Macro-mesoscopic Study Of Dual-porosity Effects On Soil Thermal Conduction

Underground space has been recognized as one kind of valuable space resource,which has been widely used as a solution to many problems like traffic jams and land shortage.Consequently,The development and utilization of underground space is of great significance to the sustainable development of the city.Simultaneously,shallow geothermal energy and geothermal energy are also precious natural resources,and has been gradually developed and used.During the collaborative development and utilization of underground space and geothermal energy,it is necessary to study the heat transfer characteristics of surrounding rock(mainly soils).Currently,researches on thermal conductivity of soil are mainly dependent on test to reveal the changing rules of soil thermal conductivity.Notably,thermal conductivity is only the macroscopic average reflection of heat conduction process through soils,and it is just a simple index to measure the overall thermal conductivity of soils.However,this index conceals not only the difference of local heat flow movement in soil,but also ignores the inhomogeneity of the whole heat flow distribution in soils.Therefore,it is not comprehensive enough to study soil heat conduction characteristics from the perspective of thermal conductivity,which lacks analysis on actual movements of heat flow in soils so that it cannot reveal the internal mechanism of the changing rules of soil heat conduction.To sum up,it is necessary to apply an insightful investigation of soil thermal conduction.Soil structure has a undisputable effect on its thermal conduction behavior.Heat flux always goes through the region of particular skeleton with lower thermal resistance showing a typical phenomenon of preferential flow.Due to the specialty of soil structures,its thermal conduction behavior is quite different from other materials with regular structure.Hence,a special focus on soil structural characteristics is need when working on its thermal conduction.In the present study,a typical structural soil is chosen as the object with dual-porosity system.Combined with the correction principle of multi-thickness,an improved steady method is developed to measure soil thermal conductivity accurately.Then a series of tests are carried out to measure the thermal conductivity of dual-porosity soils.At the same time,we develop a mesoscopic model system for soil heat conduction,including soil structure modelling and heat conduction simulation,then a systematic simulation of dual-porosity effects on soil thermal conduction behavior is employed in order to further understand the thermal conductivity of soil,deeply reveal the mechanism of soil thermal conductivity and scientifically predict soil thermal conductivity.The main results are as follows:(1)Heat conduction state of soils usually fluctuate less in underground engineering,and it is in a steady state for heat conduction.Hence,a steady method is adopted to measure soil thermal conductivity because it is closer to actual heat conduction condition of soils.However,there is a shortcoming for steady method,which corresponds to thermal contact resistance.The presence of thermal contact resistance will make the measurement smaller,which leads to an unreliable result.Thereby,a modified steady method is developed to remove the effect of thermal contact resistance based on the principle of multi-thickness test.This improved method can improve measure accuracy that will lay the foundation for the systematical tests of soil thermal conductivity.(2)Based on the improved steady method,the thermal conductivity of compacted Xia-shu soil is tested systematically.The results show that thermal conductivity will increase as water content and dry density increase.Simultaneously,when keep dry density constant,the λ-w curve presents a piecewise response with an inflection point.Obviously,the physical reason of this special phenomenon is closely related to the preferential water distribution characteristics,which can be revealed by the mesoscopic heat conduction simulation for compacted Xia-shu soil.(3)A dual-porosity model is adopted to model the characteristics of soil thermal conduction containing intra-aggregate pores and inter-aggregate pores.Based on QSGS method and integration strategy.Then,a new modeling method to construct mesoscopic structure of soils with dual-porosity system is developed.In this model,the preferential distribution of water between intra-aggregate pores and inter-aggregate pores has been paid special attention.(4)Heat conduction codes have been programmed when soil structural model has been completed by Matlab language.Meanwhile,the effectiveness of the code is verified by comparison with analytical solutions and the results of commercial software as well as experimental data.Furthermore,it also proves that the mesoscopic simulation can give an accurate prediction of soil thermal conductivity at various dry densities and water contents.It can provide an intuitive exhibition of the details of local heat conduction process.(5)Based on the model and simulation method above,the effects of dual-porosity system on thermal conductivity are investigated systematically.The results show as follows:①Keep total void ratio constant,the void ratios of intra-aggregate pores(em)and inter-aggregate pores(eM)change reversely.Thermal conductivity will decreases as the void ratio of intra-aggregate pores increases.②Keep the void ratio of intra-aggregate pores or inter-aggregate pores constant,the void ratio of the other pore-system just increases,the thermal conductivity will be more sensitive to the change of em than eM.③Keep em and eM fixed,and water is preferentially distributed within intra-aggregate pores rather than inter-aggregate pores.The response of thermal conductivity to water content just presents a piecewise rise shape.(6)In the present study,the research technology about soil thermal conduction behavior is successfully employed into Su-Tong GIL Comprehensive pipe gallery project.Twenty undisturbed fine-gained soil samples were tested to obtain their thermal conductivity,and a series of simulations were also carried out at the same time.The results show that the predicted data agree well with measured results,which indicates that the mesoscopic model can be used to predict actual thermal conductivity of soils.Besides,the above predictive model was employed into the water tunnel project of Changshu power plant.Considering it is difficult to get undisturbed samples of gassy soft-soils,a series of simulations were performed to illuminate the thermal conduction characteristics of this special soil.It has been revealed that the presence of shallow gas has a vital influence on thermal behavior of gassy soft-soils.These findings will provide necessary theoretical basis for the rational value of the thermal conductivity of this special soils in freezing construction design.