Research On The Load Bearing Property Of Bored Single Pile In Permafrost Areas

Based on the analysis of physical and mechanical characteristics of frozen soil, the working mechanism of cast-in-place pile in permafrost areas on the Tibetan Plateau is studied in this paper. The main factors affecting the bearing capacity and the stability of pile foundation in permafrost are presented. Some key technical problems of pile engineering in permafrost were discussed. These problems comprise the formation process of freezing strength between the pile and frozen soil, the variations of the pile's bearing capacity during the freezing back period and the law for distribution of ground temperature field at different ages of pile formation, etc.During the construction of concrete filling pile in permafrost, the heat provided by hydration leads to increase of the ground temperature and thawing of the frozen soil around the pile. Then, with the decrease of the hydration heat, the soil freezes back slowly. In this period, a self-balanced testing method was adopted to test the bearing capacity of the pile for two times respectively. The results indicate that the bearing capacity increases as the soil temperature decreases. From the Q-S curve of single pile, conclusion can be drawn that after about one month of soil freezing back, the pile can provide enough bearing capacity for the next step of construction.The heat of cement hydration brought into the frozen earth during concrete engineering should be taken into account. The response of soil temperature profile to the construction of engineering piles in permafrost is studied by analyzing the in-situ data. For the specific situation observed, it is found the temperature in the pile decreases to below 0℃in 20 days after reaching its maximum value; however, it may be a long process for the soil temperature around the pile to recover its original value.Based on the theory of heat conduction, a heat analysis model of the temperature field of ground around a single cast-in-place pile in permafrost was established. The variations of temperature field of ground are calculated by using the finite element method with consideration of actual boundary conditions and initial conditions of temperature. The FEM solutions and it's comparison with the measured data indicate that numerical analysis by program can simulate the recovery of the soil temperature effectively.The conclusions drawn out from this paper can provide reference for working out construction plans of cast-in-place pile in permafrost regions.