青藏直流输电工程粗粒冻土地基温度监测与分析

MONITORING AND ANALYSIS OF COARSE-GRAINED FROZEN SOIL TEMPERATURE IN QINGHAI-TIBET DC TRANSMISSION LINE ENGINEERING

为分析青藏直流输电线路工程冻土地基的冻结情况及其对基础安全稳定性的影响,在青藏高原五道梁地区对装配式原型基础冻土地基进行1个冻融过程的地温监测,结合该地区气温资料,分析粗粒冻土地温随时间变化特点和沿深度分布情况。监测结果显示:地温呈周期性波动,振幅随深度增加而减小,原状和回填冻土地基上部均存在冻融状态交替的冻融活动层;监测期内基础底部冻土处于冻结状态,基础安全稳定;原状和回填冻土最大融化深度分别为3.0和3.2 m,通过建立地温估算公式,并利用地温变化幅值、均值等结果,得到原状多年冻土上限为3.1 m,与工程勘测和监测结果一致;建立冬季时高孔隙率回填冻土地基传热方程,分析地基传热性能和与孔隙率直接相关的地基中空气自然对流速度对地基回冻的影响。研究表明:冻土回填扰动加剧地温波动的振幅和增大冻土融化深度,但影响程度和范围有限;输电线路冻土装配式基础冬季施工,在冻融活动层深度内保持地基适当孔隙率,既可在冬季加速地基回冻,又可利用土体自然固结和融沉,提高压实度,从而在暖季减弱热量向地基深部扩散,有利于地基保持冻结。

To analyze refreezing of frozen ground and its influence on foundation stability in the Qinghai-Tibet DC Transmission Line Engineering,temperature monitoring for a freezing-thawing process is carried out for the assembling prototype foundations and frozen soil in Wudaoliang area on the Tibetan Plateau.Combining with atmospheric temperature data in the area,the ground temperature changing characteristics with time and distributing along the depth of undisturbed and backfill coarse-grained frozen soil are analyzed.Monitoring results show that：（1） Ground temperature is periodic fluctuation and its amplitude decreases with increase of depth.There is a freeze-thaw state alternating layer in the upper undisturbed and backfill soil.（2） In the monitoring period,the frozen soil below foundation slab is in frozen state and the foundations are stable.（3） The maximum thawing depths of undisturbed and backfill frozen soil are 3.0 and 3.2 m respectively.Through the establishment of ground temperature estimation formula,and based on the results of ground temperature change amplitude and mean value,etc.,it is obtained that the permafrost table is 3.1 m,which is consistent with the results of engineering survey and monitoring.（4） Using heat transfer theories,ground heat transfer model of high porosity frozen soil backfilled in winter is established.The heat transfer ability and air natural convection velocity effects on refreezing of ground are analyzed.The results indicate that：（1） Backfill disturbance of frozen soil aggravates temperature fluctuation amplitude and increases the thawing depth,but the effect degree and range are limited.（2） Construction of transmission lines in frozen soil and assembly foundations in winter and keeping proper porosity of the backfill frozen soil in freeze-thaw active layer are beneficial not only to accelerate the ground refreezing,but also to increase compaction degree because of soil natural consolidation and thawing settlement.So,the frozen state in the warmer seasons could keep while heat diffusion to deeper ground weakens.