ice pad stability on sand: large-scale laboratory tests

Proceedings oh the 18th IAHR InternationalSymposium on Ice (2006)kN30201000 100 200 300 400 500Time (seconds)IncreasingverticalloadFigure 5: Horizontal load traces from one test series,for vertical loads ranging from 9 to 63 kN.surface, such that only about70% of that surface was in actualcontact with the sediments.(Since both the normal and thetangential loads are normalizedover surface area, this reductionin effective footprint does notaffect the results of our study.)When the <strong>ice</strong> was unloaded andallowed to float during the testprogram, visibility was too poor(due to the fine sediments insuspension) to allow a glance atthe bottom surface with anunderwater camera. However, upto three refrozen <strong>ice</strong> cracks a fewmillimetres in thickness running across the top surface of the block were observed later afterunloading the <strong>ice</strong>. It is uncertain how this may have affected the <strong>ice</strong>-sand interaction.)Horizontal load (kN302010F H = 0.47F V + 1.6F H = 0.39F V + 0.3600 20 40 60 80Vertical load (kN)STATICKINETICLinear (STATIC)Linear (KINETIC)Figure 6: Linear behaviour of thehorizontal/vertical load ratio. The y-intercept (uponextrapolating) represents cohesion.the <strong>ice</strong> salinity was about 5 ‰ at the beginning of testing, bureflecting the temperature fluctuations the <strong>ice</strong> had undergoneAt the end of the program, the waterwas removed from the test basin andthe <strong>ice</strong> was sectioned with a chain saw(Fig. 4). At that tim e, it had a totalthickness of 345 mm in the centre,thinning down to 260 mm along themarg ins, with a width of 1.96 m alongall edges. The second (new) <strong>ice</strong>growth phase led to a conspicuoushorizontal layering, shown in Fig. 4.The upper portion of the <strong>ice</strong> sheet wasdominated with mm-sized crystalsresembling frazil <strong>ice</strong>, which extendeddown ward in a columnar structure. Nosediments were incorporated into the<strong>ice</strong>, with the exception of thelowermost 30 to 40 mm, where the <strong>ice</strong>was slightly brownish. Interestingly,t averaged 2 ‰ at the end,throughout the test program.Friction coefficientsExamples of load traces (the cumulative response of both load cells) are shown in Fig. 5 for aseries of tests with increasing vertical load. These, and those for all other tests, display asimilar pattern of strain softening: a regular increase in horizontal load with time (ordisplacement) up to a peak, followed by a sudden drop and levelling off of the load. The peakload is that required to overcome static friction; the flat portion of the load trace is the resultof the kinetic friction. When the latter segment was not perfectly flat, an average value wasrecorded. In some instances, the load trace at that stage comprised random (as shown by someof the traces in Fig. 5) or systematic cycling, thought to result from, respectively, slip-179-