| During the construction of high rockfill dam with clay core in high altitude and cold regions in China,temperature control to reduce the freeze-thaw impact on corewall clay is a major challenge.At a negative temperature,it is common to see the compactibility degeneration of core-wall clay after short-term frozen.In addition,some structural defects(e.g.,frost heave,thawing settlement and structure crack)will lead to the working performance deterioration in strength and impermeability after freeze-thaw cycles.According to practical experience,the surface layer is able to protect the clay beneath from the turbulence of the ambient temperature.Thus,the protection measures for the exposed surface layer are the key to preventing the core-wall clay from freezethaw damage during winter construction in cold regions.At present,there are few anti-freezing measures for core-wall clay during winter construction in cold regions.Although some measures are able to effectively lower the risk of short-term freeze-thaw for core-wall clay,they will slow down the normal construction process because of the serious interference with core-wall clay compaction.In addition,these measures depend heavily on the energy supply from anti-freezing measures,and there are few hours suitable for construction at day.Therefore,when these anti-freezing measures fail to be optimized,they may be inconsistent with the requirement of continuous construction for the core-wall clay in winter.In order to handle the above problems in cold regions,this research developed a novel material namely core-wall clay incorporated PCM(PCM-clay)and proposed the technology and theoretical methods for the temperature control for core-wall clay during winter construction.This novel solution was inspired by studies on the latent heat of phase change material(PCM)in the fields of road and building.Then,the feasibility of PCM-clay as the core-wall clay and its effectiveness of temperature control during earth-rock dam construction in winter were investigated for a novel technical measure to solve the issue on temperature control of core-wall clay during winter construction.These technical measures and theoretical methods are able to accelerate the construction and realize the fine management of anti-freezing and temperature control measures,which has important value of theory and practical significance,especially for the construction and operation of super-high rockfill dam with clay core in cold regions.The main results of this dissertation can be summarised as follows.(1)A novel PCM-clay for the temperature control of core-wall clay during winter construction was developed.The preparation and construction methods of PCM-clay were introduced for the application.From the intermolecular interactions between the PCM and clay particles,the effects of PCM content to the physical and mechanical properties(e.g.,compaction characteristics,Atterberg limits,direct shear and unconfined compressive strength,and permeability)of PCM-clay were analysed.In addition,the feasibility of PCM-clay as the impervious material of an earth-rock dam was also verified.Engineering suggestions were also given about the performance improvement of PCM-clay.The PCM-clay provides a potential solution for antifreezing and temperature control problem of core-wall clay during winter construction.(2)The PCM content effect on the thermal properties(e.g.,transformation temperature,latent heat,thermal conductivity and specific heat)of PCM-clay was analysed,and the thermal reliability of paraffin-based PCM was discussed.In addition,a homogenization equivalent model for thermal parameters of PCM-clay was established to interpret the variation of thermal properties,which provides a reference for effective evaluation of equivalent thermal performance parameters for PCM-clay of any PCM content.(3)Two large-scale thermal cycling tests were designed to investigate the temperature history of PCM-clay(e.g.,surface and profiles),which verified the effectiveness of temperature control of PCM-clay in simulated and practical environment during winter construction.In addition,the heat storage-release mechanism of PCM-clay during temperature evolution of surface layer of PCM-clay was discussed,and the energy supply of surface clay from the PCM-clay beneath was verified.It provides the theoretical basis for analysing the temperature effect of PCMclay that is characterized by the layered compaction during winter construction.(4)To handle the complexity of variable control for the large-scale thermal cycling tests,a numerical simulation method was proposed for temperature control of PCMclay during winter construction,which ensures an accurate sensibility analysis of factors during the preparation of PCM-clay.According to the large-scale thermal cycling tests,the basic processes and boundary conditions of phase change heat transfer numerical simulation were provided.In addition,considering the crystallinity of PCM in porous media,a finite element model was established to accurately simulate the large-scale thermal cycling tests according to a practical earth-rock dam in cold regions,which provides an effective tool for the analysis and optimization of different PCMclay preparation solutions based on their effectiveness of temperature control.(5)The effect of PCM properties(e.g.,content,thermal conductivity,specific heat,latent heat,transformation temperature and temperature range)on the effectiveness of temperature control during winter construction was analysed,and the comprehensive influence was also discussed considering the preparation temperature of PCM-clay.In addition,an optimal design method for PCM-clay preparation was proposed with the basic principle and design flow,which provides a reasonable reference for the decision of the optimum PCM content and preparation temperature of PCM-clay under different conditions. |
