Research On The Thermosyphon Foundation Of High-Voltage Transmission Tower In Permafrost Region

So far, there hadn't been interiorly any systemic test research or numerical calculation on combination of the bearing capacity of the foundation of the transmission tower with the preservation of the frozen state of the frozen soil in the permafrost region. The direct-current (±500kV) transmission project, from Hu Lun Bei Er to Liao Ning Province, was across the permafrost region, so it was very significant and practical to study the design method to ensure the construction period, the security and the regular service of the project.To satisfy the requirement of the project, the new-model thermosyphon foundation was applied, and the thermosyphon was wrapped with concrete, so this special foundation can not only bear the load from superstructure, but also preserve the permafrost soil from thawing.The main investigation contents: According to the conditions of the permafrost region and considering the global warming, firstly, the thermal and mechanical finite element model of the thermal pipe foundation was established with the function of the thermal and nonlinear mechanical analysis of ANSYS, and then the distribution comparison between temperature fields of the thermosyphon foundation and the normal pole foundation was obtained in the permafrost region. The thermal calculation results indicated that the frozen table rose with 0.5m, and the inner temperature lowered with 2℃, in comparison with those with the normal pole foundation in the same permafrost site, which can prove the distinct permafrost-preserved effect of thermal pipes. In response to the distribution of the temperature fields of ten years in the permafrost region, it was concluded that the permafrost-preserved effect of thermal pipes was in direct ratio to the ages that they had worked for in the project. With the increase of the area-ratio and the length of the thermosyphon, a better cooling effect was obtained on the ground base; when the spacing between the thermo pipes was 5m, the cooling effective was optimum, and no matter what the change of spacing parameter was, it would reduce the work efficiency of the thermosyphon. On the basis of above thermal analysis, the appropriate design parameters of the thermosyphon foundation were attained. Secondly, in terms of the thermal calculation results, the soil body was divided into several calculating layers, and then the mechanical parameters of each one were obtained under the different conditions of the frozen or thawing state. The authors computed the load capacity of this special foundation, while it bore alternative loads between tensile and pressure(caused by wind), and horizontal loads in the frozen or thawing state in the permafrost. The result showed that the horizontal displacement was larger than the vertical displacement with two orders of magnitude, when the whole seasonally frozen layer was thawing, which showed that the horizontal loads were the control loads in this state. Under the same horizontal loads and other conditions, the the horizontal displacement of the normal pile foundation was 4 times larger than that of the thermosyphon foundation, and it proved that the thermosyphon foundation provided a stronger vertical-build-in force. The load capacity of the thermosyphon special foundation was improved much with the augment of the thermal pile's length and diameter, but the change of the elastic modulus of the piles had little influence on the load capacity.Finally, the reasonable design parameters were provided to the project, and the investigation was significant to the design of the thermosyphon foundation in the permafrost region in China.