Experimental Study On Influence Of Plasticity On Unfrozen Water Content In Frozen Soils Based On Nuclear Magnetic Resonance

Frozen soil is a four-phase medium composed of soil particles,gas,ice and unfrozen water.The mechanical properties of frozen soil such as strength and deformation are closely related to the relative content of ice and unfrozen water in frozen soil.It is very important to study the content,influencing factors and changing rules of unfrozen water for frozen soil mechanics.The plasticity index,as a key physical property of fine-grained soil,is a vital criterion for dividing soil types in engineering.Studying the influence of plasticity on the unfrozen water content,on the one hand,can reveal the changing mechanism of the unfrozen water content in various types of frozen soil,on the other hand,it is helpful to calculate the frozen depth of frozen soil foundation,which has high scientific significance and engineering value.Previous studies generally adopted natural soil as the research object when exploring the influence of a certain factor on the unfrozen water content,while various types of natural soil have large differences in particle composition,gradation,salt content,total water content and dry density.Because the influence of the above factors on the unfrozen water content is very complicated and coupled with each other,the changing laws of unfrozen water content obtained in previous studies are sometimes different or even opposite.Therefore,in order to more accurately study the influencing factors and changing laws of the unfrozen water content in frozen soil,it is necessary to manually prepare frozen soil samples,so that except for the main research factors of samples,other factors are consistent,so as to ensure the consistency of the experimental results.In this paper,two industrial standard soil materials(i.e.,ISO standard sand and GCL bentonite)and one natural soil material(i.e.,Qinghai-Tibet silty clay)are used to manually prepare saturated frozen soil samples with different plasticity indexes.Then based on the nuclear magnetic resonance(NMR),the changing rule,changing mechanism and calculating method of the unfrozen water content concerning temperature and plasticity are studied.The main research contents and results are as follows:1)For saturated bentonite-standard sand mixed soil samples and bentonite-silty clay mixed soil samples with different plasticity indexes and dry densities,multi-stage cooling tests were conducted to obtain relationship curves of unfrozen water content with temperature,i.e.,the soil freezing characteristic curve(SFCC),by using a temperature-controlled NMR instrument.It was found that the SFCC was divided into the super-cooling stage,the rapid-drop stage and the stable stage.With the increase of the plasticity of soil,the super-cooling stage became less obvious;the rapid-drop stage became gentler,that is,the unfrozen water content was larger at the same negative temperature;the stable stage was nearly unchanged.2)The distribution curves of the transverse relaxation time(T2)at different temperatures were obtained through the NMR test.It was found that the peak points of the T2 distribution curves gradually moved in the direction of decreasing pore size as the temperature decreases.In the combination of the theoretical relationship between T2 and pore size,it was concluded that the influence of plasticity on unfrozen water content was due to the differences of specific surface areas and bound water contents in soil samples with different plasticity.The greater the plasticity of soil,the greater the specific surface area,and the smaller the pore size of the water,that is,the more content of bound water.Because the freezing temperature of bound water was lower than that of free water,it was difficult for soil samples with large plasticity to freeze,and thus had a higher unfrozen water content at the same temperature.3)According to the experimental change rule of the unfrozen water content with temperature and plasticity index,the classical powerful prediction formula of unfrozen water content was improved.The improved formula overcame the problems that the original formula is not uniform in dimension and the physical meaning of the parameters is not clear and could describe the segmented characteristics of the SFCC.Subsequently,the quantitative relationship between the formula parameters and the plasticity index was established.The calculating results showed that the improved formula could predict the SFCC of different soil samples well.