混凝土的入模温度和水化热对青藏直流输电线路冻土桩基温度特性的影响

施工过程中混凝土的入模温度和水化热对多年冻土区桩基施工期间的热稳定性具有重要影响. 针对该问题，利用有限元方法定量研究了±400 kV青藏直流输电线路冻土区锥柱基础入模温度、水化热和含冰量对桩基回冻过程、温度场变化和桩底融化深度的影响规律. 结果表明：水化热影响下，桩基中心温度在第3天达到最高，桩底滞后1 d，基坑表面受其影响较小，主要受环境温度影响；第24天，桩底出现最大融化层，随着入模温度增加，融化层厚度相应增加，入模温度为6℃时融化层厚度为34 cm，15℃时为55 cm；入模温度越高，回冻时间越长，当入模温度为6℃时，完全回冻需经历52 d，15℃时，回冻时间将增加7 d. 含冰量对桩底融化深度有影响，含冰量越大底部融化深度越小；冻土年平均地温是影响桩底融化深度的重要因素，少冰高温（-0.52℃）、低温（-1.5℃和-2.5℃）冻土条件下，最大融化层厚度分别为38 cm、34 cm和25 cm. 基于上述结果，在多年冻土地区的桩基工程，建议混凝土入模温度为6~8℃，底部碎石垫层至少40 cm.

Impact of molding temperature and hydration heat of concrete on thermal properties of pile foundation in permafrost regions along the Qinghai-Tibet DC Transmission Line

Molding temperature and hydration heat of concrete significantly affect the thermal stability of cast-in-place pile foundation in permafrost regions during the construction period. In this paper, aiming at this problem, numerical method was used to research the impacts on refreezing process, thawing depth under pile bottom and temperature field of pile foundations, which were buried in permafrost along the 400 kV Qinghai-Tibet DC Transmission Line (QTDCTL). The results show that concrete temperature at pile center might reach to the highest after three days. The highest temperature at pile bottom might delay about one day. Concrete temperature at pile foundation surface has not been seriously affected by hydration heat, but mainly affected by ambient temperature. The maximum thawed depth under pile bottom appears after 24 days and increases with the increasing molding temperature. The maximum thawed depths with 6℃ and 15℃ molding temperatures are 34 and 55 cm, respectively. The higher the molding temperature, the longer the refreezing time. Refreezing time of pile foundation with 6℃ molding temperature is 52 days, while it delays one week for that of 15℃ molding temperature. Thawed depth decreases with ice content. The mean annual ground temperature is an important factor affecting thawed depth under pile bottom. The maximum thawed depth under pile bottom is 38, 34 and 25 cm in warm permafrost (-0.52℃) and cold (-1.5 and -2.5℃) permafrost areas, respectively. So the reasonable molding temperature range and the thickness of sandy gravel cushion under the pile are recommended to be 6-8℃ and at least 40 cm, respectively, according to this study.