The effect of frozen stagnant water and its impact on slope stability: A case study of Heifangtai, Gansu Province

The frequent landslides during freeze-thaw period have not only caused increasing social concern but also aroused much attention among researchers. With frequent occurrence of landslides during the transition period between winter and spring, Heifangtai of Gansu is the priority for studying the seasonal freeze-thaw action. For the purpose of exploring the problem whether the effect of frozen stagnant water exists or not and revealing the mechanism of landslides induced by freeze-thaw action, a coordinated monitoring network incorporating air temperature, ground temperature and real-time groundwater table was established. The monitored data show that seasonal freeze-thaw phenomena do exist in Heifangtai. The phenomena can be further divided into yearly cycling process which includes the partially frozen period in early winter, the totally frozen period in deep winter, the freeze-thaw period in spring, the melting period in summer, and the daily cycling process which is caused by daily changes of air temperature. The 1.0 m elevation of groundwater table due to the freezing of soils in the middle of the slope confirms the existence of the effect of frozen stagnant water. The groundwater simulation reveals that the effect of frozen stagnant water causes the groundwater table to rise by more than 3m at the toe of the slope and affects a horizontal zone by more than 30m into the slope. The results of slope stability analysis show that the coefficient of slope stability before freezing is 1.19. After freezing, the stability coefficient is reduced to 1.09 if only the groundwater table rise due to the effect of frozen stagnant water is considered. If the reduction of strength of loess after repeated freeze-thaw action is considered, the slope stability coefficient is reduced to 0.97. These data demonstrate that the dual functions of the effect of frozen stagnant water and cyclic freeze-thaw are the fundamental factors responsible for the frequent occurrence of landslides during spring.