Abstract：It is necessary to understand the effect of cast-in-place bored bridge piles in the construction of the Qinghai—Tibet railway on variations in ground temperature around the piles and further to determine the time required for refreezing after the construction of piles in order to provide scientific support for the subsequent constructions such as those of pile caps，bridge piers and super structures and for evaluation of the time schedule of construction. Aiming at the problem of refreezing of cast-in-place bored piles in permafrost regions of the Qinghai—Tibet plateau，comparative analysis are conducted of monitored data on ground temperatures in two experimental sites：one in a low temperature region with annual average ground temperature below －2.0 ℃，and the other in a high temperature unstable region with annual average ranging from －0.5 ℃ to －1.0 ℃. The preliminary characteristics have been obtained of refreezing after installation of large-diameter concrete piles in the two permafrost regions. Negative temperatures first occurred at depths of 3.5～7.5 m below the ground (－0.027 ℃～－0.32 ℃) and at the bottom of pile (－0.35 ℃) after 8 days since installation of pile in the low temperature region. However，in the high temperature unstable region，negative temperatures (－0.18 ℃) first occurred at the bottom of pile after 18 days. On the basis of ground temperature data，the authors analyzed the refreezing characteristics in different regions and some conclusions have been obtained which can be utilized as a guidance or as a reference in the construction of pile foundations in permafrost regions.
