Evolution Mechanism Of Frozen Water And Unfrozen Water In Sandstones With Different Pore Structures Under Low Temperature Condition

By using real-time low-field nuclear magnetic resonance technology to accurately measure unfrozen water in low temperature environment,the influence of pore structure in rock samples on frozen water is analyzed,which is of great significance to reveal the freezing damage mechanism of frozen rock mass.Based on this,the effects of different types of saturated sandstones on unfrozen water and frozen water in saturated rocks at real-time low temperature are studied.After vacuum saturation of three different types of sandstones,the evolution mechanism of frozen water and unfrozen water in rock at low temperature is detected in low-field nuclear magnetic equipment.The pore structure of the three sandstones is different in the microscopic electron microscope test.In order to analyze the change of unfrozen water content generated by different pores,the pore structure of different geometric shapes is printed by 3D printer to conduct model tests under the same conditions,and the test results are analyzed,and the following research results are obtained:(1)In the range of 0℃～-5℃,capillary water and granular water basically freeze,and a small part of bound water freezes;In the temperature range of-5℃～-15℃,the relaxation signals of granular water and capillary water are basically unchanged,and the content of bound water in rock decreases with the decrease of temperature,and the relaxation signals increase in a small range.The pore water inside the rock at-15℃～-20℃ maintains the same change rule as in the previous stage.The latent heat release of water ice phase transition mainly affects the binding content at-5℃～-15℃.(2)The low temperature freezing of sandstone can be divided into three stages:0～-5℃ is the initial freezing of the model structure and the freezing evolution stage;-5 ℃ ～-10 ℃ freeze rebound stage;-10 ℃ ～-20 ℃ is the stage of model freezing development.The rock damage is relatively stable when the temperature range is-5℃～-10℃.It is concluded that rock porosity can not be used as a direct judge of freezing damage.It should be divided into two stages.The dominant factor of frost heave force at 0 ℃ ～-5 ℃ is the effective freezing porosity.At temperatures below-5 ℃,the dominant factor of frost heave force is bound water content.(3)The freezing situation of unfrozen water content varies with different pore structures.In the temperature range from 0 ℃ to-20 ℃,the freezing content of unfrozen water in triangles,quadrilateral,pentagon,hexagonal and circular molds is about 1/10,1/6,1/10,1/10 and 3/10.The variation of unfrozen water content of the three sandstones is about 4/5,3/10 and 3/10.The circular pore structure is closer to the freezing evolution law of actual rock samples.(4)The overall trend of freezing degree of the five samples with different pore structures is consistent,and the freezing degree is the same at-20℃,and the circular sample can represent irregular pore structure.The freezing degree of the overall freezing of the mold is described by the linear relationship between relaxation signal and moisture quality,and the overall distribution is M-type.The validity of the existing formula is verified by the experimental data,and the effective freezing porosity can be used to replace the pore size distribution characteristic value in the freezing model.