Deformations of a highway embankment on degraded permafrost

01-Nov-0809-Feb-0920-May-0928-Aug-0906-Dec-0916-Mar-1024-Jun-1002-Oct-1010-Jan-11Total Head (m)Depth (m)01-Nov-0809-Feb-0920-May-0928-Aug-0906-Dec-0916-Mar-1024-Jun-1002-Oct-1010-Jan-11Total Head (m)Depth (m)3.4 Ground Water DataFigures 8a and 8b show total heads plotted against time(indicated here as dates) at two different depths in thestable and unstable sections respectively. (Rememberthat the silty clay is much thicker beneath the unstablesection [Figure 2].) The figures imply seasonal changes inthe gradients <strong>of</strong> total head at both sections. Ground waterconditions appear to be broadly hydrostatic duringsummer months, with the ground water levelapproximately at the ground surface. During freezing,upwards gradients develop. These are believed to becaused by upward flow <strong>of</strong> water towards suction pressures(negative potentials) at the freezing front as it movesdownwards during the winter.Stable Section at the Toe1.51Depth (m)a)3.40.51.50-0.5-1-1.5-21.510.50-0.5-1-1.5-2Unstable Section at the ToeDepth (m)8.74.5b)3.5 Ground TemperaturesFigures 9 and 10 show minimum, average, and maximumtemperature pr<strong>of</strong>iles with depth for the unstable and stablesections respectively, for years 2008-09 and 2009-10.Please note the different scales used for depths in thesefigures. They arise from the different thicknesses <strong>of</strong> thefoundation soils under the two sections, (Figure 2). Again,the original ground level is at depth ‘0’.Unstable Section, Mid SlopeTemperature (˚C)-6 -4 -2 0 2 4 6 8 10 12 14 16 18 20-10123456789101145678910MinAveMax2008-092009-10Unstable Section, ToeTemperature (˚C)-6 -4 -2 0 2 4 6 8 10 12 14 16 18 200123MinAveMaxb)a)2008-092009-10Figure 8. Total head vs. time at different depths at the toe<strong>of</strong> the a) stable section; b) unstable sectionThe effects <strong>of</strong> frost action involve a combination <strong>of</strong>frost heave during a downward advance <strong>of</strong> the freezingfront, with the accompanying formation <strong>of</strong> ice lenses; anda subsequent reduction <strong>of</strong> shear strengths and highercompressibilities when the ice melts during the springthaw (Andersland and Ladanyi, 2004).Figure 9. Annual temperature envelopes betweenNovember 2008 and October 2010 for the unstablesection at the a) mid-slope; b) toeThe figures show temperatures measured by thethermistor clusters at the toes and mid-slopes <strong>of</strong> bothsections. ‘Geothermal gradient’ is the rate <strong>of</strong> change <strong>of</strong>temperature with depth in the ground. When averagetemperatures are examined in this way, the slope <strong>of</strong> theaverage temperature pr<strong>of</strong>ile can be taken as anapproximate indicator <strong>of</strong> net heat flow into, or out from, theground surface over a period <strong>of</strong> one year.