Permafrost Engineering in Alaska. By Don Reid, Adam Price, Sabine Eisma, Casey Wagner and Logan Wallace

Transcription

1 Permafrost Engineering in Alaska By Don Reid, Adam Price, Sabine Eisma, Casey Wagner and Logan Wallace

2 Permafrost Engineering in Alaska Basic Alaskan Geography and Geology Permafrost features and distribution Permafrost Engineering Attributes Foundations in Permafrost Water Prospecting in Frozen Soils

3 Alaskan Geology Formation of Alaska Ice Age Seismic Activity Ice Fields Mountains and Minerals Aleutian Mountain Chain

4 Formation Alaska formed ~125 million years ago Two hypotheses on formation Merging of 7 plates Formed From Merger of Pacific and N. American Plates

5 Cold Past Pleistocene Ice Age 2.5 mya 9 kya Alaska connected to Asia by landbridge due to lowered sea level (el feet)

6 Seismic Activity ~4% of all energy released by earthquakes has Alaskan origin. Caused by the Pacific Oceanic plate slipping under the North American Continental plate Mostly focused around south central Alaska ttp://geology.com/novarupta/

7 Ring of Fire

8 Alaskan Tectonics Eastern Segment Central Segment

9 Ice Fields Land areas completely covered in ice, year round. Cover 28,80 square miles 4.9% of Alaska ttp://

10 Mountains and Islands 20% of Alaska s land area is over 4,500 feet above sea level Mountain ranges split Alaska into 5 sections: The Interior The Arctic The Aleutian Islands Southeast Alaska South-central Alaska Alaska is rich in minerals Some of the many minerals deposits found in Alaska: Gold, copper, silver, mercury, platinum, tin, coal.

11 Aleutian Islands In the Ring of Fire Highly active - 26 of the 57 Volcanoes have erupted since 1970 Volcanic arc is fed from the subduction of the Pacific plate sinking beneath the North American plate No trees and little to no vegetation

12 Permafrost Definition: Ground that has been continuously below the freezing point of water for two successive winters and the intervening summer Below layer of active sporadically frozen soil.

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14 Permafrost Formation Earth has a fairly constant temperature gradient (approximately 3 degrees Celsius per 100 meters). As surface temperatures go below freezing, permafrost can form to varying depths.

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16 Distribution of Permafrost Generally continuous north of Fairbanks and discontinuous in southern portions of Alaska.

17 Surface Formations Patterned Ground - A phenomenon on the active layer caused by freeze/thaw cycles. Freeze cycles churn and sort finer grain soils to the surface.

18 Surface Formations Pingos Frozen subsurface lake surrounded by permafrost. The density of the permafrost does not give way, forcing the expanding ice up, creating a blister like formation beneath the active layer.

19 Pingo

20 Mechanical Properties of Permafrost

21 Characterization of Permafrost Use soil grain characteristics, crystalline structure of ice, temperature ratio of interstitial ice to soil grains to determine mechanical properties Source: Nelson et al, Cold Regions Engineering. U. of Wash. 2008

22 Elastic Modulus Vary Widely, but higher in frozen soils Brittle, stiff, rocklike. Wave speed is greater (stress waves and seismic waves) Source: Smith, D.W. Ed Cold Regions Utilities Monograph ASCE, 1996.

23 Strength Ice Poor Soils Dominated by coarse grained or dense fine-grained soils Generally, strength is about the same as un-frozen soils (Can be slightly lower): τ = c f + σ n tanϕ Ice Rich Soils Dominated by loose fine-grained soils Bulk Density under 1000 kg/m 3 Strength dominated by cohesion of interstitial ice τ = c f + σ n tanϕ f Source: Smith, D.W. Ed Cold Regions Utilities Monograph ASCE, 1996.

24 Basic Effect Source: Nelson et al, Cold Regions Engineering. U. of Wash. 2008

25 Creep Slow progressive movement of a loaded frozen soil without volume change. (Smith, p 3-33) Slow liquid movement Short term (Primary) Creep: Strain Rate starts high, decreases. Secondary Creep: Long term, low. Constant strain rate. Tertiary Creep: Accelerating Strain Rate. Can be short. Soil is considered to have failed. Source: Smith, D.W. Ed Cold Regions Utilities Monograph ASCE, 1996.

26 Sample Issues with Permafrost Frost Heaving The freeze cycle of the active layer presses against the permafrost layer, forcing the freezing soil to expand upward. Thawing Building constructed on permafrost can offset the thermal balance causing thawing of the permafrost layer. Permafrost thawing results in settling, undermining foundations and structures. Excavation Difficulties Source: Smith, D.W. Ed Cold Regions Utilities Monograph ASCE, 1996.

27 Excavation Difficulties Permafrost excavation is similar to that of intact rock masses. Typical soil excavation techniques are inadequate Some permafrost excavation options Heated water bladders thaw the soil before excavation Steam jets Explosives Jackhammers Source: Smith, D.W. Ed Cold Regions Utilities Monograph ASCE, 1996.

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29 Permafrost and Foundations Rice, Building in the North, 4 th Edition, Anchorage: Alaska Science & Technology Foundation

30 Frequent Foundation Problems Up-heave caused by cyclic thawing and refreezing of soils. Thawing of soils around foundation is caused by either heat leaking from the foundation or by seasonally varying temperatures. Frost-Jacking: Poles/piles become unstable when bottom few feet is not placed in permanently frozen soil. Andersland and Ladanyi (1994), Frozen Ground Engineering, 2 nd Edition, John Wiley and Sons, Inc. for ASCE Coduto, Foundation Design, 2 nd Edition, New Jersey: Prentice Hall 2001

31 Solutions Excavation and backfill - used to change unsuitable soils (Fine-grained or poorly drained) into suitable soils, like coarse sand and gravel. Wooden Piles - anchored or not anchored, depended on the size of the active layer. -Should be placed well into the permanently frozen layer and should have a elevation of about 5 feet above ground level. -Should be wrapped in a low friction plastic sleeve covering the depth of the active layer to prevent frost jacking Rice, Building in the North, 4 th Edition, Anchorage: Alaska Science & Technology Foundation

32 Foundation Temperature Control To prevent thaw/freeze problems, soils around foundation are kept cold by Ventilation Mechanical Refrigeration Insulation Rice, Building in the North, 4 th Edition, Anchorage: Alaska Science & Technology Foundation

33 More Solutions Post and Pad Footing - mostly used by people who build their own homes. Easy to build, cheap and easy adjustable. Rice, Building in the North, 4 th Edition, Anchorage: Alaska Science & Technology Foundation

34 Permafrost and Water Supply in Alaska's North Alaska has an abundance of surface water sources such as lakes and rivers. The run- off rates are very high along the Pacific coast, dropping off significantly in the central and northern parts of the state. In the north, rivers and lakes freeze to their full depth in winter so are not practical water sources. 83% of Anchorage public water supply is from surface water. 100% of Fairbanks public water supply is from ground water. (USGS Alaska Annual Data Report 2005) Source: Smith, D.W. Ed Cold Regions Utilities Monograph ASCE, 1996.

35 Groundwater in Permafrost A. Suprapermafrost - Water above the permafrost (in the active layer). In the Arctic region, the active layer is 1~6 feet. Very little water accumulates in this layer, and what does is often contaminated and is frozen in winter. B. Intrapermafrost- Water within the permafrost layer- In order to stay in the liquid phase, water within permafrost layers must have high concentrations of dissolved solids. The water pockets are constantly changing and there is, as yet, no economically feasible way to access this source. C. Subpermafrost- Water below the permafrost layer. This is the most dependable place to find high quality water in large quantities. In general, the deeper the source, the warmer the water will be and therefore the least likely to freeze during extraction. Source: Smith, D.W. Ed Cold Regions Utilities Monograph ASCE, 1996.

36 Groundwater in Permafrost

37 Subpermafrost Water Alluvial- below very thick alluvial deposits where the permafrost layer is not too thick. Layered- Stratified bedrock may have water filled layers (aquifers) that have considerable yield. Wells should be situated along the axis of a syncline and the deepest, warmest one is likely to be the most productive. Fissured- Subpermafrost water in joints caused by tectonic forces or weathering. Joints caused by tectonics are quite deep and are rare in Northern Alaska. Those caused by weathering are much more common and can contain significant quantities of water. They are generally a lot shallower and therefore may be frozen in winter in areas where the permafrost is thin. Solution Channels- Found in areas of high limestone content. Also known as Karst water, it is found in the cavities in eroded carbonate bedrock In Northern Alaska, karst landscapes exist in the Brooks Mountains and the White Mountains. Source: Muller, Siemon W.,"Frozen in Time ASCE 2008

38 Other Water Sources Very deep lakes are plumbed in winter and the water stored in heated tanks. In small, remote settlements in the far north, such as Arctic islands, glacier and river ice is used as the only source of water. Source: Smith, D.W. Ed Cold Regions Utilities Monograph ASCE, 1996.

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