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Proceedings, 2012 International Snow Science Workshop, Anchorage, Alaska Proceedings, 2012 International Snow 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 ice 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 water-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] 300 Proceedings, 2012 International Snow Science Workshop, Anchorage, Alaska 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 “icicle” 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. 301 Proceedings, 2012 International Snow Science Workshop, Anchorage, Alaska 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
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