多年冻土区灌注桩的人工冷却试验研究

多年冻土区灌注桩修筑过程中,混凝土携带的热量和水化热会对周围冻土产生强烈的热扰动,由于基础在回冻前承载力非常小,因此桩基的快速回冻已成为该地区基础设施建设中备受关注的问题。为解决该问题,创新性地提出人工冷却法在多年冻土区灌注桩中的应用,以实现基础的快速降温和回冻,并在青藏高原北麓河进行现场人工冷却灌注桩试验。试验结果表明：人工冷却降温效果显著,试验进行到第2天,基础周围土体温度已经降至天然场温度以下;随着冷却试验的继续进行,基础和周围土体的温度会进一步降低;人工冷却有效降低了土体温度,并增加了桩基周围土体的冷储量,冷却试验结束后的第7个月试验桩的平均界面温度为-0.6℃,无冷却措施桩对应的平均界面温度为-0.37℃。承载力计算结果表明：人工冷却可以快速、大幅提高桩基的承载力,通过该措施的应用,试验桩的承载力可提高至2 231 kN,而无冷却措施桩对应时刻的承载力仅为549 kN;该措施有效缩短了冻土区灌注桩施工的后续等待时间,经过人工冷却的桩基承载力很高,冷却结束后即可进行上部荷载的施工。人工冷却可作为冻土地区桩基快速施工中的一种重要方法和有效途径,其不仅能够解决灌注桩对冻土产生热扰动这一难题,还能够有效缩短工程的建设周期,具有很高的工程应用价值。

Artificial Cooling of Cast-in-place Piles in Permafrost Regions

During the construction of cast-in-place piles in permafrost, the heat of hydration and the heat contained in concrete will cause strong thermal disturbances to permafrost. Since the bearing capacity of the foundation is quite small before full back-freezing, quick refreezing of the cast-in-pile foundation has become crucial to infrastructure construction in permafrost. In this study, the application of an innovative artificial cooling method was proposed to combat the abovementioned problem. This method was applied at the end of the curing period of the cast-in-place pile in permafrost, to realize the rapid recovery and formation of the bearing capacity of pile foundations in frozen soil. The field test for the artificial cooling of the cast-in-place pile was conducted in Beiluhe in the Qinghai-Tibet Plateau. The observed results exhibit a significant cooling effect that occurs due to of the artificial cooling measures. The application of small refrigeration equipment for 2 days lowered the temperature of the pile below the natural ground temperature. By increasing the cooling duration, the temperature of the pile and surrounding soil will drop further. Artificial cooling effectively reduces the temperature of the soil, increases the cold reserves of soil around the pile, and enhances the thermal stability of pile. Further, this method rapidly and significantly improves the bearing capacity of the pile foundation and enhances its long-term stability. The average surface temperature of the test pile was observed to be -0.6℃ and that of the comparison pile was -0.37℃, 7 months after the conclusion of the test. The calculated results indicate that artificial cooling can rapidly and significantly improve the bearing capacity of the pile foundation. The application of this method increases the bearing capacity of the test pile from 549 kN to 2 231 kN. Moreover, this measure considerably reduces the construction period. Since the pile foundation has a high bearing capacity after cooling, the upper load can be constructed without waiting for refreezing during the cold season. Therefore, artificial cooling has a high practical value and can fulfill the requirements of modern rapid construction.