Permafrost - Started November 15th, 2011

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Unformatted text preview: Started November 15th, 2011 !""#$"%&'&"()* !  !"#$%$#&'()*$+,(*$-&&.$/"*$0&1$+,&$2341*&$34+5()&6$ !  7&"8$2,"0+&1$9%$ Envir Sc 1 G03: Earth & the Environment Ice Beneath our Feet: Permafrost$$ *periglacial Outline 1. 2. 3. 4. 5. 6. Introduction Distribution of permafrost Ground ice, thermokarst Frost action Slope processes Environmental problems 1. Introduction What is permafrost? !  rock or soil material that has remained below 0ºC for 2 or more years Upper surface thaws in summer - active layer 1-3m deep Permafrost table - limit of seasonal thawing TALIK - unfrozen water Active layer Permafrost Why is permafrost important to Canadians? !  50% land area underlain by permafrost !  25% earth's total land area !  permafrost common in periglacial (cold climate areas) Fig. 16.38 2. Distribution of permafrost !  High latitude areas (Arctic regions) !  CONTINUOUS PERMAFROST !  up to 700 m thick !  mean annual air temps < -7ºC * N S Fig. 16.38 Distribution of permafrost.. !  Towards south (south of tree line) !  DISCONTINUOUS PERMAFROST !  thins to only few metres, patchy and sporadic !  mean annual air temps > -7ºC* 3. Ground ice !  Frozen moisture below earth's surface !  varies from small amount in pore spaces to large ice bodies Ground ice Fig. 16.39 Ground ice forms: !  Pingoes (ice-cored hills) Pingo - 20 m high - essentially a hill filled with ice, maybe groundwater fed, or a spring Fig. 16.39 Ice wedges Disturbance of ground ice !  Causes melting of the ice !  THERMOKARST !  erosion of stream banks microclimate changed landscape Fig. 16.41 thermokarstic area Landslides – e.g Mackenzie River, NWT Thaw Lakes THERMOKARST also causes.. !  Ground Subsidence Road disturbance house collapse 4. Frost action !  Freeze-thaw processes !  water expands by 9% on freezing !  can fracture rocks Produces frost-shattered screes, felsenmeer (block fields) scree slope Ice growth !  Occurs !  preferentially below stones in soil ice lifts stones - 'heaving' Heaving produces... !  Stone polygons stone strips Fig. 16.40 dierential expansion causes all this 5. Slope processes !  Active layer has high water pressures !  prone to mass movement - SOLIFLUCTION Slow downslope movement of soil- few cms/yr Produces solifluction lobes/terraces Creep !  Very slow rates of movement (<5mm/yr) !  can aect stability of built strutures 6. Environmental problems !  Frost Heave !  growth of ice in soil !  expands, lifts structures Causes displacement of roads, bridges, buildings need to get down to bedrock to get a strong foundation Minimizing impacts …..frost heave !  Avoid building over permafrost !  Mount structures on piles - anchor in bedrock Problems…..Thermokarst !  Melting of ice causes ground subsidence !  movement of heavy machinery regulated Disturbance of permafrost can take 10 to 30 years to stabilize How do you move hot oil in a pipeline over permafrost? Alaska pipeline !  Heat must not pass down into ground dissipated with air cooled 'fins' Fig. 16.42 Crosses from North slope to south coast of Alaska Mackenzie gas pipeline - A proposed 1220 km natural gas pipeline system along the Mackenzie Valley - Will cross large areas of permafrost http:// Minimizing impacts….thermokarst ! Place structures on gravel pad or fill !  add insulating matting below pad Add heat exchangers to foundations BOX 16.8 Problems….water supply and disposal !  Water and sewer services above ground !  in UTILIDORS (e.g. Unuvik) - Insulated aluminum boxes Problems…water supply and disposal !  Services placed below ground in gravel-filled trenches !  e.g. Dawson City +#*,-* !  !"#*$+,(*$-&&.:$ !  ;+"1+$+,().()'$"#34+$+,&$<)"5$&=">$ ...
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This note was uploaded on 11/28/2011 for the course ENV SCI 1G03 taught by Professor Padden during the Fall '11 term at McMaster University.

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