Study On Microstructure Change And Stress-strain Law Of Structural Loess Under Variable Temperature

As a well-known cold(thermal)source,geothermal energy widely exists in the surface soil on the earth and can be used as an energy source for heating and cooling in urban centralized buildings.Therefore,Energy Supply Piles that can use geothermal energy have become rising star among many energy-saving technologies.Its working process is a very complex process from thermal to mechanical research involving changes in the temperature field of the rock and soil around the piles,the changing temperatures have huge impact on the mechanical properties of natural loess in our country.Macroscopic and microscopic laboratory tests as well as theoretical analysis methods were adopted to study the stress-strain law of structural loess under variable temperature conditions and the microscopic mechanism was studied in this article.A binary medium model for structural loess under different ambient temperature conditions was established.The main work and research results are as follows:(1)Through the consolidated undrained shear test for structural loess and remolded loess at different temperatures(5℃,20℃,50℃,70℃)as well as different confining pressures(50kPa,100kPa,200kPa,400kPa),the results of train stress curve and strain pore pressure curve of structural loess and remolded loess under different conditions were obtained,the change law of soil strength and pore pressure under different conditions were analyzed;Meanwhile the strain-stress curves of structural loess and remolded loess were compared and analyzed and the impact of loess structure on the strain-stress curve was analyzed.The results show that under different conditions(4 confining pressures and 4 temperatures),the strain-stress curve of structural loess is in the state of softening while the strain-stress curve of remolded loess is in the state of hardening.(2)The nuclear magnetic resonance test of structured loess under different temperatu conditions was conducted and the change law of mesostructure of structured loess under different temperatures was analyzed.The study shows that the T2 spectrum distribution curve of saturated loess under different conditions presents trimodal distribution and what mainly exists inside of the soil is small-sized pores;with the increase of temperature,the pores of various sizes increase accordingly,the micro pores transform into small pores;the pore size grows slowly at lower temperatures and grows faster at higher temperatures.(3)The part of using the binary-medium model to describe the stress-strain relationship of saturated structural loess was corrected and the breaking parameter expression in the model was established.The theoretical calculation methods of the corresponding parameters in the cementing unit and the friction unit were given.By comparing theoretical calculation and experimental curve,the accuracy of the constitutive model established in this article was verified.