Effects of backfill materials on frost heave characteristic of ground heat exchanger

Based on the ground source heat pump technology (GSHP) application in facility agriculture in cold regions, the frost heave characteristics of ground heat exchanger were studied. The research, based on the freezing radius tracking and pipe surface strains measurement, was carried out by frost heave experimental system. The experimental system mainly included soil tank, surrounding soil (ground soil and backfill material), U-type heat exchange pipe and cold source circulation system. In order to reflect better the effects of soil frost heave on pipe, uniform and saturated surrounding soil was used, the initial temperature of which was 4 ℃. The effects of sand-based and clay-based backfill materials on frost heave were contrasted. The frost-susceptible natural clay was applied as ground soil and clay-based backfill material, the mixture of fine sand and bentonite was applied as sand-based backfill material. In the experiment, the cryogenic fluid circulated in the U-pipe for 100 hours continuously, the temperature of which reduced from 0 to -10 ℃. The research on the characteristics of freezing area growth, buried pipe deformation and contraction were developed. Moreover, the changes of heat exchange pipe volume and flow resistance due to pipe deformation were investigated. In this study, the X direction was defined as the line through two centers of inlet pipe and outlet pipe, the Y direction was perpendicular to X direction. It could be found in both backfill materials, the running time in the X direction was less than the Y direction at the corresponding freezing radius. This was related to the structure of U-pipe with two side-by-side pipes. The difference decreased with the increase of freezing area. In contrast, the freezing radius growth rate in the sand-based backfill (the rates in X and Y direction were 1.64 and 1.43 mm/h respectively) was slightly larger than that of the clay-based backfill (the rates in X and Y direction were 1.58 and 1.37 mm/h respectively). This indicated the freezing area in the sand-based backfill was greater than that in the clay-based backfill. As the freezing area increased, the pipe surface strains in both backfills showed gradually increasing and regular difference, which indicated the pipes' cross-sections became more and more elliptical. The major axes of elliptical cross-sections coincided with the Y direction. Meanwhile, the decrease of the mean strains on the pipe surface indicated that the pipes' cross-sections were contracting. It was found that the elliptical deformation and the contraction of the pipe in the clay-based backfill were more serious than that in the sand-based backfill. By eliminating the influence of temperature drop from the mean strains, it could be found that the frost heave effect on the heat exchange pipe in clay-based backfill could be greater than that in sand-based backfill, when the freezing area exceeded a certain range. After the temperature of U-pipe dropped from 0 to -10 ℃ within 100h, it could be found that the pipe volume reduced 0.4%, the flow resistance increased 6.5%. Consequently, the pipe deformation with ovalization and contraction could be one reason for the decrease in circulation flow rate and system efficiency.