青藏高原多年冻土区不同地温分区下大直径钻孔灌注桩回冻规律试验研究

AN EXPERIMENTAL STUDY ON REFREEZING CHARACTERISTICS OF LARGE-DIAMETER BORED PILE IN DIFFERENT PERMAFROST AREAS OF THE QINGHAI—TIBET PLATEAU

为解决青藏铁路修建过程中多年冻土区桥梁钻孔灌注桩浇筑桩身混凝土对冻土层地温的影响,把握后续工程施工,需要确定钻孔灌注桩灌注后的回冻时间,目的在于为后续工程施工提供可靠的依据,如桥墩(台)的施工、上部结构的铺架以及对工期的影响等工程实际问题。针对青藏高原多年冻土区钻孔灌注桩回冻问题,从现场实际地温测试资料出发,通过对青藏高原低温多年冻土区及高温不稳定多年冻土区 2 个试验场地实测地温资料的对比分析,初步得出 2 个不同地温分区下大直径钻孔灌注桩回冻规律:低温多年冻土区天然地温较低,桩身混凝土浇筑 8 d 后,桩身首先在地面以下 3.5～7.5 m (-0.027 ℃～-0.32 ℃)及桩底处出现负温(-0.35 ℃);高温不稳定多年冻土区天然地温较高,桩身混凝土浇筑 18 d 后,桩身首先在桩底处出现负温(-0.18 ℃)。所得到的初步结论来源于现场测试结果,对工程实践能够起到指导作用。

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.