CLIMATE CHANGE IMPACTS ON PERMAFROST ENGINEERING DESIGN
MARCH 1998
Funding for this Project Provided by:
Panel on Energy Research and Development (PERD)
Report published by the Environmental Adaptation Research Group, Atmospheric Environment Service, Environment Canada. Scientific Officer David Etkin, Environment Canada
Editors Greg Paoli, Decisionalysis Risk Consultants, Inc. Dan Riseborough, Department of Geography, Carleton University
Please address correspondence to: David Etkin (
[email protected]) Environmental Adaptation Research Group Environment Canada at the Institute for Environmental Studies, University of Toronto, 33 Willcocks Street Toronto, Ontario M5S 3E8 Tel: (416) 978-6310 Fax: (416) 978-3884
Acknowledgements The authors wish to thank Margo Burgess, Larry Dyke and Fred Wright of the Geological Survey of Canada; Barrie Maxwell of Environment Canada; Virgina Maclaren of the University of Toronto; and Alan Hanna of Agra Earth and Environmental Limited for their review comments and contribution to this document. The authors also wish to thank the Panel on Energy Research and Development (PERD) for financial support of this project.
CLIMATE CHANGE IMPACTS ON PERMAFROST ENGINEERING DESIGN CONTRIBUTORS (in alphabetical order) Elizabeth Bush Atmospheric Environment Service Environment Canada 4905 Dufferin St. Downsview, Ontario M3H 5T4
Elisabeth Hivon EBA Engineering Consultants Ltd. 14535, 118 Avenue Edmonton, Alberta T5L 2M7
[email protected]
Beth Lavender Smith & Lavender Environmental Consultants R.R. # 7, Perth Ontario K7H 3C9
Jamie Smith Department of Geography University of Toronto Sidney Smith Hall, 100 St. George St., 5th Floor Toronto, Ontario, M5S 3G3
[email protected]
David Etkin Environmental Adaptation Research Group Atmospheric Environment Service, Environment Canada, and Institute for Environmental Studies, University of Toronto 33 Willcocks Street Toronto, Ontario M5S 3E8
[email protected] Greg Paoli Decisionalysis Risk Consultants, Inc. Suite 302, 76 Park Avenue Ottawa, Ontario K2P 1B2
[email protected] Don Hayley EBA Engineering Consultants Ltd. 14535, 118 Avenue Edmonton, Alberta T5L 2M7
[email protected] Dan Riseborough Department of Geography Carleton University Ottawa, Ontario K1S 5B6
[email protected]
Branko Ladanyi Professor Emeritus Department of Civil Engineering École Polytechnique P.O. Box 6079, Station Centre-Ville Montréal, Québec H3C 3A7
[email protected] Michael Smith Chair, Department of Geography Carleton University Ottawa, Ontario K1S 5B6
[email protected]
EXECUTIVE SUMMARY This report is intended to merge current understanding of the expectation of global climate change with current engineering practice in the north. The goal of the report is to provide a consistent basis for planning of long-term projects in the North and a basis for evaluation of the potential impact of climate warming on existing facilities that were designed prior to consideration of climate change. The Intergovernmental Panel on Climate Change has concluded that some global climate change has already occurred and that increased emissions of greenhouse gases due to human activity have caused, and will continue to contribute to, this change. While there is considerable uncertainty in the magnitude and pace of change, “the least likely scenario is considered to be no change.” In the Arctic, the Mackenzie District has warmed by 1.5C over the past 100 years (with the warming being most pronounced in the winter and spring). Global circulation models suggest increases in global annual mean temperature relative to the present of 1 to 3.5C by 2100. However, the circumarctic area is expected to experience an amplified change due to the action of climatic feedback mechanisms. Canada’s Arctic regions are particularly vulnerable to global climate change for a number of reasons:
climatic changes will be amplified at high latitudes
the climate is the principal determinant of human and ecological vulnerability
the transition to a warmer climate will profoundly affect hundreds of thousands of square kilometres in the Arctic
among other impacts, widespread thinning and disappearance of permafrost is expected.
This report focuses on the impact of climate warming on the use of permafrost as a structural material in engineering design. Warming of the ground as a result of climate change will degrade the performance of many existing and planned structures including roads, foundations, embankments and open pit mines. Of particular concern are mine tailings retention structures and the use of frozen soil in the containment of hazardous waste. The report describes a screening process for considering climate change as a factor in engineered facilities. The process was designed to gauge the level of analysis required to address climate change in the design of a proposed facility. It could also be used to consider and prioritize concerns about existing facilities with respect to potential climate change impacts. The screening process also serves as a prototype for use by environmental assessment agencies in decisions regarding engineering projects in the north. In this study, the only parameter of global climate change that is considered in detail is change in the average temperature. More research is required to address the potential impacts of changes in climate variability and precipitation. Further research is also recommended to examine the potential for increased likelihood of failure among existing facilities whose design did not incorporate climate change.
TABLE OF CONTENTS 1
INTRODUCTION .......................................................................................................................................1 1.1 BACKGROUND ............................................................................................................................................1 1.2 ENGINEERING PROJECTS IN THE NORTH .....................................................................................................1 1.3 SCOPE OF THIS REPORT ..............................................................................................................................2
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CONSTRUCTION OF CLIMATE SCENARIOS FOR NORTHERN LATITUDES...........................6 2.1 2.2 2.3 2.4 2.5
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GREENHOUSE GAS EMISSION SCENARIOS ..................................................................................................6 MODELS OF CLIMATE CHANGE ..................................................................................................................6 ‘BEST ESTIMATE’ AND PESSIMISTIC SCENARIOS FOR THE YEAR 2100 .......................................................9 GLOBAL SCENARIOS AND ARCTIC AMPLIFICATION ....................................................................................9 MEAN TEMPERATURE CHANGE ESTIMATES BY SEASON, LATITUDE AND DECADE ..................................10
ENGINEERING IN PERMAFROST ......................................................................................................13 3.1 ENGINEERING CONSIDERATIONS IN PERMAFROST ....................................................................................14 3.2 PERMAFROST BEHAVIOUR AND FAILURE MODES.....................................................................................14 3.2.1 Strength...........................................................................................................................................14 3.2.2 Creep ..............................................................................................................................................15 3.2.3 Thaw Settlement..............................................................................................................................15 3.2.4 Permeability ...................................................................................................................................16 3.2.5 Frost Effects / Frost Heave.............................................................................................................16 3.2.6 Changes to Permafrost ...................................................................................................................16 3.3 MAJOR PROJECT CATEGORIES..................................................................................................................17 3.3.1 Northern pipeline design ................................................................................................................17 3.3.2 Pile foundations..............................................................................................................................17 3.3.3 Tailings disposal facilities ..............................................................................................................17 3.3.4 Bridges, Dikes and Erosion Control...............................................................................................18 3.3.5 Open Pit Mines ...............................................................................................................................18 3.3.6 Environmental Consequences.........................................................................................................18 3.4 MITIGATION STRATEGIES .........................................................................................................................18
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SCREENING PROCESS ..........................................................................................................................19 4.1 BACKGROUND ..........................................................................................................................................19 4.2 PRECEDENT FOR PROVISION OF A SCREENING PROCESS ...........................................................................20 4.3 DESCRIPTION OF THE SCREENING PROCESS ..............................................................................................21
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APPROACHES TO ANALYSIS ..............................................................................................................25 5.1 USE OF NUMERICAL MODELS ..................................................................................................................26 5.2 ACCOUNTING FOR PRECIPITATION ...........................................................................................................27
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CONCLUSIONS ........................................................................................................................................29
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REFERENCES ..........................................................................................................................................30
APPENDIX: SCREENING WORKSHEETS ..................................................................................................33
PREFACE This project was undertaken to ensure that global climate change is consistently incorporated into long-term planning in Canada’s north. The Panel on Energy Research and Development (PERD) funded a project to study this issue over a three-year period. David Etkin of the Environmental Adaptation Research Group of Environment Canada served as the Principal Investigator and assembled a working group of specialists. This document provides a condensed version of their work. The present document examines the relationship between global climate change and the long-term performance of engineered structures built on permafrost. It provides a screening process that identifies the appropriate level of analysis for incorporating climate change in planning and design. The procedure is based on sensitivity to an increase in temperature and the severity of failure consequences. A brief review of some analytical approaches is also included. The document does not provide prescriptive guidance for engineering design; nor is it intended to be used formally in environmental assessment proceedings. It does, however, provide a framework useful for both exercises. The authors encourage the further development of tools to assist in proactive planning for, and effective monitoring of, global climate change impacts in the north. The report provides a basis for further work in these areas.
1 INTRODUCTION Current practice in cold-region engineering involves due consideration of climate variability to assure long-term project reliability. For projects that rely on the properties of frozen materials, global climate change adds another layer of uncertainty. This report addresses the incorporation of global climate change issues into the designs and decisions involving engineering projects in permafrost.
1.1
Background
The past decade has seen increasing attention paid to the prospect of global climate change. A scientific consensus has emerged that increased concentrations of radiatively active gases will cause an increase in the global average temperature over the next century. The consensus is based on the greenhouse effect in which certain gases in the atmosphere absorb long-wave radiation from the earth. This absorption of radiation is a normal part of the earth’s energy balance and warms the earth by approximately 33C. Evidence of increased concentrations of greenhouse gases at many observation points in the atmosphere suggests that the greenhouse effect will be enhanced. The Intergovernmental Panel on Climate Change (IPCC) has concluded that some global climate change has already occurred (based primarily on temperature trends) and that increased emissions of greenhouse gases due to human activity have contributed and will continue to contribute to this change (IPCC, 1995). While there is considerable uncertainty in the magnitude and pace of change, “the least likely scenario is considered to be no change” (Arrhenius and Waltz, 1990). Global climate change is uncertain in both magnitude and rate. The expected significance of an enhanced greenhouse effect varies widely by region and by the type of impact. While the overall energy balance of the earth is reasonably well understood, the probability and magnitude of specific regional impacts of a changed climate are currently very difficult to predict. Uncertainty increases when focussing on smaller areas where local
variations in air and ocean circulation patterns and topographic features such as the presence of water bodies and mountains are increasingly important. Changes to the magnitude and likelihood of extreme events, though inherently difficult to detect or predict, could also have a significant impact. Various climatic feedback mechanisms will amplify a global trend of increased temperature in the Arctic. The function of the polar environment as a heat sink of relatively small area (compared to the tropics) and the presence of the Arctic inversion further magnify the effects in this region. Furthermore, the impact on polar regions is expected to be particularly important due to the high degree of dependence of the natural and human environment on climate. There are many possible types of change in the arctic environment associated with global climate change. Changes in the extent of sea ice, vegetation and wildlife patterns, for example, will have important implications for northern communities. While many changes cannot be readily abated or mitigated, facilities that are designed for long-term use are good candidates for proactive planning.
1.2
Engineering Projects in the North
Global climate change has particular significance for the permafrost environment. Permafrost is defined as a temperature condition in which earth material remains below 0C perennially. Contrary to what its name implies, permafrost is inherently unstable. If climate warming takes place in a permafrost area, the ground temperature at depth will respond with the possibility that the permafrost would be destabilized. Changes will be felt at the surface first, propagating into the ground slowly. In the North American Arctic, tens of thousands of square kilometres of permafrost are within one or two degrees Celsius of the melting point. Therefore, much of the permafrost environment would be profoundly affected by the transition to a warmer climate. In addition, the physical and mechanical properties of permafrost as an engineering material are temperature dependent, and this dependence is most pronounced at temperatures
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within one or two degrees Celsius of thawing. With a temperature increase, due to climate warming or changes in surface conditions, frozen soil will weaken. On complete thawing, it will lose its strength due to ice cementation, with implications for the stability of slopes, structures and foundations. Engineering projects in the north often depend critically upon permafrost as a foundation material, or as a contaminant barrier, among other functions. The ability of the frozen ground to carry out these functions is dependent upon:
local climatic conditions, ground temperature, soil/rock material properties, and ground ice conditions.
Under stable climate conditions, these factors can be assessed by geotechnical evaluation and professional judgement. Current engineering practice accommodates climate variability through consideration of the historical climatic record. Given the expectation of global climate change, practice must be adjusted to accommodate expected trends in climate variables relevant to the design. This requires consideration of the lifetime of the project, to determine the extent to which a climate trend will have significant impact during the useful life of the project.
1.3
Scope of this Report
This report merges current understanding of the expectation of global climate change with current engineering practice in the north. In particular, it is concerned with the development of a process to screen projects on the basis of the increased risk of failure due to climate change. The process is intended for new projects as well as for existing or abandoned facilities whose status may need to be revisited in light of climate change. Since assessment of global climate change is a resource-intensive exercise, it is not intended that every project be subject to the most rigorous analysis available. However, it is important that facilities posing significant consequences in the event of failure, and
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whose performance is sensitive to global climate change, undergo due scrutiny. The process was developed to address the broad spectrum of engineering projects and the great variability in the risks of failure (Figure 1). The screening process could be applied during environmental review proceedings to prescribe the level of analysis to be supplied by the proponent in support of a facility license application. Alternatively, a public sector agency with an inventory of existing infrastructure could use the process to identify those facilities where concern should be focused. It is important to recognize that the process does not estimate risk. Rather, it classifies a project according to: (i) the severity of the consequences of failure and (ii) the sensitivity of the failure modes to climate change (Figure 2). The sensitivity factor is based on the underlying temperature sensitivity adjusted for climate change based on the project lifetime. Due to the wide spectrum of engineering projects that are dependent upon permafrost conditions, there is considerable variety in the types and severity of consequences that result from failure. At one extreme, a temporary road may experience thaw settlement resulting in minor inconvenience or some maintenance costs. In another project, permafrost may be used as a contaminant barrier or dam structure where failure would result in considerable disruption to a community. A failure may result in adverse effects on human or ecosystem health (e.g. loss of freshwater supply, environmental contamination), socio-cultural disruption (e.g. isolation due to road outages or airstrip damage), or economic costs (e.g. damage to buildings, increased maintenance costs). Regional details of global climate change are uncertain. While global climate change is often simplified to the term global warming, on a regional basis there could be a net cooling trend, a change in the amount and seasonal pattern of precipitation, or in a change in the frequency of extreme events. Even if the exact changes in climatic parameters could be known, ranking their importance with respect to permafrost behaviour is not immediately evident. A change in the seasonal or annual variability in temperature could have a greater impact than a
change in the average annual temperature. Similarly, a change in precipitation patterns could have a greater impact on permafrost than an increase in average temperature. At the current level of understanding, most of these changes cannot be forecast with sufficient precision to influence design procedures. Given the uncertainty described above, it may be reasonable to perform only limited climate change analysis for some engineering projects. Several factors, individually and in combination, may make a project relatively insensitive to climate change: (i)
where the permafrost temperature is significantly below the thawing point,
(ii)
where the material properties are not highly dependent upon temperature,
(iii)
where the project lifetime is relatively short,
(iv)
where considerable over-design has been included for other reasons, or
(v)
where failure negligible.
consequences
are
However, the need for climate change analysis is not necessarily a simple ‘yes or no’ decision. The screening process points to an appropriate level of analysis, ranging from qualitative analysis to detailed thermal modeling involving a variety of future climate scenarios and proposed mitigation strategies. This report will also review some of the types of analyses available to consider climate change in this context.
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Climate to Surface Heat Transfer
Normal Climate
Climate Change
Surface to Permafrost Heat Transfer
Permafrost Performance
Frost Effects
Roads and Airstrips
Thaw Settlement
Foundations And Piles
Strength
Slopes and Embankments
Open Pit Mines
Creep
Permeability
Tailings Ponds And Reservoirs
Waste Containment
Societal and Environmental Context of Project
Public Health
Socio-Cultural Disruption
Ecosystem Health
Economic Costs
Figure 1: Influence of Normal and Changed Climate on Permafrost in Engineering Projects
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Project Description
Material and Initial Climate Description
Relevant Failure Modes
Baseline Material Sensitivity
Climate Change Description
Climate Change Sensitivity
Failure Consequence Description
Climate Change Induced Material Sensitivity
Consequence Assessment
Sensitivity Assessment
Y
X
X Consequences
Y
Z
Sensitivity
Z
Level of Analysis
Figure 2: Schematic of Screening Process
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2 CONSTRUCTION OF CLIMATE SCENARIOS FOR NORTHERN LATITUDES Before any assessment of the impact of global climate change can be carried out, a set of scenarios representing the expected climatic trend are required. This section describes an approach to the problem of constructing such scenarios, with the following steps:
2.1
Characterize current knowledge about the expected range of scenarios for future climate at the global scale. The scenarios are evaluated to provide a ‘best’ estimate and a ‘pessimistic’ estimate of temperature change. Characterize current knowledge about the expected range of scenarios for future climate for northern latitudes. This knowledge is based on climatological reasoning and climate models at high latitudes.
Greenhouse Gas Emission Scenarios
'Normal' concentrations of greenhouse gases in the atmosphere trap some of the earth's outgoing radiation and, on average, warm the earth's surface by 33C. Figure 3 shows recent trends in atmospheric CO2 concentrations. Increasing concentrations are certain to continue though the rate of
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increase depends upon current and future societal decisions. Emission reduction strategies, focussing on emissions from fossil fuel burning, are the subject of current debate (note that the 1990 concentration of carbon dioxide already exceeds values in the past 220,000 years). Figure 4 shows a variety of emissions scenarios (termed the IS92 scenarios) which range from optimistic to pessimistic in the achievement of emission reductions. The figure also provides the corresponding cumulative concentrations of atmospheric carbon dioxide.
2.2
Models of Climate Change
Climate models that incorporate mathematical description of the atmosphere, land, ocean, biosphere and cryosphere, are important tools for understanding climate. The most commonly cited models are General Circulation Models (GCMs). Their approach has been to produce a model that simulates the current climate, and then perturbs the modeled climate by changing one or more of the parameters. The various models use a common scenario to study climate change involving the doubling of CO2 in the atmosphere. Based on these models, the IPCC estimates a global mean temperature sensitivity between 1.5C and 4.5C, with a ‘best estimate’ of 2.5C. The long-term equilibrium mean global temperature change under the doubling (2CO2) scenario is termed the ‘climate sensitivity’ (IPCC, 1995).
Figure 3: CO2 concentrations measured at Mauna Loa, Hawaii since 1958. (Source: IPCC, 1994).
Figure 4: Total anthropogenic CO2 emissions under the IS92 scenarios and (b) the resulting atmospheric CO2 concentrations (Source: IPCC, 1995).
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Figure 5: Projected global mean surface temperature changes from 1990 to 2100 for the full
set of IS92 scenarios. A climate sensitivity of 2.5C is assumed. (Source: IPCC, 1995)
Figure 6: Projected global mean surface temperature change from 1990 to 2100. The highest temperature changes assume a climate sensitivity of 4.5C and the IS92e emission scenario; the lowest, a climate sensitivity of 1.5C and the IS92c emission scenario. The mid-range estimates assume a climate sensitivity of 2.5C and the IS92a scenario. (Source: IPCC, 1995).
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2.3
‘Best Estimate’ and Pessimistic Scenarios for the Year 2100
Given that greenhouse gas concentrations are not likely to reach the levels equivalent to a doubling of CO2 until late in the 21st century, the next step is to determine the likely trend toward this point. IPCC (1995) provides a range of global mean temperature trends. (based on the overall ‘best estimate’ sensitivity of 2.5C) under the IS92 emissions scenarios. The temperature projections assume that only 50-90% of eventual changes are realised at the time of greenhouse gas stabilisation. For the overall ‘best estimate’ sensitivity of 2.5C, the projections suggest a range of 1.3C to 2.5C with a ‘best estimate’ temperature change of 2.0C for the year 2100. These projections constitute the best available range of future global mean temperature change for this estimate of climate sensitivity (Figure 5). When these projections are expanded to include the broader range of climate sensitivities (1.5C to 4.5C), the range of global mean temperature changes extends from 0.9C to 3.5C with a best estimate of 2.0C for the year 2100 (Figure 6). For a variety of reasons, the authors of the IPCC (1996) report do not provide global mean precipitation scenarios based upon a range of GHG emissions and climate sensitivity cases. The models (not GCMs) used to predict the range of temperature responses seen in Figure 6 do not incorporate the hydrological cycle, and the precipitation results from GCMs are associated with more uncertainty than the results for temperature. GCM results regarding precipitation do agree, however, that annual mean precipitation will increase in high and mid-latitude regions as a
result of enhanced transport of water vapour to these regions (IPCC, 1996). For example, the Canadian Climate Centre (CCC) GCM, which has a climate sensitivity of 3.5C, predicts a 4% increase in mean annual global precipitation with 2CO2 (Boer et al., 1992), but percentage increases in high latitudes are expected to be greater.
2.4
Global Scenarios and Arctic Amplification
The next step in constructing a climate change scenario for a project is to convert global temperature change scenarios into more localized scenarios based on latitude. Climatological reasoning and climate models provide evidence for considerable amplification of the global warming signal in the winter with increasing latitude. General Circulation Model (GCM) simulations of temperature change at high latitudes (for continental grid points in winter) uniformly indicate significantly increased winter temperatures. Based on an assessment of these model results, we have estimated an amplification of the global temperature change as a function of latitude for continental areas. At 50N, the temperature change will be double the global mean, while at 80N the temperature change will be amplified by a factor of 3.5. Through similar reasoning, it is expected that the global warming signal will be increasingly damped in the summer with increasing latitude. The damping effect of melting sea ice excess energy is absorbed as latent heat will cause the global temperature signal to be reduced with increasing latitude. For present purposes, the summer temperature change will be equal to the global temperature change signal at 50N, with the summer global warming signal being reduced to one-quarter of the global mean at 80N.
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2.5
Mean Temperature Change Estimates by Season, Latitude and Decade
The tables below (Table 1 & Table 2) provide estimates for temperature change for summer and winter for each decade (1990 to 2100) and latitude band (50N to 85N). These values represent a synthesis of the emissions scenarios and climate model outputs (for continental grid points) with climatological reasoning based on the discussion above. The temperature change estimates assume linear warming trends and do not include the potential for climatic surprises (IPCC, 1995). For present purposes, they can be considered nominal temperature change scenarios for northern North America to the north and west of Hudson’s Bay. There is still considerable uncertainty in both the direction and magnitude of the impact of global warming on the northeastern part of North America. Table 1 provides temperature change estimates for the ‘best estimate’ scenario (corresponding to an average global temperature change of 2C by the year 2100). Table 2 provides temperature change estimates for the ‘pessimistic’ scenario (corresponding to an average global temperature change of 3.5C by the year 2100). Arctic amplification is applied to both estimates to the same degree. These scenarios have been constructed using currently available research findings and climate model outputs. Regional scenarios of climate change (including scenarios of arctic amplification) are subject to ongoing refinement. Further research should provide a better understanding of factors influencing regional deviations from the global mean scenarios.
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Table 1: Mean Air Temperature Change by Season, Latitude and Decade (C) - Best Estimate Case
Year
DJF
1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100
0.0 0.4 0.7 1.1 1.5 1.8 2.2 2.5 2.9 3.3 3.6 4.0
50 MAM JJA 0.0 0.5 0.9 1.3 1.8 2.2 2.7 3.1 3.5 4.0 4.4 4.9
0.0 0.2 0.4 0.5 0.7 0.9 1.1 1.3 1.5 1.6 1.8 2.0
Latitude (N) – Best Estimate for Climate Sensitivity (2.5C) 60 70 SON DJF MAM JJA SON DJF MAM JJA SON DJF 0.0 0.2 0.3 0.4 0.5 0.7 0.8 1.0 1.1 1.2 1.4 1.5
0.0 0.5 0.9 1.4 1.8 2.3 2.7 3.2 3.6 4.1 4.5 5.0
0.0 0.4 0.7 1.1 1.4 1.8 2.1 2.5 2.8 3.2 3.5 3.9
0.0 0.1 0.3 0.4 0.5 0.7 0.8 1.0 1.1 1.2 1.4 1.5
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.8 0.9 0.9 1.1 1.2
0.0 0.5 1.1 1.6 2.2 2.7 3.3 3.8 4.4 4.9 5.5 6.0
0.0 0.4 0.8 1.1 1.5 1.9 2.3 2.7 3.1 3.4 3.9 4.2
0.0 0.1 0.2 0.3 0.4 0.5 0.5 0.6 0.7 0.8 0.9 1.0
0.0 0.1 0.2 0.3 0.4 0.6 0.6 0.7 0.8 0.9 1.0 1.1
0.0 0.6 1.3 1.9 2.5 3.2 3.8 4.5 5.1 5.7 6.4 7.0
80 MAM JJA 0.0 0.3 0.7 1.0 1.3 1.6 1.9 2.3 2.6 2.9 3.2 3.5
0.0 0.1 0.1 0.1 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5
SON 0.0 0.3 0.5 0.6 0.8 1.1 1.3 1.6 1.8 1.9 2.2 2.4
Based on temperature projections in IPCC (1995). Assumptions: Temperature change is linearized between 1990 and 2100. Amplification of change in winter is by a factor of 2 times the global average at 50N and a factor of 3.5 at 80N. Attenuation of change in summer is by a factor of 1 at 50N and by a factor of 0.25 at 80N. Spring and Fall changes are estimated as adjustments from Winter and Summer from graphs of CCC GCM data as reported in the Canada Country Study (Maxwell, 1997).
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Table 2: Mean Air Temperature Change by Season, Latitude and Decade (C) – High Sensitivity Case
Year
DJF
1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100
0.0 0.6 1.3 1.9 2.5 3.2 3.8 4.5 5.1 5.7 6.4 7.0
50 MAM JJA 0.0 0.9 1.6 2.3 3.1 3.9 4.6 5.5 6.2 7.0 7.8 8.5
0.0 0.3 0.6 1.0 1.3 1.6 1.9 2.2 2.5 2.9 3.2 3.5
Latitude (N) – High Estimate for Climate Sensitivity (4.5C) 60 70 SON DJF MAM JJA SON DJF MAM JJA SON DJF 0.0 0.2 0.5 0.8 1.0 1.2 1.4 1.7 1.9 2.2 2.4 2.6
0.0 0.8 1.6 2.4 3.2 4.0 4.8 5.6 6.4 7.2 8.0 8.8
0.0 0.6 1.2 1.9 2.5 3.1 3.7 4.4 5.0 5.6 6.2 6.9
0.0 0.2 0.5 0.7 1.0 1.2 1.4 1.7 1.9 2.1 2.4 2.6
0.0 0.2 0.4 0.5 0.8 0.9 1.1 1.3 1.5 1.6 1.9 2.0
0.0 1.0 1.9 2.9 3.8 4.8 5.7 6.7 7.6 8.6 9.5 10.5
0.0 0.7 1.3 2.0 2.6 3.3 3.9 4.6 5.2 5.9 6.6 7.2
0.0 0.2 0.3 0.5 0.6 0.8 1.0 1.1 1.3 1.4 1.6 1.8
0.0 0.2 0.3 0.6 0.7 0.8 1.1 1.2 1.4 1.6 1.8 2.0
0.0 1.1 2.2 3.3 4.5 5.6 6.7 7.8 8.9 10.0 11.1 12.3
80 MAM JJA 0.0 0.6 1.1 1.7 2.3 2.8 3.4 3.9 4.5 5.0 5.6 6.2
0.0 0.1 0.2 0.2 0.3 0.4 0.5 0.6 0.6 0.7 0.8 0.9
SON 0.0 0.4 0.7 1.2 1.6 1.9 2.3 2.6 3.0 3.5 3.8 4.2
Based on temperature projections in IPCC (1995). Assumptions: Temperature change is linearized between 1990 and 2100. Amplification of change in winter is by a factor of 2 times the global average at 50N and a factor of 3.5 at 80N. Attenuation of change in summer is by a factor of 1 at 50N and by a factor of 0.25 at 80N. Spring and Fall changes are estimated as adjustments from Winter and Summer from graphs of CCC GCM data as reported in the Canada Country Study (Maxwell, 1997).
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3 ENGINEERING IN PERMAFROST Figure 7 below, shows a typical temperature profile of permafrost in equilibrium with the climate at the surface. The ground surface, in contact with atmospheric processes, experiences the greatest annual temperature range. The top of permafrost is defined as the
depth in the ground where the maximum temperature just reaches 0C. The annual temperature range decreases with depth, down to about 20 m (the depth of "zero annual amplitude". Below this, the temperature increases steadily with depth (at a rate determined by the local geothermal gradient) until the temperature reaches 0C, at the base of permafrost.
Figure 7: Typical Temperature Profile of Permafrost
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Near the surface, the mean annual temperature profile shifts to lower temperatures between the surface and the base of the active layer. This shift, referred to as the "thermal offset", is the result of changes in thermal properties between frozen and unfrozen soil. One consequence of this effect is that stable permafrost can exist where the mean annual ground surface temperature is above 0C. When the climate changes, the permafrost temperature regime will move toward a new equilibrium with the new climate. Changes will be felt at the surface first, propagating into the ground slowly. The time required to achieve a new equilibrium in the ground will be on the order of centuries or longer, since in permafrost a large quantity of heat is required to melt any ice.
2. Climate warming should be taken into account in engineering design where appropriate and where the effects would represent an important component of the geothermal design. Designs that preserve thaw-unstable permafrost, or which rely on the increased strength, decreased permeability, or other properties of frozen material, will require some consideration of the changes in permafrost conditions because of climate change. Below is an overview of the engineering behaviour of permafrost materials that are likely to be altered under climate change, followed by a partial list of project types that will be affected by these changes. This section concludes with a discussion of mitigation techniques that can be used to minimize the adverse effects of change.
3.2 3.1
Engineering Considerations in Permafrost
Permafrost can provide a stable base for engineered structures if its properties and behaviour are accommodated in design and construction. Design strategies depend on whether permafrost is considered ‘thawstable’ or ‘thaw-unstable.’ Thaw-unstable permafrost loses a significant amount of strength upon thawing, due to the presence of significant amounts of “excess" ice (see Thaw Settlement, below). Where thaw-unstable permafrost is present, design strategies involve either the preservation of freezing ground conditions, or the removal of unsuitable materials. Ground that does not contain ice, which will not settle excessively upon thawing, and that is not underlain by thawunstable materials, is considered thaw-stable and can accommodate conventional designs. A survey performed by Nixon (1994) among 10 professional engineering firms, concluded that: 1. Climate warming may be a factor in engineering design, if its effects go beyond those anticipated in an existing conservative approach.
14
Permafrost Behaviour and Failure Modes
Permafrost, by definition, includes both soil and rock. The behaviours discussed in this section are primarily those of frozen soil. Permafrost weakening due to temperature increase can be expressed in terms of its strength and its creep behaviour.
3.2.1 Strength The strength of the soil is its ability to resist applied forces. In frozen soils, the loss of strength on thawing may be the most serious problem associated with warming soil. Even when the ground does not thaw there can be a significant loss of strength in soil that is warmed towards 0C. While the strength of frozen soil is temperature sensitive (especially within a few degrees of 0C), a frozen soil has a higher longterm strength than the same soil when unfrozen. In addition to the temperature dependence indicated above, the strength of frozen soil is also time dependent. Frozen soil has a higher shortand long-term strength than unfrozen soil, primarily because ice cements the soil mass together. This ice component of strength decreases as the ice in the soil deforms plastically in response to very low loads. Figure 8 shows schematically how the compressive strength of
Figure 8: Schematic of the variation of frozen soil strength with temperature change (Source: Andersland and Ladanyi, 1994).
frozen soil declines as it approaches 0C. It shows also that the higher the frozen soil temperature, the more strength loss the frozen soil will experience for a given temperature increase. In order to estimate this strength loss, Ladanyi (1995, 1996) has proposed a Strength Sensitivity Index, defined as the ratio of strength loss over the initial frozen soil strength, for a given temperature increase. He found, for example, that an ice-rich silty soil at –7C would, for a temperature increase of 1C, undergo a strength loss of 7.5%, while the same frozen soil at initial temperature of -3C, would lose about 15% of its original strength.
3.2.2 Creep The plastic time-dependent deformation of soils under load is called creep. After loading for periods of months and longer, soil creep causes soil movement at very low loads. The rate at which soils creep at a given load increases significantly as permafrost temperatures approach 0C. In fine-grained soils, the increase in creep deformations close to 0C could be greater than would be predicted for pure ice. Engineered structures that must account for creep settlements, such as piles, may be compromised if creep rates increase because of soil warming. For example, Ladanyi (1995) has estimated that, for a typical ice-rich silt, a temperature
increase of 1C, say from -2C to -1C, would result in approximately 3.5 times higher creep deformations and creep rates of the soil at the same applied load.
3.2.3 Thaw Settlement Frozen soil containing excess ice (also called "ice-rich" soil) is susceptible to thaw settlement. "Excess ice" is ice in the soil that exceeds the pore capacity of the soil after thawing. Thaw settlement occurs when soil containing excess ice thaws. Ice is unevenly distributed in the ground (both vertically and horizontally). Excess ice is present in permafrost because of a number of thermodynamic and geological processes, making it difficult to predict its extent and volume in the field. Where it is present, excess ice content typically decreases with depth. Most is found within 5 metres of the surface, with a few exceptional areas where it extends as deep as 60 metres. Extensive areas of permafrost contain little or no ice: such material is thaw-stable. If the mean annual ground temperature (MAGT) is only slightly below 0C, a small temperature increase may initiate ground warming sufficient to degrade permafrost completely. In ice-rich soils, resulting subsidence and loss of bearing capacity can cause extensive damage to structures. Even in cold permafrost (below -5C), a temperature rise of one or two degrees may cause a substantial deepening of the active
15
(annual thaw) layer, which can initiate thaw settlement. Once settlement is initiated, permanent subsidence of the ground surface results, as excess water is able to drain away. Thaw settlement causes an uneven surface relief, called thermokarst topography, which can severely impact land uses. Thermokarst depressions may trap snow in winter or may fill with water, increasing and accelerating the initial warming (Williams and Smith 1989, Chapter 4). Such settlement is presently responsible for damage to buildings, roads and runways, and increases in thaw settlement would result in severe maintenance and repair problems. Some roads and structures might have to be rebuilt or abandoned. Greater depths of gravel padding would be needed to preserve permafrost under roads and structures. Special concern might be directed to existing water-retaining structures, especially in areas of warm or thaw-sensitive permafrost. Based on experimental data, the unit thaw settlement can be estimated for typical permafrost soils from their basic physical properties, such as the bulk density and the total moisture content. Statistical correlations and empirical formulas obtained mainly in Northern Canada can be found in Ladanyi (1994).
3.2.4 Permeability The permeability of a material indicates the ease with which fluids can pass through them. The rate at which fluids (usually water) can move through a material is proportional to the material permeability and to the gradient of fluid pressure. In general, unfrozen coarsegrained materials have higher permeabilities than fine-grained materials. Saturated, frozen materials have extremely low permeabilities. Facilities such as sewage lagoons, tailing ponds, and reservoirs that contain fluids in the ground rely on a low-permeability barrier to impede liquid losses. In unfrozen ground, this is usually achieved using low-permeability unfrozen soil materials. Where permafrost is present, it is possible to achieve this low 16
permeability with the use of frozen soil materials that would not be suitable once thawed.
3.2.5 Frost Effects / Frost Heave Fine-grained soils that undergo freezing will expand due to the frost heave process, also known as ice segregation. Frost heave draws water from the unfrozen soil toward the freezing soil, causing ice lenses to form and the frozen soil to expand. This can impose uplift stresses and other destructive movements on foundation materials within the active layer. The amount of frost heave that occurs varies depending on local temperature and moisture conditions, the physical characteristics of the soil, and the ground water level. Local variations in these conditions cause uneven frost heave, resulting in stresses on buried structures due to differential movement. In general, the amount of frost heave uplift is proportional to the thickness of the active layer and is greatest in saturated, moderately finegrained soils. Changes that increase the depth of the active layer and the ground water level may increase the amount of frost heave to which engineered structures are subjected.
3.2.6 Changes to Permafrost A steady increase of ground surface temperature will result in two related effects on the ground temperature: 1. An increase in the mean annual temperature at the ground surface, which will slowly propagate to depth and, depending on latitude, produce either a thinning or a complete disappearance of the permafrost layer. Permafrost temperatures will change at the surface first, with changes propagating to deeper ground slowly. The time required to achieve a new equilibrium in the ground will be on the order of centuries or longer, since in permafrost a large quantity of heat is required to melt any ice. 2. A change (increase or decrease) in the annual amplitude of seasonal ground temperature variation, damped with depth, and affected by related changes in precipitation (snow cover), groundwater hydrology and vegetation.
The effects of warming of the mean annual temperature on permafrost will be different for cold and warm permafrost. In cold permafrost, the effect would be to warm it and possibly to change the depth of the active layer. Thawing at the base of permafrost would start several centuries later and would proceed at a rate of about a centimetre or more per year (Nixon 1994, Esch and Osterkamp 1990). In warm permafrost, the effects of a few degrees warming in the mean annual temperature would be greater. Since most of this permafrost is within a few degrees of thawing, it should eventually disappear. Although many centuries will be required for the permafrost to disappear entirely, increases in the active layer and thawing of the warmest permafrost from the top would begin almost immediately (Nixon 1994, Esch and Osterkamp 1990). Changes to permafrost conditions because of global warming will be accompanied by changes in most or all of the above behaviours concurrently. The linkage between changes in the permafrost environment and the technical consequences of climate warming has been summarized by Esch and Osterkamp (1990) as follows.
Increases in annual thaw (active layer): a) Thaw settlements during seasonal thawing. b) Increased frost heave forces on pilings. c) Increased total and differential frost heaving during winter.
Warming of permafrost body at depth: a) Increase in creep rate of existing piles and footings. b) Increased creep of embankment foundations. c) Eventual loss of adfreeze bond support for piling. Development of thaw zones (taliks): a) Decrease of effective length of piling in permafrost. b) Progressive landslide movements. c) Progressive surface settlements.
3.3
Major Project Categories
A partial list of engineering activities that would be affected by climate warming follows (primarily from Nixon, 1994).
3.3.1 Northern pipeline design Pipeline design will require consideration of increased frost heave and thaw settlement (Nixon, 1990b). Slope stability will also be an issue in discontinuous permafrost (Nixon, 1990b).
3.3.2 Pile foundations Piles are load bearing columns embedded in the ground, relying on some combination of end bearing, friction and freezing to the soil to carry structural loads. Design of piles in permafrost must account for settlement due to creep. The settlement of shallower pile foundations could be accelerated by climate warming over the design life of a structure. However, deeper piles for heavier structures would be less affected in the same time period (Nixon, 1990b). As adhesion forces decline with progressive warming of permafrost, problems may arise with the bearing capacity of piles, which are widely used in northern construction. Deepening of the active layer will reduce the effective embedment length of piles in permafrost, while frost heave forces might be increased (Esch and Osterkamp, 1990). For example, Nixon (1990a) has mathematically simulated the creep settlement of a pile embedded 6m in frozen silty clay, at a temperature of -1.3C, carrying a load of 200 kN. He found that a soil temperature warming rate of 0.1C per year would, after 25 years, result in a 30% increase in creep settlement of the pile.
3.3.3 Tailings disposal facilities Tailings disposal facilities might be affected (negatively or positively) by climate warming, due to the long-term effects on frozen or unfrozen tailings. Layers that freeze during winter disposal may thaw out many years later, releasing excess water and contaminants into the ground. Climate warming could play a significant role in the rate of melting of such materials.
17
3.3.4 Bridges, Dikes and Erosion Control Many effects on bridges, pipeline river crossings, dikes and erosion protection structures are related to changes in surface hydrology rather than changes in permafrost conditions per se. Precipitation changes and changes to soil permeability would alter runoff patterns, and possibly the ice water balance in the surface-active layer. It would be very difficult to assess the effects of these changes on engineering structures.
3.3.5 Open Pit Mines Open pit mine wall stability may be affected where steep slopes in permafrost overburden have been exposed for long periods of time. The engineering issues would revolve around deepening thaw, with consequent increased pore pressures in the soil/rock, and resulting loss of strength and reduction in pit wall stability.
3.3.6 Environmental Consequences Woo et al. (1992) and Maxwell (1997) summarize a number of environmental consequences of permafrost degradation because of climate warming. The erosion of lake, river and reservoir shorelines may be expected to increase because of permafrost thawing and a longer open water season. Increased sediment transport in rivers could shorten the operating life of hydroelectric projects, for example. The expected rise in sea level accompanying global warming could lead to accelerated rates of coastal retreat in permafrost regions, and combined with thaw settlement as permafrost melts, could also result in the inundation of presently low-lying areas. Osterkamp (1984) points out that most of the petroleum-producing areas of the Arctic and coastal Alaska are presently at very low elevations.
3.4
Mitigation Strategies
Current practice in permafrost engineering incorporates mitigative measures for disturbance to permafrost (due to changes in
18
local surface conditions) even under the assumption of a constant climate. These measures may be employed to deal with climate warming as well. Vyalov et al. (1993) suggest that the effects of climate warming can be neutralized by recognizing that in the northern climates the average annual air temperature is always several degrees lower than the average annual temperature of permafrost. This natural temperature difference can be used for cooling the permafrost by various techniques. One approach is pre-construction ground cooling by snow removal. Field observations and numerical simulations show that the annual average ground temperature can be decreased considerably by clearing the surface of vegetation and snow. Ongoing cooling during service life can be achieved through several strategies, such as the traditional ventilated basement. Ventilated basements elevate the structure above the ground surface, preventing snow accumulation in winter and shielding the surface from solar radiation. Ventilation is usually natural, with forced air circulation sometimes used. Experience shows that this can achieve a 1 to 3C decrease in the average annual ground temperature. Ventilation ducts can also be installed in the ground under a building, with natural or forced air circulation. The most effective measure to cool the ground is the use of thermosyphons and thermal piles, in which self-activated refrigeration technology is used to extract heat from the ground. In special cases, artificial cooling of foundations may be needed. The best results in improving the frozen ground load bearing capacity can be attained by combining these measures. Even without climate change, there are often uncertainties about the net effect of changes to the microclimate (e.g. snow clearance or altered snow accumulation, changes to surface cover, foundation installations, etc.). Buried temperature sensor cables, monitored at regular intervals, can ensure that undesirable changes to the permafrost temperature regime are detected before significant problems arise.
to climate change effects.
4 SCREENING PROCESS This section describes a step-by-step process to assess the appropriate level of analysis in considering global climate change as a factor in permafrost-related projects. The process is intended to consider the sensitivity of the project to climate change as well as the consequences associated with failure in a systematic way.
4.1
2. To allow projects with low climate change sensitivity, and without potential for severe consequences, to forego detailed analysis of climate change impacts. 3. To provide guidance for project scrutiny by regulatory bodies and review panels. This screening process has been designed to place a project on a two-dimensional continuum of sensitivity and consequences (see Figure 9).
Background
The screening process is intended to satisfy the following objectives: 1. To ensure that projects with substantial impacts are duly scrutinized with respect
High
Water Retention Dam in Continuous Permafrost (15 year life)
Failure Consequences
Temporary Road on Continuous Permafrost
Low Low
Gold Mine Tailings in Discontinuous Permafrost (50 year life)
Highway over Discontinuous Permafrost
Climate Change Sensitivity
High
Figure 9: Two dimensions of consideration in screening permafrost-related projects
19
4.2
Precedent for Provision of a Screening Process
Risk-based screening of installations has become part of safety management activities in a number of industries and organizations. An early formalized approach is that of the Zurich Insurance Company Zurich Hazard Analysis method for classifying risks of hazardous facilities such as chemical plants and dams for insurance underwriting purposes (Zurich, 1987). One of the first standards for risk analysis, produced by the Canadian Standards Association (CSA-Q634-91) includes a generic instrument for prescribing the level of analysis for identified risks. The CSA screening process is illustrated in Figure 10.
Likelihood
Frequent Probable Occasional Remote Improbable
In explicit standards for risk assessment, risk consists of two dimensions: likelihood and consequences. The likelihood dimension, for present purposes, corresponds to the sensitivity of the project to climate change. This reflects the fact that climate change is an incremental hazard, in that it exacerbates existing threats to permafrost, thus providing an incremental increase in the likelihood of failure. The consequence dimension is captured as a qualitative measure of the significance of an induced failure. Note that this is a highly subjective interpretation of an event that could have profound human health, environmental, sociological and financial impacts. The following section describes the adaptation of the CSA process for present purposes.
Consequence Negligible C C D D D
Risk Level D C B A
Minor A B B C C
Major A A B B C
Analysis Prescribed Not Required Qualitative Semi-quantitative Detailed Quantitative
Figure 10: Screening process from CSA-Q634
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Catastrophic A A A A B
4.3
Description of the Screening Process
The consideration of climate change in engineered projects on permafrost terrain is based on three main sources of information: 1. A description of the project purpose, location, temporal scope and situation visa-vis citizens, the natural environment and its socio-economic value, including any benefit of the project to the community bearing the risk. 2. A technical description including: the current state of the permafrost (composition, ice-content, and temperature), the function demanded of it, the failure modes and consequences, and sensitivity to various climatic variables. 3. Description of the effects and timing of potential climate change at global and regional levels.
The above information must be assembled in summary form in order to screen the project. This information is used to perform qualitative riskbased screening as illustrated in Figure 2. Step 1: Identification of Relevant Failure Modes The potential failure modes depend on the function of the permafrost in the project. Various project types and the relevant failure modes are indicated in Table 3. Given the relevant failure modes, and the project description, a set of failure scenarios can be generated. These scenarios can be qualitatively rated as being: catastrophic, major, minor or negligible. For the purposes of this process, severity should be considered in as wide a perspective as possible and should include the potential for socio-economic or cultural impacts. The labelling of the severity of the scenarios corresponds to item ‘X’ in Figure 2.
Table 3: Index of Relevant Failure Modes by Project Type
Project Type Failure Mode
Thaw Settlement Loss of strength / creep Increased Permeability Accelerated frost effects
Tailings Ponds, Dams YES
Open Pit Mine
Roads
Foundations / Piles
Slopes, Embankments
NO
YES
YES
YES
YES
YES
NO
YES
YES
YES
NO
NO
NO
NO
NO
NO
YES
YES
NO
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Step 2: Categorization of Climate Change Sensitivity As mentioned in Section 3, the strength, creep and thaw-settlement properties of frozen soils are sensitive to temperature change, particularly at temperatures close to the melting point. The overall sensitivity of permafrost is dependent upon the type of soil, its ice content, and on an estimate of the ground temperature at the end of the project’s life. The final temperature is determined by considering the current permafrost temperature and adjusting for the duration of the project’s life. This screening process employs a conservative simplifying assumption that the ground temperature change will change at the same rate and magnitude as the air temperature. In reality, ground temperature response will lag behind air temperature changes. The time lag effects will vary with depth and other factors such that there is not a unique relationship between air and ground temperature changes. Sensitivity depends primarily on anticipated consequences of thaw settlement and loss of strength due to active layer deepening, reduction of bearing strength and creep
resistance due to warming of frozen ground. The classification used here combines all of these effects into a single sensitivity classification. A more refined classification scheme for sensitivity could be differentiated by failure mode. The effect of active layer deepening will be greater at warmer temperatures, although thaw settlement and strength effects will be more serious at lower temperatures for soils that contain excess ice. Temperature sensitivity is usually greater for more fine-grained soils as a result of the higher unfrozen water content. Marine soils with salinity will exhibit temperature sensitivity at lower temperatures because of the freezing point depression of the pore water. Soils have been classified by physical characteristics (predominantly grain size), or by material origin (with physical characteristics assumed). Table 4 provides a qualitative assessment of sensitivity based on the soil type and temperature, separated into four permafrost zones. The concept of permafrost zones (Vyalov, 1993) has been slightly modified to reflect North American practice. Zone 1, for example, which is considered unstable should cover the temperature range of 0C to -2C. The temperature of permafrost is defined as the constant temperature below the zone of seasonal influence. The sensitivity is classified as high, medium or low.
Table 4: Designation of Material Sensitivity by Zone and Soil Type
Type of Soil
Zone 4 ( T -7C )
Any Soil containing massive ice
M
H
H
H
Peat and Organic
L
M
H
H
Lacustrine (silt or clay)
M
M
M
H
Morainal soils (till)
L
L
L
M
M
M
H
H
L
L
L
M
L
L
M
M
Marine Soils with Salinity Alluvial and Glaciofluvial (sand or gravel) Frost-shattered rock
22
Permafrost Temperature Zone Zone 3 Zone 2 ( -7 T -4C ) ( -4 T -2C )
Zone 1 ( -2 T 0C )
In section 2, two estimates for temperature changes are provided for each combination of decade, latitude and season. The first estimate (found in Table 1) is considered to be a ‘best estimate’ of temperature change. The second estimate (found in Table 2) is considered to be a more extreme (though plausible) value for temperature change. Note that these estimates do not include other possible relevant impacts of climate change such as changes in precipitation. While the influence of snowcover is important in determining the effect of climate change on permafrost, precipitation change is not included in the screening process. This is due to the relatively low confidence that can be placed in regional GCM forecasts of precipitation, and because the net effect of precipitation change on permafrost conditions will depend on local conditions to a significant degree. Changes in total precipitation will combine with seasonal temperature change to alter the proportion and timing of precipitation that falls as snow. Environmental change will also cause changes in the evolution of snowpack properties (densification, depthhoar development, etc.) through the snowcover season. The combined effect of these changes to the snowcover at particular sites may be to augment or offset the change in temperature. Using the final year of the design life of the project, locate the best estimate and high estimate for temperature change from Tables 1
and 2 for the appropriate latitude. Add the estimated temperature change to the initial permafrost temperature to arrive at the final temperature for each material. Locate the permafrost material and final temperature in Table 4. Note the material sensitivity at this temperature (high, medium or low). The final sensitivity measure, based on the material and final temperature, corresponds to item ‘Y’ in Figure 2. Example: Consider a project whose design life extends to the year 2030. The project is located at 60N. The designated temperature changes (from Table 1 and 2) are: Designated Temperature Change (C) Year 2030 at 60N
Winter
Summer
Best Estimate (from Table 1)
1.8
0.5
High Estimate (from Table 2)
3.2
1.0
The project utilizes soil classified in Table 4 as Peat and Organic. The initial permafrost temperature is -5C. For screening purposes, apply the best estimate of winter temperature change, yielding a final permafrost temperature of –3.2C (Zone 2) for the best estimate. Using Table 4, the sensitivity of the material to climate change is classified as high.
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Sensitivity (Y) High Medium Low
Risk Level D C
B
A
Consequence (X) Negligible C D D
Minor B C C
Major A B C
Catastrophic A A B
Analysis Prescribed No action required. Perform qualitative analysis. Apply expert judgement. Document result of evaluation. Perform quantitative evaluation for projects with limited precedence in: design, function, or construction method. In addition to requirements for ‘C’: Perform limited quantitative analysis. Use engineering judgement for input parameters. Monitor permafrost performance. Perform full quantitative evaluation for projects with limited precedence. Perform detailed quantitative analyses. Refine input parameters with additional investigation and testing. Perform full scale monitoring programme with periodic evaluation of performance. Independent expert review required.
Figure 11: Adaptation of CSA-Q634 screening framework to permafrost-related projects.
Step 3: Combination of Consequence and Sensitivity Based on the qualitative measures of consequence (X) and sensitivity (Y), a prescribed level of analysis can be derived from the chart in Figure 11 (adapted from Figure 10). Based on the prescribed level of analysis (A, B, C, or D in Figure 11), recommendations for analysis are provided (see section 5). Experts employed to perform the screening of projects are also free to document reasons for a higher or lower level
24
of analysis based on factors not identified in this screening process. The judges should be encouraged to document their use of the instrument. The process described in this section is captured in the worksheets provided as an Appendix.
5 APPROACHES TO ANALYSIS The screening process presented in this document is intended to guide decisions regarding the appropriate level of analysis required to demonstrate sound design. In most cases the effort required to account for climate change effects will not exceed what would normally be undertaken, and may possibly be incorporated into an existing design process with little modification. Choices made in analysis need to be explicit and justified so that, in principle, model results can be replicated. Numerical modeling of climate change effects should be undertaken using detailed numerical modeling for projects at risk level A, and for projects at risk level B for which design principles have limited precedence. Analytical modeling of climate change effects should be sufficient for all projects at risk level C, and for projects at risk level B for which design principles have sufficient precedence. Modeling the impact of environmental change requires accounting for the combination of time dependent ground temperatures and temperature dependent properties and behaviour. Methods to combine these properties will vary depending on the critical properties and behaviours being modeled. In some cases, it will be sufficient to show that the ground will not warm beyond a critical temperature, based on the temperature dependence of a critical property such as strength or permeability. In other cases, it may be necessary to account for changing soil properties, together with their effect on the structure, through time. The most complicated analyses may need to account for changes to the configuration of the ground (due to thaw settlement, creep, and/or frost heave) and their effects on the project. Modeling techniques to account for the consequences of degradation of permafrost are well established, since effects such as thaw settlement are already a common consequence of microclimatic or surface disturbance.
Geothermal models vary from simple single equations representing idealized cases (e.g., the Stefan equation for thaw depth), to three dimensional numerical simulations replicating virtually all relevant details of a ground thermal problem. Choosing the appropriate level of analysis depends on how critical the thermal condition of the ground is to the problem under investigation. Thermal, mechanical, and other physical properties, as well as information about surface and other boundary conditions, are needed as input to any analysis. Properties for a specific site or material can be known with a high level of confidence only through site investigation and laboratory testing. With increasing levels of uncertainty, some properties can be estimated from correlation with known physical properties of site materials or from geological inferences. Similarly, detailed information on vegetation, seasonal snow cover, and other conditions can be obtained at specific sites. Thus, model predictions can be based on more complete information at site scales where intensive investigation of terrain conditions can be justified. For site or route selection problems, limited knowledge of model parameters will increase the uncertainty in model results. At any level of analytical effort, reporting on results should include the following:
Model name and reference (source)
Discussion of the assumptions of the model (e.g., excluded processes, simplified boundary conditions, treatment of transient effects)
Justification for choice of model
Identification of model parameters
Parameter values
Sources for parameter values (estimates from material type/physical properties/tests on samples)
Discussion of the effect assumptions on results
of
model
Analytical solutions exist for many thermal problems involving freezing or thawing (e.g., Lunardini 1981, Andersland and Ladanyi, 1994). 25
These generally provide idealized conditions (uniform soil properties, simple surface temperature conditions) so that results may include errors related to the underlying assumptions of the model. More detailed thermal predictions for permafrost can be made using computer based (“numerical”) geothermal model. Analytical solutions exist also for estimating the effects of warming on mechanical properties of frozen soils and their implications on the design of engineering facilities in permafrost areas. (e.g., Andersland and Ladanyi, 1994).
5.1
Use of Numerical Models
Numerical methods can provide reasonable predictions for complex ground thermal problems. These methods have been embodied in many numerical geothermal models, which may be considered standard tools for the analysis of permafrost conditions. Such models have the advantage that heat transfer relations between the atmosphere (climate) and the various terrain elements are explicit, facilitating the investigation of permafrost responses to alternative climate scenarios. Numerical geothermal simulation requires specification of the following:
Problem geometry (1-, 2-, or 3dimensional). The boundaries of a geothermal problem usually extend beyond/below the immediate range for which results are required. For example, a simulation of seasonal freezing behaviour will normally require that the lower simulation boundary be several metres below the base of the active layer.
Thermal properties of the subsurface materials, to the depth of the boundaries defined for the problem under consideration. Such properties may be determined from direct testing, estimated from relationships with measured field physical properties (Farouki 1982), or based on more limited knowledge of the ground materials (e.g., soil or rock type).
26
The temperature dependence of properties may be accommodated.
Initial thermal conditions. The temperature of each point within the simulation boundaries must be specified at the start of a simulation. This information may be available from site investigations for all or part of the ground, or may be estimated from information available about permafrost thickness, local climate conditions, etc.
Thermal conditions at boundaries. Boundary conditions within the ground (at the bottom, as well as at the sides in 2- or 3dimensional problems) are usually easiest to specify, typically with an assumption that heat will flow through these boundaries at a constant rate. At the ground surface, the assumed course of temperatures must be specified. This may be established directly, or may be determined as the function of various possible surface heat transfer schemes. The snowcover may be treated more or less realistically as a transient layer with its own thermal behaviour, or it may be subsumed within a seasonal transfer function such as the n-factor. Cooling by natural or artificial ventilation can be treated in two or more dimensions as a special “internal” boundary condition.
In addition, it may be important to consider changes through time. Thaw settlement due to permafrost degradation, or frost heave in the active layer, will change the problem geometry (and thermal properties) through time. Inclusion of such effects will be important in some cases, and will require the use of models specifically designed to accommodate these effects. In thawunstable soils, slopes and embankments may be modified to prevent thaw settlement or to ensure that thaw settlement is limited to acceptable levels. Accounting for the effects of thaw settlement on slope geometry will only be necessary where thaw-unstable ground is expected to thaw. In general, the changes in soil geometry due to frost heave will be small compared to the effect of thaw settlement. The inset provides a suggested checklist for reporting of numerical results.
Program/Model Name: Author/Source: Model Description: for Precipitation 5.2 Accounting Type: AccountinFinite g forDifference the effect ofElement; changes in / Finite snowcover in a permafrost thermal modeling 1-, 2-, or 3-dimensional: exerciTreatment se will require: of Thermal Properties: (Temperature Dependent/Bimodal) Treatment of Latent Heat of Fusion (e.g., apparent heat capacity) ccountingof Hydrology for seasonal (monthly) ATreatment anges in ofthe precipitation as predicted chTreatment Mechanical Properties: (Temperature Dependent/Bimodal) GCMs. ofBecause of the low resolution byTreatment Special Conditions: and low Frost region Heave al accuracy of current CM Thaw output, most modeling efforts G Settlement Heat Sources/Sinks creaInternal se or decrease current precipitation in by an amount scaled to the data Settlement/Creep peInput Parameters rcentage change predicted by GCMs. For each parameter, give data source and individual parameter values.
Accounting for the change in snowfall as a Specifications sult Grid of the changing temperature regime. re Time Stepping The approach taken should match the level Thermal Boundary (Upperfor / Lower / Lateral) used Conditions in accounting ofsophistication Source (estimated / extrapolated / measured) and Values temperature change. The simplest Initial Temperature Profile roach uses 0 C as a threshold, with app Thermal Properties precipitation falling as snow when the Mechanical Properties temperature falls below this value. A more Strength sophisticated approach could incorporate Creep stic generation of daily temperatures stoc haThaw settlement/consolidation parameters precipitation Frost heave parameters and using the statistical properties of the precipitation-temperature Figure relationship. 4Supporting details for reporting modeling results. For example, the relationship in Figure 12 (from Riseborough and Smith, 1993) between mean daily temperature and precipitation type for Fort Simpson NWT, was used to modify the snowfall regime in a climate change modeling exercise. To model snowcover under climate change, they:
Dyke et al. (1997) discuss a geothermal study (Geo-Engineering, 1992) that considered changes to both temperature and snowcover conditions. The transient response of permafrost to surface warming was investigated for locations in the Mackenzie Valley using the Atmospheric Environment Service’s GCM outputs for mean monthly temperatures and precipitation. Both Randomly selected years from the climate linear and exponential rates of temperature record. Figure 12: Precipitation a function of daily mean temperature. Fort Simpson increase type fromas1CO 2 to 2CO2 conditions were (1963-1989). Source: (Riseborough and Smith, 1993). record weather examined for a 50 year period. Although Adjusted daily temperatures and precipitation amounts according to GCM precipitation increases under 2CO2 conditions, predictions, over an assumed transition the effect in winter is to produce more rain rather period, than snow (because of higher air temperatures), so that snow cover decreases. The reduced snow Assigned precipitation as snow or rain cover reduces its insulating effect, diminishing probabilistically, using the relationship in the impact of the warmer winter air temperatures. Figure 12. Despite this result, however, this study also examined a scenario where snow cover was arbitrarily increased by 10%, since confidence in
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GCM precipitation predictions is much less than for temperature predictions.
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6 CONCLUSIONS The expectation of climate change and the further amplification of this change at high latitudes suggests that engineering designs should consider climate change where indicated by the sensitivity of the material and the severity of consequences. The screening process presented in this document provides a prototype for systematically considering this issue. The process relies on conservative assumptions with respect to the response of frozen ground to air temperature change. Overall, however, the process is not necessarily conservative because factors such as changes in precipitation and snow cover are not considered in the current version. Further research into the relative importance of temperature and precipitation changes would be valuable to the overall understanding of climate change impacts on permafrost engineering design. Further refinement of the process would facilitate its use in the screening stage of environmental assessment proceedings For existing facilities, global climate change may significantly shorten their operating life. The screening process in this report provides a means for prioritizing facilities for investigation and monitoring. For certain projects the potential for adverse consequences is perpetual. Hazardous waste containment in frozen ground and the retention of toxic mine tailings are two examples. The potential impact of climate change on their performance in the very long term must be thoroughly scrutinized.
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7 REFERENCES Andersland, O.B. and Ladanyi, B. (1994). An Introduction to Frozen Ground Engineering. Chapman & Hall, New York, 352pp. Arrhenius, E. and Waltz, T., (1990). “Minimizing the greenhouse effect.” In: Managing Natural Disasters and the Environment, A. Kreimer and M. Munashighe (Eds.) pp. 40-49. The World Bank.
IPCC (1994). Climate Change 1994: Radiative Forcing of Climate Change and an Evaluation of the IPCC IS92 Emission Scenarios. Cambridge University Press, Cambridge, UK. IPCC (1995). Climate Change 1995: The science of climate change. Technical Summary. Cambridge University Press. Cambridge, UK. Johnston, G.H., Ed. (1981). Permafrost, Engineering Design and Construction. J.Wiley & Sons, New York.
Boer, G.J., N.A. McFarlane, and M. Lazare (1992). Greenhouse Gas-induced Climate Change Simulated with the CCC SecondGeneration General Circulation Model. Journal of Climate, 5(10), 1045-1077.
Ladanyi, B. (1994). La conception et la réhabilitation des infrastructures de transport en régions nordiques. Rapport RTQ-94-07, Ministère des transports du Québec, 123 p.
CSA (1991). CSA-Q634-91: Risk Analysis Requirements and Guidelines. Canadian Standards Association. Rexdale, Ontario.
Ladanyi, B. (1995). Civil engineering concerns of climate warming in the Arctic. Transactions of the Royal Society of Canada, Sixth Series, Vol. VI, pp. 7-18.
Dyke, L.D., Aylsworth, J.M., Burgess, M.M., Nixon, F.M., Wright, F. (1997). Permafrost in the Mackenzie Basin, its influence on landaltering processes, and its relationship to climate change. In: Mackenzie Basin Impact Study, Cohen, S. [Ed.]. pp. 112-117. Atmospheric Environment Service, Downsview, Ont. Esch, D.C. and Osterkamp, T.E. (1990). “Cold regions engineering: Climatic warming concerns for Alaska.” J. of Cold Regions Eng. 4(1):6-14. ASCE. Farouki, O.T., (1982). Evaluation of methods for calculating soil thermal conductivity. U.S. Army Cold Regions and Engineering Laboratory, Hanover, New Hampshire, Report 82-08, 90 pp. Geo-Engineering (M.S.T.) Ltd. (1992). Potential impact of global warming in permafrost in the Mackenzie Valley. Geological Survey of Canada, Open File 3017. Haas, W.M. and Barker, A.E. (1989). Frozen gravel: A study of compaction and thawsettlement behavior. Proc. 5th Cold Regions Engrg Symp., St.Paul, Minnesota, pp. 308-319. Hoffert, M.I. and Covey, C., (1992). “Deriving Global Climate Sensitivity from Paleoclimate Reconstructions.” Nature 360, 573.
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Ladanyi, B. (1996). A strength sensitivity index for assessing climate warming effects on permafrost. Proc. Eighth Int. Conf. On Cold Regions Engineering, Fairbanks, Alaska, ASCE, New York. Leroueil, S., Dionne, G. et Allard, M. (1990). Tassement et consolidation au dégel d’un silt argileux à Kangiqsualujjuak. 5th Canad. Permafrost Conf, Collection Nordicana No.54, Laval University, Québec, pp. 309-316. Lunardini, V. (1981). Heat Transfer in Cold Climates. New York: Van Nostrand Reinhold, 731 pp. Maxwell, B. (1997). Responding to Global Climate Change in Canada’s Arctic. Volume II of the Canada Country Study: Climate Impacts and Adaptation. Atmospheric Environment Service, Environment Canada. Downsview, Ontario. McRoberts, E.C., Law, T.C. and Moniz, E. (1978). Thaw settlement studies in the discontinuous permafrost zone. 3rd Int. Conf. on Permafrost, Edmonton, pp. 700-706. Nelson, R.A., Luscher, U., Rooney, J.W. and Stramler, A.A. (1983). Thaw strain data and thaw settlement predictions for Alaskan silts. Proc. 4th Int. Conf. on Permafrost, Fairbanks, AK, pp. 912917.
Nikiforoff, C. (1928). The perpetually frozen sub-soil of Siberia. Soil Science 26:61-81.
U.S.Army C.R.R.E.L. Special Rep. 93-22., Hanover, NH
Nixon, J.F. (1990a). “Effect of climate warming on pile creep in permafrost.” J. of Cold Regions Eng. 4(1):67-73. ASCE.
Sayles, F.H. (1968). Creep of frozen sands. U.S. Army C.R.R.E.L., Hanover, NH, Tech.Rep. 190.
Nixon, J.F. (1990b). “Seasonal and climatic warming effects on pile creep in permafrost.” 5th Canadian Permafrost Conference. Collection Nordicana No. 54 Laval University, Quebec, pp. 335-340. Nixon, J.F. (1994). Climate change as an engineering design consideration. Report for Canadian Climate Centre. Atmospheric Environment Service, Environment Canada, by Nixon Geotech Limited, Calgary, March 1994. Osterkamp, T., (1984). “Potential impact of a warmer climate on permafrost in Alaska.” in The potential impacts of carbon dioxideinduced climate changes in Alaska. Misc. Publication 83-1, School of Agriculture and Land Resource Management, University of Alaska-Fairbanks. pp. 106-113. Riseborough, D.W. and Smith, M.W. (1993). Modelling permafrost response to climate change and climate variability. Proceedings, 4th Intl. Symposium Thermal Engineering and Science for Cold Regions, pp. 179-187.
Shabalova, M.V. and Können, G.P. (1995). Climate change scenarios: comparisons of paleoreconstructions with recent temperature changes. Climatic Change 29:409-429. Williams, P.J. and Smith, M.W. (1989). The Frozen Earth: Fundamentals of Geocryology. Cambridge University Press, Cambridge, England. 306pp. Woo, M.K., Lewkowitz, A.G. and Rouse, W.R. (1992). “Response of the Canadian permafrost environment to climatic change.” Phys. Geog. 13(4):287-317. Vyalov, S.S., Gerasimov, A.S., Zolotar, A.J. and Fotiev, S.M., (1993). “Ensuring structural stability and durability in permafrost ground areas at global warming of the Earth’s climate.” 6th Int. Permafrost Conference. Beijing, China. South China University of Technology Press. pp. 955-960. Zurich (1987). A Brief Introduction to the “Zurich” method of Hazard Analysis. Zurich Insurance Company.
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APPENDIX: SCREENING WORKSHEETS
33
Consideration of Climate Change in Engineered Projects Involving Permafrost JUDGE INFORMATION
Name of Judge
Date
Title Affiliation Address Judge’s Relationship (if any) to Project and/or Proponent:
PROPONENT DESCRIPTION
Proponent Representative Address
Project Title and Brief Description:
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PROJECT DESCRIPTION
Documentation on which this description is based:
Brief Site Description:
Latitude:
Longitude
Nearest Populated Point: Municipal or Regional Authority:
Purpose of Project:
Project Beneficiaries Principal Risk Bearers
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PERMAFROST DESCRIPTION
Functions to be provided by frozen material (e.g., structural, contaminant barrier, road foundation, dam, etc.)
Description of Soils (including ice content):
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IDENTIFICATION OF FAILURE MODES
Description of Project (for use with table below):
Project Type Failure Mode
Tailings Ponds, Dams
Open Pit Mine
Roads
Foundations / Piles
Slopes, Embankments
Thaw Settlement
YES
NO
YES
YES
YES
YES
YES
NO
YES
YES
YES
NO
NO
NO
NO
NO
NO
YES
YES
NO
Loss of strength / creep Increased Permeability Accelerated frost effects
List all combinations of failure modes and the associated materials:
Failure Mode
Material
1. 2. 3. 4. 5.
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DESCRIPTION OF FAILURE MODES
For each combination of failure mode and associated material, describe the functional consequence of failure for each failure mode (e.g., collapse, leaking, road surface damage): 1.
2.
3.
4.
5.
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CONSEQUENCE CLASSIFICATION
For each combination of failure mode and associated material, describe the broader consequences (and the bearer of the consequence) of failure. Include human health, environmental, socio-economic, financial and any other consequence of public concern. For each combination, classify the consequences in one of the four categories. 1.
Catastrophic
Major
Minor
Negligible
Catastrophic
Major
Minor
Negligible
Catastrophic
Major
Minor
Negligible
Catastrophic
Major
Minor
Negligible
Catastrophic
Major
Minor
Negligible
2.
3.
4.
5.
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CLIMATE CHANGE SENSITIVITY
Final Year (for purpose of design life) Latitude
Designated Temperature Change (C) Year
N
at
Winter
Summer
Best Estimate (from Table 1) High Estimate (from Table 2)
As a conservative estimate of ground temperature change, assume that the ground temperature change will be equivalent to the best estimate of air temperature change in winter (from the table above). For each combination of failure mode and associated material, indicate the initial and final permafrost temperature, the final permafrost zone and the final material sensitivity (from Table 4). Climate Change Sensitivity
Number Example
Failure Mode Permeability
Material Alluvial
Temperature (Initial Final) -5 -3
Final Zone (1,2,3,4) 2
Sensitivity (Low, Med, High) Low
Discuss the potential for sensitivity to changes in vegetation, precipitation and/or changes in depth and timing of snow cover:
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REQUIRED LEVEL OF ANALYSIS
For each combination of failure mode and material, enter the consequence level and sensitivity level into the table on Form 9. In the last column, enter the letter (into the last column) corresponding to the prescribed level of analysis from the table below.
Sensitivity (Y) High Medium Low Risk Level D C
B
Consequence (X) Negligible C D D
Minor B C C
Major A B C
Catastrophic A A B
Analysis Prescribed No action required. Perform qualitative analysis. Apply expert judgement. Document result of evaluation. Perform quantitative evaluation for projects with limited precedence in: design, function, or construction method. In addition to requirements for ‘C’: Perform limited quantitative analysis. Use engineering judgement for input parameters. Monitor permafrost performance. Perform full quantitative evaluation for projects with limited precedence.
A
Perform detailed quantitative analyses. Refine input parameters with additional investigation and testing. Perform full scale monitoring programme with periodic evaluation of performance.
Independent expert review required.
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REQUIRED LEVEL OF ANALYSIS (cont’d)
Number Example
Failure Mode Permeability
Material Alluvial
Consequence Level Major
Sensitivity Level Low
Level of Analysis C
Describe the degree of precedence for this type of engineering design including consideration of location, temperature, function and materials:
Further Recommendations for Analysis:
Judge’s Signature:
End of Worksheets
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Date:
