Study On Engineering Characteristics And Temperature Control Of Phase Change Material And Gravel Mixed Soil For Core Wall Anti-freezing

Gravelly soil core rock-fill dam construction faces the freeze-thaw challenge of clay in cold regions.A negative temperature would cause the compactibility degeneration of frozen core-wall material.In addition,some cracks after freeze-thaw cycles will lead to the deterioration in strength and impermeability.In practice,it is recommended to pause the construction progress or cover with thermal insulation materials at a negative temperature to prevent core wall material from freezing.However,the release and reclaim process of thermal insulation measures need extra work and interfere the core-wall compaction process,which would reduce construction efficiency with a high risk of project delay.Based on heat storage/release property of phase change material(PCM),this dissertation developed a phase change material incorporated into gravelly soil to improve the construction efficiency in a cold period.The influence of phase change material on mechanical properties,impermeability and temperature control performance of core wall gravel soil was analyzed through the mechanical tests,temperature control experiments and heat transfer simulation.The feasibility of gravelly soil incorporated with PCM as core-wall material and antifreezing material during winter construction were verified.This research will provide a potential application of PCM to improve the anti-freezing performance and extend the winter construction time of earth-rock dam,which has great significance for accelerating the construction progress of gravelly soil core wall rockfill dams in cold regions.The main results of this dissertation can be summarized as follows.(1)The engineering properties of phase-change gravelly soil were determined by geotechnical tests,including the physical indicators of gravel-mixed gravelly soil,the maximum dry density and the optimal moisture content of phase-change gravelly soil with different PCM contents,which providing a control standard for test samples preparation.The strength and impermeability performance of phase-change gravelly soil with different PCM contents were analyzed.The results indicate that the shear strength,unconfined compressive strength and impermeability of phase change gravelly soil are significantly higher than those of the gravelly soil without PCM.(2)The thermal properties(e.g.,thermal conductivity and specific heat)of phasechange gravelly soil with different PCM contents were analyzed by the thermal constant test.Thermal cycling test of phase-change gravelly soil was conducted to investigate the surface temperature and temperature profile at different depths.In addition,the heat storage-release mechanism during temperature evolution of surface layer of phasechange gravelly soil was revealed.The effectiveness of temperature control of phasechange gravelly soil during winter construction was verified.(3)According to thermal parameters of the phase-change gravelly soil verified by the temperature control test,a finite element simulation of phase-change heat transfer was carried out under construction conditions.The influence of different ambient air temperatures on temperature control performance of the phase-change gravelly soil during winter construction was analyzed.The simulation result shows that the phasechange gravelly soil with 6% PCM content is able to anti-freeze under different ambient air temperatures,which increases the positive temperature time of the phase-change gravelly soil during construction.It is helpful for selecting an appropriate temperature for core wall material preparation and providing certain guidance for reasonably lifting time of upper pavement layer,thereby providing a new way to improve the anti-freezing problem and fasten construction of core wall soil material in cold regions.