Deformations of a highway embankment on degraded permafrost

28-Mar-0906-Jul-0914-Oct-0922-Jan-1002-May-1010-Aug-1018-Nov-1026-Feb-11Vertical (m)Depth (m)28-Mar-0906-Jul-0914-Oct-0922-Jan-1002-May-1010-Aug-1018-Nov-1026-Feb-11Vertical (m)Depth (m)06-Jul-0914-Oct-0922-Jan-1002-May-1010-Aug-1018-Nov-1026-Feb-11Vertical (m)there is about 0.08m <strong>of</strong> cumulative settlement at midslopeand 0.04m at the toe.The lateral movements at the ground surface (againfrom the settlement plates) in Figures 5a, 5b, and 5c showsomewhat similar trends, though seasonal effects areperhaps only apparent at the toes <strong>of</strong> the <strong>embankment</strong>s.Again, the stable section did not move much in the two040123415114.414.41 2 2 31 2 31 2 3 4 4 5 6 7a) -0.151 2 3 4 5 6 71 - SurfaceSettlement2 - DeepSettlement3 - Thermistors4 - Stand Pipe5 - SlopeInclinometer6 - Piezometers7 - ExtensometerThe toe <strong>of</strong> the <strong>embankment</strong> shows the highest amount <strong>of</strong>frost-heave during the freezing season while the shouldershows almost no frost heave, probably due to the gravelfill on the shoulder. Gravel can be considered to be anon-frost susceptible material (Koyama and Sasaki 1967).It drains freely by gravity and does not lead to upwardmovement <strong>of</strong> capillary moisture.Elevation0.050.00-0.05-0.100.100.050.00-0.05-0.10SurfaceSettlement14.414.4SurfaceSettlementa)Date Elevationb)StableUnstableStableUnstable8b)0.100.050.00StableUnstableElevation Date-0.0514.4SurfaceSettlementc)20-0.10Figure 3. Instrumentation at a) stable section; b) unstablesectionyears <strong>of</strong> observation – the scatter in the data is probablyabout equal to the precision <strong>of</strong> the observations. All threeplates at the unstable section show cumulativemovements, about 0.07m at the shoulder and mid-slope,and 0.03m at the toe.‘Frost heave’ <strong>of</strong> the road <strong>embankment</strong> is associatedwith the formation <strong>of</strong> ice lenses in seasonally frozenground. Heave results when the water freezes, attractsadditional water by capillary action, and forms ice lenses.Lenses form in all soil types by the addition <strong>of</strong> waterduring stationary or slow movement <strong>of</strong> the freezing frontinto the soil but are most common in silts and fine sand.The supply <strong>of</strong> water and ease <strong>of</strong> water movement(hydraulic conductivity) determines the extent andthickness <strong>of</strong> ice lenses. Frost heave occurs at the frostline, with T ≤ 0°C (Andersland and Ladanyi, 2004).The seasonal heaves seen in Figure 4 for theunstable section are believed to be due to frost heave.DateFigure 4. Vertical movements (settlements) at shoulder;mid-slope; and toe <strong>of</strong> the <strong>embankment</strong>3.2 Lateral Displacements from Shallow ExtensometersVibrating wire extensometers with a gauge-length <strong>of</strong> 1.0mwere installed at the toes <strong>of</strong> both test sections at a depth<strong>of</strong> 0.8m. The instruments were intended to monitor lateraldeformations at shallow depths and hopefully to relatemovements at the toe to lateral spreading and longitudinalcracking <strong>of</strong> the road surface. Unfortunately, theextensometer at the stable section failed shortly afterinstallation. Figure 6 shows horizontal strains at the toe <strong>of</strong>the unstable section plotted against time over a 28 monthperiod in 2008-2011.