geotechnical implications of climate impacts in permafrost · 2018-03-14 · this presentation? •...
TRANSCRIPT
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Geotechnical Implications of Climate Impacts in Permafrost John Thornley, PE & Brenton Savikko, PE Golder Associates, Inc.
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What is Permafrost?
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How Do Changes in Permafrost Affect Geotechnical Engineering?
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What are the Tools We Use?
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What are the Tools We Use?
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What are the Tools We Use?
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What are the Tools We Use?
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What are the Tools We Use?
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Are There Other Tools?
• Climate Data • Air Temperatures, AFI, ATI
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Air Temperatures
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Air Thawing Index
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Air Freezing Index
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SNAP – Scenarios Network for Alaska & Arctic Planning
• A database created by UAF of free climate data for Alaska, including high resolution monthly climate data for ~1901-2100.
• 5 world recognized Global Climate Models used to simulate historical and future climate conditions
• 3 Emission Scenarios to model various climate trends
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SNAP – Scenarios Network for Alaska & Arctic Planning
• Final product is ‘downscaled’ climate data on 2 km
by 2 km grid for entire state which accounts for land features such as slope, elevation and coastline, as well as knowledge of local climate experts.
• SNAP purpose is to help people plan in a changing climate.
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SNAP – Emission Scenarios
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SNAP – Scenarios Network for Alaska & Arctic Planning
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SNAP – Scenarios Network for Alaska & Arctic Planning
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SNAP – Scenarios Network for Alaska & Arctic Planning
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• Thicker active layer • Warmer permafrost • Shorter winter season • Degrading permafrost where initially warm • Increased coastal erosion • Changes in river flooding patterns • Increased risk of slope instability
Climate Impacts
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Change in Conditions = Change in Permafrost
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Increased Coastal Erosion
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Riverbank Erosion
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Failing Infrastructure
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Failing Infrastructure
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Failing Infrastructure
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Increased Risk of Slope Instability
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Increased Risk of Slope Instability
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Basic Principles I - Preserve the frozen condition II - Design for thaw if the settlements can
be kept within allowable limits Variations Preconstruction thawing Preconstruction freezing of degraded
permafrost
Permafrost Foundation Design Principles
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Permafrost Pile Design
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• If a pile design is based on 30°F
• At 31°F, only 50% remains
• At 31.5°F, only 25% • So, why not design for
the unfrozen case? 0%
25%
50%
75%
100%
30°F 31°F 32°FTemperature
Pile Capacity is Very Sensitive to Warming
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What happens if the permafrost degrades
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Pile Performance in Degrading and Thaw-unstable Permafrost
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• Post and pad footings • Driven pile (steel pipe or H) • Drilled and slurried timber or steel pile • Drilled and slurried thermopile • Torqued steel screw piles • Steel space frame (Triodetic®) • At-grade with insulation and subgrade cooling
Foundation Design Options
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• Bearing on permafrost below the depth of future thaw
• Connection with pad must resist frost heave forces on post
• Can be insulated and cooled if fill material is available
Post and Pad Foundations
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• Keep frozen or design for settlement and downdrag loads
• Must be deep enough to resist frost heave
• Installation is sensitive to soil type and ground temperature and requires heavy equipment
Driven Piles
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• Allows for verification of permafrost condition
• Easy to measure temperatures • Must be deep enough to resist
frost heave • Requires drill and slurry • Pipe piles or secondary pipes
allow for future cooling systems
Drilled and Slurried Piles
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• Provides long term cooling of the soil and highest adfreeze strength
• Removes frost heave • Requires drill and slurry • Can cause frost heave below
the tip
Drilled/Slurried ThermoPiles
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Measured Temperatures
0'
5'
10'
15'
20'
20°F 25°F 30°F 35°F
10/710/1010/1210/1410/1810/20
Air Temperatures
0°F
10°F
20°F
30°F
40°F
4-Oct 8-Oct 12-Oct 16-Oct 20-Oct
ThermoPile Cooling
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• Provides positive support if perimeter thaw is controlled
• Used for concrete and timber floors, fuel tanks and water tanks
• Requires a sand or gravel fill or super-thick EPS
• Snow drifting is aggravated • Frost heave is a risk if unfrozen
zones are present • Design for the warmer winters
At-Grade Systems
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• How sensitive is the project to climate impacts and what are the consequences of potential failures
• The relationship between sensitivity and consequences defines the risk that climate impacts pose to the project
• The degree of sensitivity and the severity of the consequences are used to establish the level of climate analysis
A Risk Based Evaluation
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• Initial permafrost temperatures • Soil’s sensitivity to temperature changes • Changes in surface conditions • Lifetime of the project • Level of over-design or safety margin
Sensitivity to Climate
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• Level A - Quantitative, numerical w/ peer review and field monitoring
• Level B - Limited quantitative w/ field monitoring
• Level C - Qualitative w/ professional judgment
• Level D - Analysis of climate impacts is not needed
Level of Analysis
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Our goal as Alaskan designers should be to reduce Alaska’s vulnerability to
climate impacts
• Our factors of safety should reflect our level of ignorance and the importance of the project
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What Can You Take Home From This Presentation? • Climate will impact your foundation • It is more cost effective to address the potential effects
of thawing permafrost during design rather than 5 to 10 years down the road
• The engineering design teams that you select should anticipate climate impacts for the life of the structure
• You should engage your design team and discuss their climate assumptions
• There are current tools to aid in developing appropriate climate assumptions
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Acknowledgements
• Duane Miller, Rick Mitchells, and others from Golder • David Lockard of AIDEA • Arctic Foundations, Inc. • The Internet
QUESTIONS?