provide for long-term additional icefall event in April of 2011, although monitoring and maintenance treatments and the 2011 event was completely contained within budgetary demand. Such a system does include a the locally widened roadside ditch and snow “Climate and Presence of Water” ranking category mobile trail area (Figures 3 and 4 above). (or similar), and may consider the general affects Although there were no safety incidents as a of ice accretion on rock slope; however, currently result of these icefall events, the Maine icefall is not explicitly considered in RHRS’s. Such Department of Transportation (MaineDOT) took a system could be adapted to include icefall an aggressive approach after evidence of icefall, explicitly, or a similar icefall hazard rating system and the authors completed an extensive icefall could be created, using RHRS as an analog.

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Given the facts presented above, most rockfall sufficient for icefall, given that ditches may fill ditches in the United States and Canada are not with ice and be dipping toward the highway in designed considering icefall catchment. the winter months; ii. Impact Barriers – High-energy impact barriers 9. ICEFALL MITIGATION STRATEGIES and fences, similar to those used for rockfall, can be fashioned to accommodate falling ice Icefall mitigation strategies, much like rockfall debris, and particularly if material is falling mitigation, can entail source zone or impact zone from beyond the back-slope and possess a remedial treatments. Source zone treatments are significant horizontal trajectory or the implemented with the aim of either removing the catchment area decreases in depth due to hazard or securing material such that it never accumulation of snow and ice. Such barriers exists the source area. Impact zone treatments can be installed directly within the impact allow for failure of the material at the source, but zone or anywhere between the source zone limit the horizontal movement. and impact zone, to capture falling up-slope 9.1. Source Zone Treatments material. Engineered wall or berms could also Given the technology currently available, source be considered. Barriers can be expensive and zone icefall mitigation treatments can include: design must consider aesthetic impacts, maintenance requirements, and need to be i. Removal – Ice removal via mechanical means designed with various snow and ice loading including machine or manual scaling, or considerations. Secondary ice throw from possibly by use of impact cannon if block is primary impact also needs to be considered free-hanging on at least one side. If not for roadside barriers; completed by experienced contractors/operators, this can be dangerous; iii. Drapery – Conventional and high-strength rockfall drapery can be fashioned for use to ii. Topographic Alterations – Includes slope re- limit small to moderate volume ice accretions design and/or re-grade, taking into on rock slopes, although the mechanics of the consideration up-slope source area(s), with application would be markedly different than the intent of either limiting the accumulation for most typical slope drapes. Intent would be volume of snow and ice or adding geometric do design drape such that it can be lifted to features that allow for capture and retention. “shake” attached ice free. Drapery has the This also can include synthetic or natural propensity to actually attract ice due to its vegetative topographic enhancements, like increased surface area and permeability, so strategically-placed woody vegetation. Costs such drapes will require periodic servicing to associated with re-design/re-grade can be remove attached ice. This treatment would not significant, although results are highly be applicable for large volume ice flows, as effective; the material would simply adhere to the face iii. Enhanced Surface and Subsurface Drainage and adhesive forces too significant to – As part of any re-grading efforts, alternative overcome. Given its application here, such a surface water drainage paths/areas can be treatment could be considered for both rock designed to minimize presence of water on and icefall, and could be applied in both the the slope. Internal (subsurface) rock slope source and impact zone. drains can also be installed in specific cases, if warranted. 10. CONCLUSIONS 9.2. Impact Zone Treatments Icefall hazards embody “ghost” like attributes, and Given the technology currently available, impact present significant difficulties for the civil (and zone icefall mitigation measures can include: even mining) engineering community and for i. Dedicated Catchment Area – A designed hazard mitigation planners alike. Evidence of catchment area, one which considers both ice icefall can be observed one day, and gone the and rockfall, is highly effective at capturing next due to changing climatic conditions. falling slope debris. Difficulties with such a Documented icefall occurrence is somewhat rare remedial approach may include road shifting in relation to rockfall. Given increased or trim blasting in order to create horizontal anthropogenic construction in remote areas and width needed for such a ditch. Catchment long-term forecasts for increased precipitation due areas designed only for rockfall may not be to climate change, the increasing risk of icefall

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impacts to engineering structures and the public is currently still the best defense for mitigating the very real. Currently, given limited case histories of potentially damaging effects from icefall. documented impact, constrained public budgets, As a final remark, the assessment of ice and dearth of icefall design criteria from the accumulation on slopes requires an engineering community, government regulatory interdisciplinary approach, with input from the bodies and civil engineers alike have had very geotechnical engineering, avalanche hazards, and little to work with. However, moving forward, the cold-regions engineering practitioners. Hazards expectation is that with increased vigilance, public directly adjacent to the slope would be considered education, and most importantly, civil engineering by geotechnical engineers; however, if frozen technical evaluation, that there will be more tools accumulation is large-scale (e.g. regional) and not for regulators and design engineers alike. just localized, avalanche hazard specialists may Based on our experience evaluating icefall provide useful insight into when snow will undergo hazards, we advocate that State and Federal thermodynamic changes into ice, or alternatively, authorities (e.g. FHWA, DOT’s) consider including where up-slope frozen deposits have slope the affects of ice accumulation on rock slopes, as profiles that are conducive to large run-out icefall part of State-specific RHRS’s. For States where or ice slide events. such a program exists, capital investment costs will be minimal. For States where no such mechanism currently exists, such a program ACKNOWLEDGEMENTS should be initiated. The capital investment required for such a venture could be relatively We wish to thank Haley & Aldrich, Inc. for its significant if starting from scratch. In order to support during manuscript preparation and Maine minimize up-front financial impact, an initial risk- DOT for their continued willingness to tackle such based evaluation (preliminary ranking program) technically challenging natural and geologic could be completed for rock slopes with observed hazards. ice build-up and/or documented evidence of icefall, and short to intermediate term solutions can be adapted. Given the relationship between REFERENCES: ice accumulation and rockfall, the most efficient Hoek, E., and Bray, J.W., 1981: Rock Slope global monitoring mechanism would be to Engineering, London Institute of Mining and consider both ice and rock in any long-term Metallurgy, E & FN Spon, 358 pp. monitoring program. Pierson, L.A., Gullixson, C.F., Chassie, R.G., Ice accumulation and icefall mitigation 2001: Rockfall Catchment Area Design Guide, technologies are still in their infancy, particularly Oregon Department of Transportation and Federal for geotechnical features like rock slopes. Source Highway Administration, 78 pp. zone treatments, as described herein, can require significant expense, construction time, and Pierson, L.A., 1991: The Rockfall Hazard Rating aesthetic impacts. Impact area treatments, for System, Oregon Department of Transportation, 11 example adjacent to a roadway, can also be pp. expensive and unsightly, and can require Varnes D. J., 1978: Slope movement types and significant long-term maintenance. All designed processes, Landslides, Analysis and Control. mitigation strategies need to take into Transportation Research Board, Sp. Rep. No. consideration reduced reliability due to snow and 176, pp. 11–33. ice loading and snow/ice thickness variability. For the case of civil excavations, the slope design http://www.thenorthernview.com/news/115565779 process needs to take icefall into consideration in .html, Greyhound bus is hit by falling ice on highway, order to avoid costly “after-the-fact” remedial February 8 2011 edition. treatments like re-excavation/redesign, scaling, http://articles.cnn.com/2010-08 01/us /washington. mesh drapery, or high-energy impact barriers. ice.caves.death_1_caves-ice-patch-family- For proposed rock slopes and those slopes that outing?_s=PM:US, Falling ice kills 11 year old girl will be trim blasted for remedial purposes, the on family outing, August 1 2010 edition. roadside ditch must be designed with both rockfall http://www.terracedaily.ca/show7605a0x300y1z/G and icefall in mind. Long-term slope monitoring is REYHOUND_STRIKES_FALLING_BLOCK_OF_I CE_ON_HIGHWAY_16, February 5 2011.

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