Download PDF Spring Bridge Issue on Engineering and Climate Change March 15, 2020 Volume 50 Issue 1 The seven articles in this issue cannot cover all engineering-related aspects of climate change, but they highlight several areas of concern. Permafrost Engineering on Impermanent Frost Monday, March 16, 2020 Author: William E. Schnabel, Douglas J. Goering, and Aaron D. Dotson The Arctic is often considered ground zero for climate change because arctic air temperatures are rising at approximately twice the rate compared to the rest of the globe (Meredith et al. 2019). However, the diverse facets of arctic climate change (e.g., sea ice loss, hydrologic changes, permafrost thaw) affect the built environment to a greater extent than one might assume based on warming air temperatures alone. Impacts include erosion of arctic shorelines (Jones et al. 2018), altered river dynamics (Toniolo et al. 2017; Zheng et al. 2019), increased wildfire risk (Hu et al. 2015), and decreased foundational integrity of terrestrial infrastructure (Nelson et al. 2001; Raynolds et al. 2014). Climate change in Alaska’s Arctic promises many of the same outcomes as it does worldwide, such as community displacement (Marino 2012; Rawlings 2015), ecological disturbances (Tape et al. 2018; Wolken et al. 2011), and profound economic disruption (Melvin et al. 2016). But a key element unique to cold regions is the impact of a warming climate on permafrost. Arctic Infrastructure and Permafrost Infrastructure is constructed with permafrost as a bearing base in much of the Arctic, and thawing of those soils can diminish their bearing capacity. Thus, one distinguishing outcome of arctic climate change is that engineers must give more consideration to protecting permafrost beneath built structures. Indeed, the Intergovernmental Panel on Climate Change (IPCC) recently estimated that by 2050, 70 percent of arctic infrastructure will be located in areas considered to be at risk from permafrost thaw and potential ground subsidence (Meredith et al. 2019). This increased risk will likely be felt in Alaska, where much of the infrastructure is located in permafrost areas (figure 1). Figure 1 Effective design, construction, and maintenance of -arctic infrastructure often require protection of permafrost from thawing and erosion. Arctic engineers have learned that lesson the hard way. During construction of the World War II–era Alaska Highway, for instance, engineers were not well prepared for challenges created by ice-rich permafrost, and soon noted that the ostensibly solid structural roadbed provided by permafrost soils can rapidly transform into mire in response to construction-related surface disturbances (figure 2). Figure 2 Ice-rich permafrost is common to Alaska. On some portions of the Arctic Coastal Plain—the main area of oil production—the upper sections of permafrost can contain up to 90 percent ice by volume (Kanevskiy et al. 2013). Structures in these areas can survive only if the underlying permafrost is protected from thawing. Permafrost temperature also plays an important role in infrastructure stability because the bearing capacity of frozen soil is greatly decreased as permafrost temperatures rise toward the melting point. Thus, arctic engineering has evolved into a field -driven largely by both the desire to protect soil in its frozen state and the need to predict the effects of localized thaw and find engineering solutions for maintaining foundation stability. In a warming Arctic, these objectives are challenged by interrelated processes that inch foundational soils ever closer to a state of thaw. Conversations regarding permafrost thaw often focus on global or regional processes, but it is the permafrost directly beneath or proximal to the infrastructure footprint that is most relevant to designers. While the permafrost may well thaw completely throughout some regions over the coming decades, it may persist in colder regions, with thaw limited primarily to soils near the ground surface. In either case, regional or localized thaw beneath built infrastructure can pose significant risk to structural integrity. Permafrost Characteristics Permafrost (perennially frozen ground) underlies about 25 percent of the Earth’s terrestrial surface in the Northern Hemisphere. It is defined as ground (soil or rock with or without ice present) that remains below 0°C for at least 2 consecutive years. As a result of the changing climate, permafrost around the globe is getting warmer; a recent study reported that global average permafrost temperature increased by 0.29±0.12°C between 2007 and 2016 (Biskaborn et al. 2019). Permafrost regions are characterized in zones according to the extent of permafrost: continuous (frozen soils underlying 90–100 percent of the surface), discontinuous (50–90 percent), sporadic (10–50 percent), and isolated (<10 percent). As illustrated in figure 1, Alaska’s permafrost is distributed across all four zones. Permafrost is insulated from the atmosphere by vegeta-tion and an active surface layer of soil that thaws annually in the summer and refreezes in the winter. In a continuous permafrost zone, the active layer may be as thin as 30–50 cm, whereas in the warmer zones the active layer may be several meters thick. Large ice accumulations do not build up in the active layer, but the upper layers of the permafrost immediately beneath the active layer often contain large amounts of ground ice. In some cases only relatively thin ice lenses are finely distributed throughout the soil structure; in others large coherent massive-ice bodies of meters or more (such as ice wedges) may occur.[1] Thawing of massive-ice bodies is a particularly acute hazard for structures, as it can create voids in the subsurface leading to subsidence of the surface features (thermokarst). In recent decades, widespread thawing of ice wedges has been observed in many regions across the Arctic, including northern Alaska (Jorgenson et al. 2006; Liljedahl et al. 2016). As a general rule, permafrost with high ice content is considered thaw-unstable because of its loss of strength upon thaw. It is this loss of soil structural integrity that leads to physical instability of the ground surface and the potential failure of surface infrastructure. Engineering Challenges and Mitigation Techniques A significant engineering challenge associated with infrastructure in permafrost areas is that of providing a solid, enduring foundation for structures. Even ice-rich permafrost can provide an adequate foundation for most infrastructure if thawing can be avoided. Heated Structures Warm structures such as heated buildings or warm pipelines must be separated from ice-rich permafrost so that their heat does not induce thawing. Often these types of structures are separated from the ground surface by ventilated space and situated on a foundation with pilings that are frozen into the permafrost (figure 3). In the warmer discontinuous permafrost zone, thermal piles are often used (figure 4A). They contain a thermosiphon (gravity-assisted heat pipe) cooling system that enhances winter-time cooling of the piling and surrounding permafrost, helping to ensure that the permafrost remains frozen and enhancing the frozen soil-piling surface (adfreeze) bond strength that provides vertical support for the piling and its load. Figure 3 An alternative to a pile-supported building is to construct the building on-grade with insulation and a cooling system installed below the building footprint to avoid permafrost thaw (figures 4B,C). In these cases, cooling can be provided by mechanical refrigeration, ventilation ducts, or thermosiphon cooling systems designed to use low winter air temperatures to intercept and dissipate heat leaving the base of the building. Figure 4 Unheated Structures Linear structures such as roads, airports, railways, or other unheated structures can sometimes be located in permafrost areas with only minimal consideration of protection of the permafrost from thaw. This is particularly true in the continuous permafrost zone, where unheated structures are less likely to cause enough warming to induce thawing. In these areas it is possible to design linear structures with an embankment height that ensures that the annual summer thaw will not penetrate the permafrost. In the discontinuous permafrost zone, conditions are generally warmer and more advanced mitigation techniques may be required (figure 4D). The mere placement of roads or rail embankments can cause permafrost thaw simply through surface disturbances that remove native vegetation and warmer surface temperatures, especially in the case of black asphalt roadways. Snow accumulation along embankments also leads to significant increases in ground temperatures due to the insulating properties of the snow itself. As a result, road maintenance costs are generally higher in warmer areas with discontinuous permafrost compared to the costs in colder con-tinuous permafrost areas. Water and Wastewater Services The provision of piped community water and waste-water services has historically been a challenge in permafrost-prone regions, and that challenge is exacerbated in a warming Arctic. In communities with buried pipes, the surrounding permafrost must be thermally protected from the warm flowing liquid by insulation of the pipes. This challenge can be mitigated by using insulated arctic pipe or heavily insulated utilidors above the ground surface. Given permafrost-related design considerations and the relatively low number of residences in many -arctic communities, buried and/or aboveground piped systems often entail capital costs that make such systems unattain-able (USARC 2015). Approximately 20 percent of homes in rural Alaska lack piped water and wastewater services (Thomas et al. 2016), and for communities that do have piped systems, climate change tends to intensify the challenge of maintaining perma-frost stability beneath utilities. Arctic communities impacted by perma-frost thaw may experience a greater rate of negative health outcomes due to lack of sufficiently available in-home water (Thomas et al. 2016). Warm Permafrost Regardless of infrastructure type, the thermal balance between permafrost foundation soils and overlying infrastructure is frequently a delicate one. In the discontinuous permafrost zone, permafrost temperatures are often within 1°C of the ice melting point. Surface disturbances due to construction activities tend to swing that balance toward permafrost thaw. In recent years, that situation has been exacerbated by warmer atmospheric air temperatures, which result in a deepening active layer and thawing of the upper (often ice-rich) layer of permafrost, causing ground surface instability and thermokarsting. Warmer temperatures also render cooling systems (thermal piles, thermosiphons, or air ducts) less effective by reducing the air freezing index available to chill the permafrost, and they reduce the ability of a given embankment to contain the annual thaw and protect underlying permafrost. For pile foundations, warmer permafrost can be detrimental because of the sensitive adfreeze bond strength, which decreases rapidly as the soil-foundation interface warms. While some amount of climate warming can be accommodated via more conservative designs, in some cases it may be difficult or impossible to adjust to a wholesale permafrost regime shift (e.g., widespread thaw). Beneficial Technologies Engineers have been designing infrastructure in permafrost regions for over 100 years. Successful designs predict and avoid localized permafrost thaw resulting from the construction process, and mitigate thermal imbalances produced by the built infrastructure itself. In a warming Arctic, additional predictive and observational capabilities are required to accommodate the shifting nature of the ambient conditions. Such technologies will be useful for addressing the following questions: What landscape-scale changes are anticipated in the vicinity of and over the design life of the infrastructure component? What are the permafrost characteristics and thaw stability of the soils across the entire infrastructure footprint? What are the fine-scale thermal processes that will likely impact structural stability? Fortunately, advances in cyberinfrastructure, high-performance computing, and observational and predictive technologies have enhanced engineers’ ability to assess the characteristics of land proximal to planned arctic infrastructure. For example, a National Science Foundation–sponsored effort, the Permafrost Discovery Gateway (https://permafrost.arcticdata.io), will provide a widely available browser-based platform for visualizing and exploring big data with a focus on satellite images of arctic regions. The gateway will allow users to interact with historical or predicted geospatial time series to identify changes down to the submeter scale. Such changes include ice wedge degradation, surface water coverage, thaw slumps, and erosion of ocean, river, and lake shorelines. The Arctic Environmental and Engineering Data and Design Support System (Arctic-EDS; under development) is intended to inform engineering design in arctic regions. Funded by the US Department of Defense Environmental Security Technology Certification Program, Arctic-EDS will develop and deploy online technologies presenting design-relevant environmental data for use in web-based maps, modules, and notebooks. Up-to-date georeferenced data collated by state and federal agencies will be curated for arctic infrastructure design use and combined in a single online hub. Beta tests of Arctic-EDS are expected to commence in summer 2021, and final product release is expected in early 2023. Recent advances in geophysical techniques have made it possible to more fully characterize soils beneath the footprint of planned infrastructure. While traditional techniques such as drilling will likely remain a key component of geotechnical investigations, geo-physical and remote sensing methods can allow engineers to better understand subsurface conditions between the boreholes. For example, electrical resistivity has proven effective for identifying massive subsurface ice bodies as well as characterizing the physical state of interstitial water (Mollaret et al. 2019; Trochim et al. 2016). By employing a geophysical survey over the entire footprint at the outset of a geotechnical investigation, designers can identify optimal locations for the placement of boreholes and make more informed inferences about the characteristics of soils not physically sampled. Advances in computational capabilities and modeling techniques will allow engineers to better predict thermal processes and the resulting structural impacts associated with new infrastructure built on permafrost soils in a warming climate. Commercial thermal modeling software can be used to help understand the details of heat transfer in arctic soils beneath planned infrastructure. The models generally include specific routines for simulating the complexities of the ground surface energy balance, have phase change routines that are customized for the soil types often found in permafrost regions, and can incorporate cooling systems such as thermosiphons or air ducts in the analysis. They are capable of simulating the progression of permafrost thaw that may occur as a response to either climate change or the placement of warm infrastructure. As such, they provide tools that are becoming a critical component of the design process for infrastructure in a warming arctic. Conclusion Engineers managing the effects of climate change in the Arctic face many of the same challenges as engineers worldwide, including challenges associated with rising sea level, erosion, flooding, wildfire, and social displacement. However, the increased risk of structural damages related to warming permafrost is unique to the Arctic and similar cold regions. Because permafrost thaw can result from a host of disturbances (e.g., construction activities, infrastructure-related thermal inputs, hydrologic changes, wildfires, or increased ambient temperatures), it is often difficult to discern the specific cause of the thaw. What is certain is that ambient permafrost temperatures are rising (Biskaborn et al. 2019), thus increasing permafrost’s susceptibility to thaw resulting from any type of disturbance. Arctic engineers have developed numerous techniques to prevent or mitigate infrastructure damage related to permafrost thaw, most often involving efforts to keep the soils frozen. Passive techniques generally employ mechanisms to restrict heat flux from the infrastructure to the underlying ground; in some cases active measures are employed to facilitate cooling. In all instances, warmer ambient temperatures impose additional challenges to maintain soils in a frozen state. Infrastructure design in a warming Arctic can be enhanced through improvements in the ability to observe and predict regional and local changes in permafrost properties. Advances in remote sensing, modelling, design support systems, and imaging techniques, and continued development thereof, can aid engineers now and in the future. In addition to its negative impacts, climate change is promoting accessibility and generating renewed interest in arctic development. Going forward, arctic engineers can expect to not only manage existing infrastructure but also design and maintain new infrastructure associated with anticipated development. The pursuit of technological advances should therefore continue, as engineers seek to design stable infrastructure on an increasingly unstable landscape. Acknowledgments We acknowledge and appreciate Yuri Shur, Mikhail Kanevskiy, and John Zarling of the University of Alaska Fairbanks (UAF) Institute of Northern Engineering and Kevin Bjella of the US Army Corps of Engineers Cold Regions Research and Engineering Laboratory for their thorough review and suggested edits to this article. In addition, several images used in this report were graciously provided by Yuri Shur, as well as Ben Jones of the UAF Water and Environmental Research Center. 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[1] Massive ice refers to large masses of ground ice, including ice wedges, buried ice, and large ice lenses. An ice wedge is a vertical triangular ice mass (pointing down, flat surface on top) formed as a result of the thermal cracking of the ground and often having dimensions of several meters or more; an ice lens is a horizontal ice formation that can heave overlying rock or soil upward. About the Author:Bill Schnabel is dean of the College of Engineering and Mines at the University of Alaska Fairbanks (UAF). Doug Goering is dean emeritus of the UAF College of Engineering and Mines. Aaron Dotson is interim vice provost for research and professor of civil engineering at the University of Alaska Anchorage.