Deformations of a highway embankment on degraded permafrost

Depth (m)Depth (m)Depth (m)Depth (m)Stable Section, Mid SlopeTemperature (˚C)-6 -4 -2 0 2 4 6 8 10 12 14 16 18 20-10Min AveMax12342008-092009-105Stable Section, ToeTemperature (˚C)-6 -4 -2 0 2 4 6 8 10 12 14 16 18 200AveMaxMin12342008-092009-10Figure 10. Annual temperature envelopes betweenNovember 2008 and October 2010 for the stablesection at the a) mid-slope; b) toeFigures 9 and 10 show average temperature pr<strong>of</strong>ilesover two years at the four instrumented locations, that isat the toes and mid-slopes <strong>of</strong> the two test sections. Nosub-zero temperatures were observed during 2008-09 and2009-10. The average temperatures at all four locationsdecrease with increasing depth. This indicates a net heatflux into the foundation soils. It also supports theobservations in an earlier section that initially frozen soilhas thawed during the lifetime <strong>of</strong> the soil - there has beengeneral warming <strong>of</strong> the <strong>embankment</strong>s and theirfoundations.Figure 11 shows the monthly temperature pr<strong>of</strong>iles forone cycle <strong>of</strong> cooling and heating between November 2009and October 2010. The two graphs in Figure 11 containdata from instrument clusters at a) mid-slope and b) toefor the unstable section. (For reasons <strong>of</strong> space andclarity, we are not including observations from 2008-2009and 2010-2011.)With increasing depths, seasonal differences intemperature become less. The depth <strong>of</strong> zero annualtemperature amplitude is the depth at which the seasonalvariations <strong>of</strong> temperatures are essentially zero(Andersland and Ladanyi, 2004). Figures 11a and 11bshow that the depth <strong>of</strong> zero annual amplitude at thea)b)unstable section is about 8 – 9m below the original ground012345678910Unstable Section, Mid Slope, Nov 09-Oct 10Temperature (˚C)-6 -4 -2 0 2 4 6 8 10 12 14 16 18 20-10a)123402-Nov-09 15-Nov-09505-Dec-09 21-Jan-10619-Feb-10 15-Mar-10714-Apr-10 15-May-10815-Jun-10 16-Jul-10916-Aug-10 13-Sep-101025-Sep-10 04-Oct-1011Unstable Section, Toe, Nov 09-Oct 10Temperature (˚C)-6 -4 -2 0 2 4 6 8 10 12 14 16 18 2002-Nov-0905-Dec-0919-Feb-1014-Apr-1015-Jun-1016-Aug-1025-Sep-1015-Nov-0921-Jan-1015-Mar-1015-May-1016-Jul-1013-Sep-1004-Oct-10Figure 11. Monthly temperature pr<strong>of</strong>iles betweenNovember 2009 and October 2010 for the unstablesection at the a) mid-slope; b) toelevel.The respective temperatures pr<strong>of</strong>iles for the mid-slopeand toe converge at about the same depth, although thereis approximately a 1.6°C difference between the twovalues. The soil underneath the <strong>embankment</strong> is colderthan soil at the same depth underneath the toe. It isbelieved that this can be explained by the previouspresence and thawing <strong>of</strong> a frost bulb underneath the<strong>embankment</strong> or alternatively by differences in thermalconductivity between gravel in the <strong>embankment</strong> and clayin the foundation soils. This observation could have beenverified if thermistor strings had been installed under thecentre <strong>of</strong> the <strong>embankment</strong>. Perhaps unfortunately, safety,snow clearing, and grading made this impossible. Coolertemperatures beneath the <strong>embankment</strong> may also be dueto an insulating cover <strong>of</strong> snow beneath the toe, combinedwith the effects <strong>of</strong> show clearing on the roadway.4 SUMMARYb)