Laboratory testing on heat transfer of frozen soil blocks used as backfills of pile foundation in permafrost along Qinghai-Tibet electrical transmission line

Generally, construction for pile foundation in permafrost has to be carried out in winter to minimize the thermal distribution to the underlying or surrounding permafrost. Thus, there exists a problem that it is hard to meet the stipulated requirement to the compaction degree of the backfilled frozen soil blocks around the pile foundation excavated quickly. In order to study the effect of froze soil blocks on the heat transfer process between pile and permafrost during the construction of the Qinghai-Tibet electrical transmission line in winter, some laboratory tests were carried out for the highly porous frozen soil blocks and the naturally compacted thawed soil body, respectively. In addition, the thermal conductivities were calculated under different temperature gradient according to the measured thermal data. Results show that the convective heat transfer occurs in the highly porous frozen soil blocks at negative temperature corresponding to winter time, which is favorable for refreezing the pile foundation and lowering permafrost temperature. However, backfilling the highly porous frozen soil blocks hardly meet the requirement of compaction degree. It has dual effect on the stability of tower foundation depending on the specific site conditions such as permafrost temperature, ice content, soil type, permeability, hydraulic condition, and embedded depth of pile. Results also show that the equivalent thermal conductivity of the frozen soil blocks is over five times more than that of the thawed soil body on average. This is because the convective heat transfer occurs in frozen soil blocks in winter, which has stronger heat exchange effectiveness and can diminish refreezing time. Tests have revealed the process of heat transfer of frozen soil blocks used as fills around the pile foundation in permafrost, verified its thermal semiconductor effect, and accumulated and expanded data of the thermal conductivity.