Prediction and measurement of ground heat transfer in a power transmission line foundation

This thesis analyzes ground heat transfer, particularly the freezing and thawing processes, in a power transmission line foundation. Better understanding of these heat transfer processes is essential for development of thermal protection techniques to overcome the foundation problems caused by permafrost degradation and active layer freezing and thawing cycles. These problems become worse with global warming and they can cause severe damage to a wide range of infrastructure in northern regions of Canada and elsewhere.The experiment setup and predictive models are validated by comparing the measured temperatures with predicted results. Then they are used to study ground heat transfer in the tower foundation under different conditions, including detailed measurements of the seasonal freezing and thawing in the active layer above a permafrost table in the test cell. Results of measurements and predictions have found significant thermal effects of the tower footing in a finite ground region around it. The tower footing induces additional temperature increases in the summer and temperature decreases in the winter, which enhance the freezing/thawing cycles and cause an adverse effect on the stability of the tower foundation. This thesis also found that thermal effects of the tower footing vary seasonally and also spatially at different locations in the foundation.Based on the results of these analytical and experimental studies, several thermal protection techniques are proposed, including thermal barriers, ground cooling with air ventilation, and a snow/solar shade. A new thermal barrier made of phase change materials has been studied. It was found that thermal barriers reduce the amplitude of temperature cycles in the foundation and thereby alleviate damage caused by the freezing/thawing cycles. But they cannot entirely prevent thermal effects of the metal tower footing. Other thermal protection techniques must be employed to form a comprehensive thermal protection strategy for the tower foundations. It is anticipated that this research will provide useful new insight of practical utility in the design and maintenance of power transmission line foundations and other similar structures in permafrost regions.Analytical models are developed to predict ground temperatures in the foundation under different atmosphere and ground surface conditions. A variable heating strength finite line heat source model (VHS model) is developed to predict the transient thermal effects of the metal tower, thereby overcoming limitations of past line heat source models. An experimental system is also developed for ground heat transfer measurements. It includes a test cell to simulate a simplified tower foundation, as well as temperature and thermal property measurement equipment.